Stratospheric ozone depletion has contributed to the recent tropical La Niña-like cooling pattern

Stratospheric ozone depletion has contributed to the recent tropical La Niña-like cooling pattern | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Stratospheric ozone depletion has contributed to the recent tropical La Niña-like cooling pattern Yue Dong, Lorenzo M. Polvani, Yen-Ting Hwang, Mark R. England This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-5073590/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 22 Apr, 2025 Read the published version in npj Climate and Atmospheric Science → Version 1 posted 6 You are reading this latest preprint version Abstract Despite the continuous global warming, over the past several decades, the tropical East Pacific has experienced a cooling trend whose origin remains an area of active research. Mounting evidence has linked tropical sea-surface temperature (SST) patterns to changes in the Southern Ocean via remote teleconnections. Using a fully-coupled global climate model, we demonstrate that stratospheric ozone depletion can produce a La Niña-like tropical SST trend pattern resembling recent observations. This tropical response initially arises from mid-latitude ocean adjustments to ozone-driven surface wind anomalies, which then enhance in the tropics via positive cloud feedback and wind-evaporation-SST feedback. Our finding suggests that the observed La Niña-like tropical SST trend pattern may have been, in part, caused by the formation of the ozone hole in the late 20th century. It also implies that ozone recovery in the coming decades will likely contribute to a future weakening or reversal of the observed tropical SST trends. Earth and environmental sciences/Climate sciences/Atmospheric science Earth and environmental sciences/Climate sciences/Climate change Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Introduction Despite widespread warming on a global scale, the observed sea-surface temperature (SST) trend pattern over recent decades features a remarkable cooling in the tropical East Pacific 1 – 2 (EP; Fig. 1 a-c). This La Niña-like tropical SST trend pattern has been linked to a broad range of observed climate changes, including the anomalously low value of effective climate sensitivity derived from observations 3 – 4 , the strengthening of the Walker Circulation 5 – 6 , and the persistent drying trend in the American Southwest 7 – 8 . Climate models, on the other hand, tend to project an El Niño-like warming pattern – the opposite of the one recently observed – to emerge in the future under rising greenhouse gas (GHG) forcing 1 , 9 . The fact that models generally fail to reproduce the observed La Niña-like historical warming pattern adds considerable uncertainty to the model projections. Despite its importance, the causes of the observed tropical SST trend pattern remain elusive. While model large ensembles suggest an important role of internal variability in modulating the decadal changes in tropical SST patterns 10 – 11 , single forcing simulations also reveal contributions from anthropogenic aerosol forcing 12 – 14 and GHG forcing 15 . For example, increased aerosol emissions could drive large-scale atmospheric circulation changes featuring enhanced northern hemisphere tropical trader winds or slow timescale tropical ocean adjustment that both lead to tropical cooling 12 – 14 . CO 2 increase may also cause a transient tropical cooling response driven by tropical eastern Pacific upwelling – the so-called ocean thermostat mechanism 15 – 16 . Beyond mechanisms involving tropical local processes, mounting evidence is pointing to the role of the remote impact from the Southern Ocean (SO) via teleconnections. A La Niña-like pattern of tropical SST response has been shown in a hierarchy of model simulations with idealized 17 – 18 or realistic SO cooling 19 – 20 . Similar results are found in experiments with SO non-thermal forcings (i.e. other than SST and surface heat fluxes), notably Antarctic wind anomalies 21 and Antarctic ice-sheet melting 22 , confirming the SO remote impacts on the tropics. Building on the modeling evidence, a recent paper 23 , has proposed that the observed La Niña-like tropical SST pattern may have arisen from the formation of the Antarctic ozone hole via the Southern Annular Model (SAM) and its impact on surface winds and SO SST. Indeed, the SAM– associated with the strengthening of surface westerlies over the SO 24 – has become more positive in the late 20th century 25 – 26 , largely due to stratospheric ozone depletion 27 – 28 . Using observational reanalysis, that study 23 noted that the monthly SST anomalies associated with the SAM feature a cooling response in the SO and tropical EP, and hypothesized that stratospheric ozone depletion could have contributed to the formation of the observed La Niña-like tropical pattern through the SO-tropics teleconnections. That hypothesis, however, is open to several questions in the proposed linkage between the ozone hole and tropical cooling. First, the observed tropical cooling trend has only emerged since the 1980s, while stratospheric ozone depletion began in the 1960s and stopped in the 2000s owing to the Montreal Protocol 29 . As a result, the observed SAM increased over the second half of the 20th century but has stopped since the 2000s 30 – 31 , leaving it unclear how the ozone-driven SAM trend would have caused the delayed tropical cooling trend. Second, it is well established that stratospheric ozone depletion drives the positive SAM trend only in austral summer (DJF) 32 – 33 ; yet the observed tropical and SO cooling are present across all seasons (Fig. 1 a-c). Further understanding is thus needed to link the highly seasonal signatures of the ozone hole with the annually persistent observed SST trends. Third, most proposed theories have focused mainly on atmospheric pathways (e.g., surface mean-wind advection and cloud feedback 18 , 21 ), it remains unclear how oceanic adjustments in the SO, if any, contribute to the remote tropical response. Addressing these questions and understanding the mechanisms of remote impacts of the ozone hole requires new modeling experiments to unequivocally isolate the effect of stratospheric ozone depletion. In this study, we accomplish this by using two ensembles with the fully-coupled Community Earth System Model version 1 (CESM1). The first one, denoted “ALL”, comprises 20 members taken from the CESM1 Large Ensemble Project 34 , integrated with the same historical forcings for 1955–2005 and RCP8.5 forcing for 2006–2020. The second one, denoted “xO3S”, is a new 20-member ensemble performed in this study. These simulations are identical to “ALL” except for stratospheric ozone concentrations, which are held fixed at 1955 levels (before the advent of ozone depletion), such that no ozone hole forms in the late 20th Century (see Methods). Although we fix global stratospheric ozone, historical changes in stratospheric ozone are the largest over the South Pole and in the austral spring months when the Antarctic ozone hole forms (Fig. S1 ). Contrasting the ensemble means of these two sets of experiments thus enables us to unequivocally isolate the forced response to stratospheric ozone depletion alone, which we refer to as “OZONE” = “ALL” – “xO3S” in the rest of the paper. Results Remote Tropical Response to Stratospheric Ozone Depletion We start by showing CESM1 ensemble-mean global SST trend patterns during the recent decades 1980–2014. The ALL ensemble (Fig. 1 d-f), like all other CMIP models 1 , simulates broad warming trends and fails to reproduce the observed cooling trends in the tropical EP and the SO. On the contrary, a cooling trend response arises from stratospheric ozone depletion (Fig. 1 g-i, ALL – xO3S) and is most significant in the tropical and subtropical EP, bearing a considerable resemblance to the observed SST trend pattern. This La Niña-like tropical SST response pattern is further accompanied by a strengthened tropical zonal gradient in sea-level pressure (SLP) and southeasterly trade winds (Fig. S2), which occur in all seasons consistent with observations. To further illustrate the robust tropical response to stratospheric ozone depletion, we show the 1980–2014 trends in the tropical SST zonal gradient and the Pacific Walker Circulation (PWC) index in all individual members in ALL and xO3S ensembles and observations (Fig. 2 ). As expected, observations suggest a La Niña-like tropical SST trend pattern and an enhanced PWC trend, while the vast majority of the ALL ensemble members produce a much weaker or even opposite SST and SLP gradient, with only one member resembling the observed SST. More importantly, model biases are even larger in the xO3S ensemble, meaning that the simulated tropical climate would have trended even more toward an El Niño-like SST pattern and a weaker PWC, if stratospheric ozone depletion had not occurred. The ensemble means of tropical SST zonal gradient and the PWC index between the ALL and xO3S ensembles are significantly different, supporting the robust La Niña-like tropical response to stratospheric ozone depletion. The SST trend pattern (Fig. 1 ) and the tropical zonal gradients (Fig. 2 ) confirm a remote impact of stratospheric ozone depletion on the tropics over recent decades in our model. However, unlike in the mechanism hypothesized by the previous study 23 , the annual-mean tropical cooling response is not accompanied by a significant SO cooling trend, which only occurs weakly in the southeast Pacific sector in DJF (Fig. 1 h). Hence, focusing now on the response to stratospheric ozone alone, we ask: How does the tropical surface cooling from stratospheric ozone depletion come about? Are there changes in the SO involved in – and crucial to – the ozone-driven teleconnections, as previously proposed? Southern Hemisphere extratropical response to stratospheric ozone depletion We start by considering the SH atmospheric responses to stratospheric ozone depletion in our simulations. Following the development of the Antarctic ozone hole (Fig. 3 a) – most dramatically between the 1970s and the 2000s – we see a significant stratospheric cooling trend over the South Pole, which is largest in the austral summer DJF (Fig. 3 c), following the season of the largest ozone loss (blue line in Fig. 3 a). This cooling is due to the reduced absorption of ultraviolet solar radiation by ozone. The resulting atmospheric temperature anomalies cause a poleward shift in the SH mid-latitude jet (which is largest in DJF, Fig. 3 d). While a detailed mechanism remains elusive, the lag between the ozone hole and the jet response is well established in observations 25 , 35 and robustly simulated in models 27 , 28 , 36 . At the surface, the strengthening and pole-ward shift of the westerlies is characterized by the positive phase of the SAM 35 and, as one expects, the SAM trends are only significant in DJF 33 (Fig. 3 b). Accompanying the positive SAM trend and enhanced surface westerlies around Antarctica, the SO SSTs also exhibit highly seasonal responses to ozone depletion. During 1970–2004 (the maximum ozone depletion period, Fig. 3 a) in DJF, the ozone hole, likely via the positive SAM trends 37 , yields a significant SST cooling response in the Pacific and Indian Ocean basins and a warming response in the Atlantic basin (Fig. S3b); in JJA, SST warms in most of the basins (Fig. S3c). As a result, the annual-mean SST trend response is mostly insignificant and weak around Antarctica. The patterns and seasonality of the SO SST response also hold for the more recent decades 1980–2014 (Fig. 1 g-i). These results are consistent with recent observational studies 38 – 39 : although the observed positive SAM trend can produce some SO cooling and Antarctic sea-ice expansion, this SAM-associated SO SST and sea-ice trends are confined to DJF and much weaker than observations. These results imply a limited role of ozone depletion in fully explaining the observed SO cooling trends. Other proposed candidates, such as SO natural variability 40 – 41 , freshwater input from Antarctic ice-sheet melting 42 – 43 , or remote influence from tropical Pacific decadal variability 44 – 45 , thus warrant further investigation to understand the observed trends around Antarctica. In summary, our model simulates the expected DJF tropospheric and surface responses to ozone depletion in the SH extratropics. These include the positive SAM trend associated with the shift in mid-latitude jet and the corresponding SO SST cooling pronounced in the Pacific basin. Turning to the remote tropical response, the key question thus becomes: does the tropical SST response (Fig. 1 ) in the model result from the ozone-induced SH extratropical changes? If so, does it arise from the regional and seasonal SO SST cooling anomalies as previously proposed, or are there any other mechanisms that might cause the tropical response? Potential pathways connecting the Southern Ocean and the tropical Pacific First, to examine how the tropical EP cooling response develops, we consider a Hovmöller diagram of the SST response to ozone depletion averaged over the eastern Pacific basin (see box in Fig. 1 and Methods). A 15-year running average is applied to isolate the low-frequency (decadal to multidecadal) SST response from potential high-frequency variabilities. Focusing on the eastern Pacific allows us to capture the maximum cooling response in both the SO and the tropical Pacific shown in the trend maps (Fig. 1 ). Starting from the annual-mean (Fig. 4 a), the Hovmöller diagram shows no evidence of a connection between the tropical and SO SST response, as there is no robust SST response in the SO on the annual mean. Yet, significant cooling emerges in the subtropical eastern Pacific in the late 1990s, consistent with the trend map over recent decades (Fig. 1 g). Turning to DJF, however, the Hovmöller diagram (Fig. 4 b) reveals an interesting communication of cold SST anomalies from the SO to lower latitudes. The SO (in the eastern Pacific basin) experiences cooling throughout the second half of the 20th century, starting from the 1970s along with the onset of the ozone hole. The negative SST anomalies are mostly confined to the high latitudes until the 1990s; after that, the SO SST anomalies expand to lower latitudes, driving a maximum cooling trend in the subtropical and tropical EP between the 1990s and the 2010s. This time-evolving response thus suggests that the tropical cooling response could be in part initiated by the SO changes in DJF. Once the SST cooling anomalies reach the subtropics in the late 1990s, they appear in all seasons and therefore stay in the annual mean (c.f. Figure 4 a and b), suggesting that there are other local processes further amplify the subtropical cooling initiated from the high latitudes. To further understand the processes contributing to the significant annual-mean subtropical cooling, we perform a mixed-layer heat budget analysis (see Methods) for the SST trends over recent decades (1980–2014, as in Fig. 1 ), following previous studies 17 , 19 , 20 . The heat budget analysis shows that latent heat flux driven by surface wind changes in the central and eastern Pacific (Fig. 5 b) and shortwave radiative flux off the coast of South America (Fig. 5 c) are the two leading contributors to the simulated subtropical EP cooling response (Fig. 5 a). The wind-driven latent heat term represents a coupled mechanism, where increases in surface wind stress cause surface cooling via evaporation, posing an anomalous SST meridional gradient that is conducive to further strengthening surface winds and amplifies surface cooling – the process known as the wind-evaporation-SST (WES) feedback 46 . The shortwave radiative flux term reflects subtropical low-cloud feedback, where surface cooling in the eastern Pacific atmospheric subsidence regions increases low cloud cover via strengthening lower tropospheric stability 3 , 4 , 47 . The increased low-cloud cover enhances the reflection of incoming shortwave radiation, further amplifying surface cooling. Both processes exert strong positive feedback for enhancing the magnitude of the subtropical EP cooling, consistent with other modeling studies 18 – 19 . Furthermore, these feedbacks are not directly ozone-forced and less season-dependent, thus playing a key role in maintaining the subtropical cooling response in all seasons (Fig. 4 a and Fig. 5 a). While the WES feedback and subtropical cloud feedback appear to make major contributions to the magnitude of the simulated tropical SST response, it remains unclear what sets the timescale of the tropical SST cooling response following the onset of the ozone hole. The eastern Pacific SST Hovmöller diagrams (Fig. 4 b) show that the tropical SST cooling occurs after the 1990s and is pronounced only after the 2000s, while the SO SST cooling begins decades earlier in about 1970s, following the maximum ozone depletion and its significant surface wind response (Fig. 3 ). This delayed timescale thus suggests that – in addition to the previously established atmospheric pathways (e.g., surface mean-wind advection in the southeast Pacific 18 , 21 ) that operate on timescales of several years – oceanic pathways are also crucial to the teleconnection in our fully-coupled simulations. Indeed, the surface heat budget analysis (Fig. 5 ) confirms that oceanic processes, following the WES feedback and cloud feedback, make additional positive contributions to the equatorial and mid-latitude SST cooling trends (Fig. 5 d). Our further analysis finds that ocean Ekman advection only explains the tropical part (not shown), implying that the midlatitude oceanic process is likely associated with ocean upwelling. Although this ocean dynamic contribution is overall smaller in magnitude (Fig. 5 d) compared to the other two leading terms, it may shed light on the critical timescale of the communication between high-latitude and low-latitude oceans. Next we will focus on the midlatitude oceanic process and its potential role in the delayed tropical response to ozone depletion. To examine the oceanic process in midlatitudes, we show four time-slices of 15-year-mean DJF ocean potential temperature and meridional overturning circulation (MOC) response in the Pacific sector, evolving from 1980 to 2010 (Fig. 6 a-d). Throughout the period, a positive (clockwise) MOC response is found in the SO centered at 60°S, driven by the anomalous westerlies associated with the positive SAM response (Fig. 6 f). This SO MOC consists of northward (cold) advection at the surface causing surface cooling, and upwelling of subsurface warmer waters (Fig. S4) on the polar branch causing subsurface warming 37 . Interestingly, accompanied with this positive MOC response is an opposite MOC response centered around 45°S, which has been reported also in other modeling studies 37 , 48 , 49 . This counter-clockwise MOC is driven by the anomalous easterlies in midlatitude arising from the poleward shift of the jet (Fig. 6 f). Although this mid-latitude MOC is weaker than the high-latitude MOC, it appears to also affect SST via two distinct stages, similar to the two-time-scale mechanism proposed for the SO MOC 37 . First, on a fast timescale, its surface branch yields surface warming , via a poleward Ekman advection from warmer low latitudes to colder high latitudes. Such a warming effect is most evident in the 15-yr average centered in 1990 (note the surface warm blob at 50°S in Fig. 6 b). As time evolves, this fast surface wind-driven advection is overwhelmed by an opposite cooling effect, as the slow upwelling becomes more dominant in the second stage (at 40°S). The upwelling branch (at ~ 40°S) brings colder subsurface water to the surface (Fig. S4), driving slow surface cooling at the equatorward edge after the 2000s (note the surface cooling blob at 40°S in Figs. 6 c and d). Once the surface cooling emerges in the mid-latitudes after the 2000s, it propagates to the tropics quickly (Fig. 4 b), enhanced by atmospheric positive feedbacks from latent heat changes and shortwave cloud radiative effect (Fig. 5 ). In summary: by investigating the surface energy budget and time-evolving ocean response, we find that the remote SST response to ozone depletion results from several processes. The low-latitude SST cooling, occurring decades after the emergence of the SO response, is likely initiated by the midlatitude ocean circulation adjustment, rather than the fast advection of SO SST anomalies as previously proposed. Specifically, the ozone hole causes a poleward shift in midlatitude jet; the resulting surface wind anomalies, consisting of westerlies on the poleward side and easterlies on the equatorward side, drive a dipole pattern of MOC in the Pacific (Fig. 6 f). The midlatitude (counter-clockwise) MOC affects SST via two stages: on the fast timescale, southward warm advection associated with easterlies yields midlatitude surface warming (pronounced in ~ 1990s); on the slow timescale, the equatorward branch of upwelling brings subsurface colder waters to surface, yielding subtropical surface cooling a decade after the onset of the ozone hole (pronounced after 2000s). Although the wind-driven MOC response is stronger in DJF (c.f. Figure 6 e and f), its resulting low-latitude SST changes are persistent in the annual mean, as they are further amplified by other positive feedbacks in the tropics, including the WES feedback and cloud feedback (Fig. 5 b, c). Discussions In this study, we have provided new modeling evidence that stratospheric ozone depletion can produce remote impacts on tropical oceans, causing SST cooling trends in the tropical East Pacific. Our all-but-stratospheric ozone (“xO3S”) experiments unequivocally isolate the surface cooling effect of stratospheric ozone depletion from all other radiative forcing agents. The results imply that stratospheric ozone depletion is a plausible contributor to the formation of the recently observed La Niña-like tropical warming pattern. The main conclusion of our study broadly confirms the hypothesized impact of the ozone hole on tropical SST trends in ref 23 . However, using specifically designed model simulations, we have been able to elucidate the mechanisms whereby the Antarctic ozone hole causes remote tropical SST cooling trends. Specifically, we emphasize the following points and address the three questions posed at the beginning of this paper: The lagged timescales between the ozone hole (maximum between the 1970s − 2000s) and the tropical SST cooling (pronounced over 1980–2015 in observations): While ozone depletion in our model produces a remote impact, the tropical SST response emerges several decades after the onset of the ozone hole in 1970s. This delayed response suggests that oceanic processes play a critical role in initializing the low-latitude SST response. Our results show that midlatitude upwelling is a plausible mechanism, which itself results from ozone-induced surface wind changes. The distinct seasonality of the ozone hole and its response: While ozone depletion and its extratropical impacts are limited to austral summer (DJF), the tropical SST response persists across all seasons. This is because atmosphere-ocean coupling in the tropical Pacific provides positive feedbacks (e.g. shortwave cloud feedback and WES feedback) that maintain and enhance the seasonal SST changes. The associated atmospheric and oceanic processes: while the simulated magnitude of the subtropical and tropical SST cooling response to ozone is largely determined by shortwave cloud feedback and latent heat flux associated with wind speed changes (the WES feedback), other oceanic processes also make a contribution, particularly the midlatitude upwelling. This ocean circulation response is driven by the shift of midlatitude jets forced by ozone depletion and appears to be critical in setting the timescale of the teleconnection from high latitudes to low latitudes. Our findings support the argument that tropical SST patterns could be influenced by remote forcing from the polar regions. Unlike previous modeling studies that used heat flux anomalies to force the SO SST to change 17 – 20 , our simulations employ realistic stratospheric ozone concentrations and allow the atmosphere and oceans to freely respond, providing independent evidence for the teleconnections in the coupled system. Furthermore, while the SH extratropical impact of stratospheric ozone depletion has long been reported, investigations on its remote impact on lower latitudes have been limited to subtropical precipitation 50 – 51 . Our study establishes a remote impact of stratospheric ozone depletion (mostly over the South Pole) on the tropical oceans, highlighting a novel long-range impact of the ozone hole on tropical and global climate change. While the goal of our study has been to bring out the specific role of stratospheric ozone depletion, our results in no way exclude the possibility that other processes may also have contributed to the observed tropical Pacific cooling 2 . These may include, and are not limited to, anthropogenic aerosol emissions in the northern hemisphere 12 – 14 , transient response to CO 2 forcing 15 , natural variability intrinsic to the Pacific Ocean 10 , the Atlantic Ocean 52 or the Southern Ocean 40 – 41 , as well as Antarctic ice-sheet melting 22 . Model biases in tropical SST trend patterns may stem in part from these additional processes. For example, the transient cooling response to CO 2 forcing may be muted due to mode biases in their tropical ocean mean states 15 ; additional cooling from Antarctic ice-sheet meltwater is not accounted for in current models, due to the lack of interactive ice-sheet coupling 22 , 42 , 43 . Our study suggests that the failure to simulate the observed tropical cooling trends may also be associated with remote response to ozone depletion. Models might underestimate the tropical cooling response to ozone depletion due to their too-weak subtropical low cloud feedback 19 – a common bias in CMIP models 53 – thus biasing their simulated historical trends. One caveat of this study is that our results are based on a single model. A recent study 54 reported a similar finding using CMIP models, where a La Niña-like SST trend response to ozone is found in CMIP historical ozone-only simulations. Although this provides additional evidence to support our results, we note that those CMIP ozone-only simulations differ from our simulations in several aspects. For example, most CMIP6 models do not have a large ensemble (> 10 members) of ozone single-forcing simulations available; some models confound stratospheric and tropospheric ozone, considering the total column of ozone in their simulations. In contrast, our study provides the first set of large-ensemble all-but-stratospheric ozone simulations (to our knowledge) that allow us to robustly separate the forced response to stratospheric ozone depletion. Future confirmation from other models with different physics or different spatial resolutions will be valuable to establish the robustness of our findings. Finally, and most importantly, we remind the reader that, unlike other GHGs which have continuously increased for many decades and are projected to keep increasing in the coming decades, the Antarctic ozone layer – which was rapidly thinning out in the late 20th century 55 – has started to heal 29 and is projected to recover in the late 21st century as a consequence to the Montreal Protocol 56 . If, as our findings suggest, the ozone hole has indeed been a key driver of the recent La Nina-like tropical SST trends, its closing in the coming decades would contribute to the projected weakening of this trend pattern. It may further drive a reversal towards a more El Niño-like pattern, alongside many other mechanisms that favor this long-time reversal 2 . In that case, the ozone-driven tropical El Niño-like pattern would influence future climate change in an opposite way than the past several decades, including a potentially higher global effective climate sensitivity and more precipitation in the southeast US. Understanding and quantifying how stratospheric ozone, together with other anthropogenic forcings and natural variability, shape future warming patterns is thus critical for accurately constraining future climate change. Methods Model and simulation setup All simulations analyzed in this study are carried out by the Community Earth System Model (CESM1), with the Community Atmospheric Model version 5 (CAM5), at the 1-degree horizontal resolution. The “ALL” ensemble, consisting of 20 members, is taken from the CESM1 Large Ensemble project 34 . All ensemble members are forced by the same historical forcings for 1955–2005 and RCP8.5 forcings for 2006–2020, except their initial conditions differ slightly. The “xO3S” ensemble (20 members) is identical to ALL, except their stratospheric ozone is held fixed at the 1955 values. This is accomplished as follows: given the vertical distribution of ozone, at each grid box we fix ozone only at levels where its concentration exceeds 150 ppbv 57 – 58 . This simple procedure avoids having to define a tropopause and leaves tropospheric ozone trends (which cause a substantial radiative forcing and are unrelated to the ozone hole) identical to the one in the historical runs. Note that the large seasonal cycle of stratospheric ozone is retained in the xO3S runs. Recall that CESM1 has no interactive ozone chemistry, and thus stratospheric ozone and ozone-depleting substances (ODS) are prescribed independently in the model. In the “ALL” and “xO3S” ensembles, ODS concentrations are identical and time-dependent, such that the difference between the two ensembles reflects the impact of stratospheric ozone depletion alone , without including the radiative effect of ODS as greenhouse gases, which is large and can cause considerable tropical warming 59 – 60 . Analysis The forced response to stratospheric ozone depletion is obtained by taking the ensemble mean difference between ALL and xO3S. Linear trends in any variable of interest are computed using the ordinary least squares (OLS) regression method. The significance of trends is evaluated using the student t-test at the 95% confidence level at each grid box. The significance of responses (used in Fig. 4 and Fig. 6 ) is evaluated as whether the mean of the response from all 20 ensemble members at each grid box is statistically different from zero at the 95% confidence level. Indices used for regional analysis The tropical SST zonal gradient (Fig. 2 a) is computed as the difference between SST averaged over 110°E − 180°E, 10°S − 10°N and 210°E − 280°E, 10°S − 10°N (West minus East). A positive (negative) SST gradient refers to a La Niña-like (El Niño-like) SST pattern. The tropical SLP zonal gradient (Fig. 2 b), i.e., the Pacific Walker Circulation index 61 , is computed as the difference between SLP averaged over 180E − 280°E, 5°S − 5°N and 80°E − 160°E, 5°S − 5°N (East minus West). A positive (negative) SLP gradient refers to a strengthened (weakened) Pacific Walker circulation. The SAM index (Fig. 3 b) is computed as the difference between zonal-mean SLP at 45°S and 60°S 26 . The SST Hovmöller diagram (Fig. 4 ) is computed for SST averaged over the eastern Pacific basin defined as 220°E − 290°E (box in Fig. 1 panels g-i). Observations In Fig. 1 , we used SST observation from Extended Reconstructed SST data version 5 (ERSSTv5) 62 . In Fig. 2 , we used SST observations from ERSSTv5 and the COBE SST dataset 63 , and SLP observations from the ERA5 64 and JRA-55 65 reanalysis data. In Fig. S2, we used SLP and 850hPa winds from ERA5 64 reanalysis data. Mixed-layer heat budget analysis We perform a mixed-layer heat budget analysis to decompose the contributions to SST trends, following the method widely used in previous studies 17 , 19 , 20 , 66 . We consider the surface energy budget as: $$\:\rho\:{C}_{P}H\frac{\partial\:T}{\partial\:t}=\:{SW}^{{\prime\:}}+\:{LW}^{{\prime\:}}+\:{LH}^{{\prime\:}}+\:{SH}^{{\prime\:}}+\:{OD}^{{\prime\:}}\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(1\right)$$ where the left hand side represents the mixed-layer heat storage, with \(\:\rho\:\) being the density of ocean, \(\:{C}_{P}\) the specific heat of the ocean, \(\:H\) the ocean mixed-layer depth and \(\:T\) is surface temperature. The right hand side represents the mixed-layer heat budget terms, with \(\:SW\) surface shortwave flux, \(\:LW\) surface longwave flux, \(\:LH\) latent heat flux, \(\:SH\) sensible heat flux, and \(\:OD\) ocean heat transport convergence due to ocean dynamics (all heat fluxes are defined as positive downward). \(\:{\prime\:}\) represents the trend between 1980–2014. At the multi-decadal timescale that we consider in this study, the left hand side (the tendency term) is close to zero. Based on the linearized bulk formula for evaporation, latent heat changes associated with Newtonian cooling can be formulized as \(\:\:\alpha\:\stackrel{-}{LH}{T}^{{\prime\:}}\) , where \(\:\alpha\:\equiv\:\frac{{L}_{v}}{{R}_{v}{T}^{2}}\:\approx\:\:0.06/K\) , \(\:{L}_{v}\) is the latent heat of vaporization, \(\:{R}_{v}\) is the gas constant for moist air, \(\:\stackrel{-}{LH}\) is the climatological mean \(\:LH\) . Such that Eq. 1 can be rewritten into a diagnostic equation for the SST trend: $$\:{T}^{{\prime\:}}=-\frac{{SW}^{{\prime\:}}+\:{LW}^{{\prime\:}}+\:{LH}_{W}^{{\prime\:}}+{LH}_{RH}^{{\prime\:}}+{LH}_{T}^{{\prime\:}}+\:{SH}^{{\prime\:}}+\:{OD}^{{\prime\:}}}{\alpha\:\stackrel{-}{LH}}\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(2\right)$$ where \(\:{LH}_{W}^{{\prime\:}}\) , \(\:{LH}_{RH}^{{\prime\:}}\) , \(\:{LH}_{\varDelta\:T}^{{\prime\:}}\) represent latent heat trend changes due to changes in surface wind speed (W), changes in surface relative humidity ( \(\:{RH}_{0}\) ) and changes in air-sea temperature gradient ( \(\:\varDelta\:T\:=\:{T}_{a}\:-\:{T}_{s}\:\) with \(\:{T}_{a}\) being the surface air temperature and \(\:{T}_{s}\:\) the surface skin temperature), respectively: $$\:{LH}_{W}^{{\prime\:}}\:=\:\stackrel{-}{LH}\frac{W{\prime\:}}{\stackrel{-}{W}}\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(3\right)$$ $$\:{LH}_{RH}^{{\prime\:}}\:=-\frac{\:\stackrel{-}{LH}{RH}_{0}^{{\prime\:}}}{{e}^{\alpha\:\stackrel{-}{\varDelta\:T}}-\stackrel{-}{{RH}_{0}}}\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(4\right)$$ $$\:{LH}_{\varDelta\:T}^{{\prime\:}}\:=\frac{\:\alpha\:\stackrel{-}{LH}\:\stackrel{-}{{RH}_{0}}\:{{\Delta\:}T}^{{\prime\:}}}{{e}^{\alpha\:\stackrel{-}{\varDelta\:T}}-\stackrel{-}{{RH}_{0}}}\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\:\left(5\right)$$ Overbar denotes the climatological mean values. Declarations Data Availability The “ALL” ensemble is taken from the CESM1 LENS project 33 , available at https://www.cesm.ucar.edu/community-projects/lens/data-sets. The “xO3S” dataset is available upon request from the corresponding author. ERSSTv5 30 and COBE4 8 SST observations are available from NOAA PSL at https://psl.noaa.gov/data/gridded/data.noaa.ersst.v5.html and https://psl.noaa.gov/data/gridded/data.cobe2.html, respectively. ERA5 49 SLP data is available from the Copernicus Climate Data Store at https://cds.climate.copernicus.eu/#!/search?text=ERA5%20monthly%20single%20levels%26type%3Ddataset. JRA55 50 SLP data is obtained from the NCAR Data Archive at https://rda.ucar.edu/datasets/d628001/. Code Availability The Python code used to generate figures is available upon request from the corresponding author. Acknowledgments YD is supported by the NOAA Climate and Global Change Postdoctoral Fellowship Program, administered by UCAR's Cooperative Programs for the Advancement of Earth System Science (CPAESS) under award NA210AR4310383. LMP is supported, in part, by an award from the US National Science Foundation to Columbia University. YTH is supported by National Science and Technology Council (NSTC 112-2111-M-002-016-MY4). MRE is supported by Royal Commission for the Exhibition of 1851. We would like to acknowledge high-performance computing support from Cheyenne (doi:10.5065/D6RX99HX) provided by NCAR's Computational and Information Systems Laboratory, sponsored by the National Science Foundation. Competing Interests The authors declare no financial or non-financial competing interests. Author contributions YD designed and performed the research; MRE and YD conducted the xO3S simulations. YD analyzed data and wrote the initial draft. 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II , 93 (1), 5–48. Kang, S. M., Xie, S. P., Shin, Y., Kim, H., Hwang, Y. T., Stuecker, M. F., … Hawcroft,M. (2020). Walker circulation response to extratropical radiative forcing. Science advances , 6 (47), eabd3021. Additional Declarations No competing interests reported. Supplementary Files Dongrn2SI.pdf Cite Share Download PDF Status: Published Journal Publication published 22 Apr, 2025 Read the published version in npj Climate and Atmospheric Science → Version 1 posted Editorial decision: Accepted 22 Mar, 2025 Reviews received at journal 21 Mar, 2025 Reviewers agreed at journal 21 Mar, 2025 Reviewers invited by journal 21 Mar, 2025 Submission checks completed at journal 21 Mar, 2025 First submitted to journal 20 Mar, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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England","email":"","orcid":"","institution":"University of Exeter","correspondingAuthor":false,"prefix":"","firstName":"Mark","middleName":"R.","lastName":"England","suffix":""}],"badges":[],"createdAt":"2024-09-11 21:57:51","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-5073590/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-5073590/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41612-025-01020-0","type":"published","date":"2025-04-22T15:57:23+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":79084706,"identity":"71789804-cce5-47bb-ad10-4bcf58c9e742","added_by":"auto","created_at":"2025-03-24 09:04:22","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":4823818,"visible":true,"origin":"","legend":"\u003cp\u003eObserved and simulated global SST trend patterns during 1980 – 2014. (top row) ERSSTv5 observation; (middle row) CESM1 ALL ensemble mean taken from CESM1 LENS; (bottom row) CESM1 simulated response to stratospheric ozone depletion (ALL minus xO3S ensemble mean). From left to right are annual-mean, DJF and JJA seasonal means. Stippling indicates where the linear trends are statistically significant at the 95% confidence level. Box in the bottom panels refers to the eastern Pacific area considered for zonal-mean analysis in Fig. 4, defined between 220°E - 290°E, 20°N - 80°S.\u003c/p\u003e","description":"","filename":"Fig1.png","url":"https://assets-eu.researchsquare.com/files/rs-5073590/v1/dafaefc33f0473aa3a35dc61.png"},{"id":79082593,"identity":"89e52ee6-d200-4c26-bf45-16ab90cbff2a","added_by":"auto","created_at":"2025-03-24 08:40:22","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":293948,"visible":true,"origin":"","legend":"\u003cp\u003eSimulated and observed tropical zonal SST gradient (West minus East) and zonal SLP gradient (East minus West; see Methods). Blue and orange dots denote all 20 members from ALL (all transient historical forcing) and xO3S (no ozone hole) ensembles, respectively. Red markers denote SST observations and atmospheric reanalysis data (Methods).\u003c/p\u003e","description":"","filename":"Fig2.png","url":"https://assets-eu.researchsquare.com/files/rs-5073590/v1/571e77d4c8b180001ce437c7.png"},{"id":79083186,"identity":"b545d562-6e12-4b68-aadd-ffcb3f3cbf66","added_by":"auto","created_at":"2025-03-24 08:48:22","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":1436445,"visible":true,"origin":"","legend":"\u003cp\u003eImposed ozone forcing and simulated SH extratropical atmospheric response. (a) Timeseries of column-integrated stratospheric ozone concentration anomalies southward of 60°S between ALL and xO3S. Blue denotes SON seasonal mean and black denotes annual mean. (b) Timeseries of the simulated SAM response to ozone depletion. Blue denotes DJF seasonal mean and black denotes annual mean. Colored shading represents 0.5 standard deviation across all ensemble members. Colored straight lines denote the linear regression over 1970 – 2004, with the solid line (blue) for a statistically significant trend in DJF SAM and the dashed line (black) for a statistically insignificant trend in annual mean SAM. (c) Simulated DJF zonal-mean atmospheric temperature trend response and (d) DJF zonal-mean atmospheric zonal wind trend response during 1970 – 2004 to ozone depletion. In (c, d), black contours are DJF climatological mean levels, stippling indicates where the linear trends are statistically significant at the 95% confidence level.\u003c/p\u003e","description":"","filename":"Fig3.png","url":"https://assets-eu.researchsquare.com/files/rs-5073590/v1/7276776443db57c96c379345.png"},{"id":79083179,"identity":"f1e692ff-390a-4e3d-9b92-01cfdeb05e97","added_by":"auto","created_at":"2025-03-24 08:48:22","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":256608,"visible":true,"origin":"","legend":"\u003cp\u003eHovmöller diagram of the simulated low-pass filtered (15-yr running averaged) SST response to ozone depletion, averaged over the eastern Pacific, in (left) annual mean and (right) DJF. Stippling indicates where the ensemble-mean response is statistically significant (see Methods).\u003c/p\u003e","description":"","filename":"Fig4.png","url":"https://assets-eu.researchsquare.com/files/rs-5073590/v1/ecb6d5dd93d3335695fd5b5f.png"},{"id":79083181,"identity":"5261550e-e281-4367-921c-5c3e4d3ea0f4","added_by":"auto","created_at":"2025-03-24 08:48:22","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":1856158,"visible":true,"origin":"","legend":"\u003cp\u003eMixed-layer heat budget analysis for the simulated annual-mean Pacific SST trend response to ozone during 1980 – 2014. (a) the total SST trend response (sum of all terms, see Methods), the contributions from (b) latent heat fluxes due to changes in surface wind speed, (c) shortwave radiation, (d) ocean dynamics, (e) longwave radiation, (f) sensible heat fluxes, (g) latent heat fluxes due to changes in relative humidity and (h) latent heat fluxes due to changes in atmospheric stability.\u003c/p\u003e","description":"","filename":"Fig5.png","url":"https://assets-eu.researchsquare.com/files/rs-5073590/v1/50b06097514da542ded7fe70.png"},{"id":79082606,"identity":"3c2a5a09-1faa-4cb8-bb63-45d3fbf60d50","added_by":"auto","created_at":"2025-03-24 08:40:22","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":1065797,"visible":true,"origin":"","legend":"\u003cp\u003eSimulated oceanic response (in the Pacific) to ozone depletion. (a-d) Zonal-mean DJF response of ocean potential temperature (shading) and meridional overturning circulation (contour) in the Pacific basin, averaged over 15-yr periods centered in 1980, 1990, 2000, and 2010, respectively. Positive (negative) contours denote clockwise (counterclockwise) MOC responses. (e, f) Hovmöller diagram of the low-pass filtered (15-yr running averaged) MOC response at 10 m below surface (shading) and surface winds (contour) in the Pacific, in (e) annual mean or (f) DJF. Red (blue) represents positive (negative) MOC response; solid (dashed) contours represent surface westerlies (easterlies).\u003c/p\u003e","description":"","filename":"Fig6.png","url":"https://assets-eu.researchsquare.com/files/rs-5073590/v1/5378d14f7a8d01f4be4f918e.png"},{"id":81570238,"identity":"9160a6a0-41c4-412c-927b-3bf72ca0103b","added_by":"auto","created_at":"2025-04-28 16:12:56","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":8304781,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5073590/v1/222fef39-4f5c-4836-a17a-fcbe4e7f5cf4.pdf"},{"id":79082587,"identity":"60e3e465-6872-405b-a926-b2b8dd600a35","added_by":"auto","created_at":"2025-03-24 08:40:21","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"supplement","size":1777189,"visible":true,"origin":"","legend":"","description":"","filename":"Dongrn2SI.pdf","url":"https://assets-eu.researchsquare.com/files/rs-5073590/v1/c3a2e1d10fb38cf1366c9c72.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Stratospheric ozone depletion has contributed to the recent tropical La Niña-like cooling pattern","fulltext":[{"header":"Introduction","content":"\u003cp\u003eDespite widespread warming on a global scale, the observed sea-surface temperature (SST) trend pattern over recent decades features a remarkable cooling in the tropical East Pacific\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e (EP; Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea-c). This La Ni\u0026ntilde;a-like tropical SST trend pattern has been linked to a broad range of observed climate changes, including the anomalously low value of effective climate sensitivity derived from observations\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e, the strengthening of the Walker Circulation\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e\u003c/sup\u003e, and the persistent drying trend in the American Southwest\u003csup\u003e\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e\u003c/sup\u003e. Climate models, on the other hand, tend to project an El Ni\u0026ntilde;o-like warming pattern \u0026ndash; the opposite of the one recently observed \u0026ndash; to emerge in the future under rising greenhouse gas (GHG) forcing\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e,\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e. The fact that models generally fail to reproduce the observed La Ni\u0026ntilde;a-like historical warming pattern adds considerable uncertainty to the model projections.\u003c/p\u003e \u003cp\u003eDespite its importance, the causes of the observed tropical SST trend pattern remain elusive. While model large ensembles suggest an important role of internal variability in modulating the decadal changes in tropical SST patterns\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e\u003c/sup\u003e, single forcing simulations also reveal contributions from anthropogenic aerosol forcing\u003csup\u003e\u003cspan additionalcitationids=\"CR13\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e and GHG forcing\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e. For example, increased aerosol emissions could drive large-scale atmospheric circulation changes featuring enhanced northern hemisphere tropical trader winds or slow timescale tropical ocean adjustment that both lead to tropical cooling\u003csup\u003e\u003cspan additionalcitationids=\"CR13\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e. CO\u003csub\u003e2\u003c/sub\u003e increase may also cause a transient tropical cooling response driven by tropical eastern Pacific upwelling \u0026ndash; the so-called ocean thermostat mechanism\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e\u003c/sup\u003e. Beyond mechanisms involving tropical local processes, mounting evidence is pointing to the role of the remote impact from the Southern Ocean (SO) via teleconnections. A La Ni\u0026ntilde;a-like pattern of tropical SST response has been shown in a hierarchy of model simulations with idealized\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u003c/sup\u003e or realistic SO cooling\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. Similar results are found in experiments with SO non-thermal forcings (i.e. other than SST and surface heat fluxes), notably Antarctic wind anomalies\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e and Antarctic ice-sheet melting\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e, confirming the SO remote impacts on the tropics.\u003c/p\u003e \u003cp\u003eBuilding on the modeling evidence, a recent paper\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e, has proposed that the observed La Ni\u0026ntilde;a-like tropical SST pattern may have arisen from the formation of the Antarctic ozone hole via the Southern Annular Model (SAM) and its impact on surface winds and SO SST. Indeed, the SAM\u0026ndash; associated with the strengthening of surface westerlies over the SO\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e \u0026ndash; has become more positive in the late 20th century\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e, largely due to stratospheric ozone depletion\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e. Using observational reanalysis, that study\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e noted that the monthly SST anomalies associated with the SAM feature a cooling response in the SO and tropical EP, and hypothesized that stratospheric ozone depletion could have contributed to the formation of the observed La Ni\u0026ntilde;a-like tropical pattern through the SO-tropics teleconnections.\u003c/p\u003e \u003cp\u003eThat hypothesis, however, is open to several questions in the proposed linkage between the ozone hole and tropical cooling. First, the observed tropical cooling trend has only emerged since the 1980s, while stratospheric ozone depletion began in the 1960s and stopped in the 2000s owing to the Montreal Protocol\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e. As a result, the observed SAM increased over the second half of the 20th century but has stopped since the 2000s\u003csup\u003e\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e, leaving it unclear how the ozone-driven SAM trend would have caused the delayed tropical cooling trend. Second, it is well established that stratospheric ozone depletion drives the positive SAM trend only in austral summer (DJF)\u003csup\u003e\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e; yet the observed tropical and SO cooling are present across all seasons (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea-c). Further understanding is thus needed to link the highly seasonal signatures of the ozone hole with the annually persistent observed SST trends. Third, most proposed theories have focused mainly on atmospheric pathways (e.g., surface mean-wind advection and cloud feedback\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e), it remains unclear how oceanic adjustments in the SO, if any, contribute to the remote tropical response.\u003c/p\u003e \u003cp\u003eAddressing these questions and understanding the mechanisms of remote impacts of the ozone hole requires new modeling experiments to unequivocally isolate the effect of stratospheric ozone depletion. In this study, we accomplish this by using two ensembles with the fully-coupled Community Earth System Model version 1 (CESM1). The first one, denoted \u0026ldquo;ALL\u0026rdquo;, comprises 20 members taken from the CESM1 Large Ensemble Project\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e, integrated with the same historical forcings for 1955\u0026ndash;2005 and RCP8.5 forcing for 2006\u0026ndash;2020. The second one, denoted \u0026ldquo;xO3S\u0026rdquo;, is a new 20-member ensemble performed in this study. These simulations are identical to \u0026ldquo;ALL\u0026rdquo; except for stratospheric ozone concentrations, which are held fixed at 1955 levels (before the advent of ozone depletion), such that no ozone hole forms in the late 20th Century (see Methods). Although we fix \u003cem\u003eglobal\u003c/em\u003e stratospheric ozone, historical changes in stratospheric ozone are the largest over the South Pole and in the austral spring months when the Antarctic ozone hole forms (Fig. \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). Contrasting the ensemble means of these two sets of experiments thus enables us to unequivocally isolate the forced response to stratospheric ozone depletion alone, which we refer to as \u0026ldquo;OZONE\u0026rdquo; = \u0026ldquo;ALL\u0026rdquo; \u0026ndash; \u0026ldquo;xO3S\u0026rdquo; in the rest of the paper.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Results","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eRemote Tropical Response to Stratospheric Ozone Depletion\u003c/h2\u003e \u003cp\u003eWe start by showing CESM1 ensemble-mean global SST trend patterns during the recent decades 1980\u0026ndash;2014. The ALL ensemble (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed-f), like all other CMIP models\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, simulates broad warming trends and fails to reproduce the observed cooling trends in the tropical EP and the SO. On the contrary, a cooling trend response arises from stratospheric ozone depletion (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eg-i, ALL \u0026ndash; xO3S) and is most significant in the tropical and subtropical EP, bearing a considerable resemblance to the observed SST trend pattern. This La Ni\u0026ntilde;a-like tropical SST response pattern is further accompanied by a strengthened tropical zonal gradient in sea-level pressure (SLP) and southeasterly trade winds (Fig. S2), which occur in all seasons consistent with observations.\u003c/p\u003e \u003cp\u003eTo further illustrate the robust tropical response to stratospheric ozone depletion, we show the 1980\u0026ndash;2014 trends in the tropical SST zonal gradient and the Pacific Walker Circulation (PWC) index in all individual members in ALL and xO3S ensembles and observations (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). As expected, observations suggest a La Ni\u0026ntilde;a-like tropical SST trend pattern and an enhanced PWC trend, while the vast majority of the ALL ensemble members produce a much weaker or even opposite SST and SLP gradient, with only one member resembling the observed SST. More importantly, model biases are even larger in the xO3S ensemble, meaning that the simulated tropical climate would have trended even more toward an El Ni\u0026ntilde;o-like SST pattern and a weaker PWC, if stratospheric ozone depletion had not occurred. The ensemble means of tropical SST zonal gradient and the PWC index between the ALL and xO3S ensembles are significantly different, supporting the robust La Ni\u0026ntilde;a-like tropical response to stratospheric ozone depletion.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe SST trend pattern (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) and the tropical zonal gradients (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e) confirm a remote impact of stratospheric ozone depletion on the tropics over recent decades in our model. However, unlike in the mechanism hypothesized by the previous study\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e, the annual-mean tropical cooling response is not accompanied by a significant SO cooling trend, which only occurs weakly in the southeast Pacific sector in DJF (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eh). Hence, focusing now on the response to stratospheric ozone alone, we ask: How does the tropical surface cooling from stratospheric ozone depletion come about? Are there changes in the SO involved in \u0026ndash; and crucial to \u0026ndash; the ozone-driven teleconnections, as previously proposed?\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eSouthern Hemisphere extratropical response to stratospheric ozone depletion\u003c/h3\u003e\n\u003cp\u003eWe start by considering the SH atmospheric responses to stratospheric ozone depletion in our simulations. Following the development of the Antarctic ozone hole (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea) \u0026ndash; most dramatically between the 1970s and the 2000s \u0026ndash; we see a significant stratospheric cooling trend over the South Pole, which is largest in the austral summer DJF (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec), following the season of the largest ozone loss (blue line in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). This cooling is due to the reduced absorption of ultraviolet solar radiation by ozone. The resulting atmospheric temperature anomalies cause a poleward shift in the SH mid-latitude jet (which is largest in DJF, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed). While a detailed mechanism remains elusive, the lag between the ozone hole and the jet response is well established in observations\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e,\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e and robustly simulated in models\u003csup\u003e\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e,\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e,\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e\u003c/sup\u003e. At the surface, the strengthening and pole-ward shift of the westerlies is characterized by the positive phase of the SAM\u003csup\u003e\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e\u003c/sup\u003e and, as one expects, the SAM trends are only significant in DJF\u003csup\u003e\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eAccompanying the positive SAM trend and enhanced surface westerlies around Antarctica, the SO SSTs also exhibit highly seasonal responses to ozone depletion. During 1970\u0026ndash;2004 (the maximum ozone depletion period, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea) in DJF, the ozone hole, likely via the positive SAM trends\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e, yields a significant SST cooling response in the Pacific and Indian Ocean basins and a warming response in the Atlantic basin (Fig. S3b); in JJA, SST warms in most of the basins (Fig. S3c). As a result, the annual-mean SST trend response is mostly insignificant and weak around Antarctica. The patterns and seasonality of the SO SST response also hold for the more recent decades 1980\u0026ndash;2014 (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eg-i). These results are consistent with recent observational studies\u003csup\u003e\u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e\u003c/sup\u003e: although the observed positive SAM trend can produce some SO cooling and Antarctic sea-ice expansion, this SAM-associated SO SST and sea-ice trends are confined to DJF and much weaker than observations. These results imply a limited role of ozone depletion in \u003cem\u003efully\u003c/em\u003e explaining the observed SO cooling trends. Other proposed candidates, such as SO natural variability\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e, freshwater input from Antarctic ice-sheet melting\u003csup\u003e\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e, or remote influence from tropical Pacific decadal variability\u003csup\u003e\u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e45\u003c/span\u003e\u003c/sup\u003e, thus warrant further investigation to understand the observed trends around Antarctica.\u003c/p\u003e \u003cp\u003eIn summary, our model simulates the expected DJF tropospheric and surface responses to ozone depletion in the SH extratropics. These include the positive SAM trend associated with the shift in mid-latitude jet and the corresponding SO SST cooling pronounced in the Pacific basin. Turning to the remote tropical response, the key question thus becomes: does the tropical SST response (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e) in the model result from the ozone-induced SH extratropical changes? If so, does it arise from the regional and seasonal SO SST cooling anomalies as previously proposed, or are there any other mechanisms that might cause the tropical response?\u003c/p\u003e\n\u003ch3\u003ePotential pathways connecting the Southern Ocean and the tropical Pacific\u003c/h3\u003e\n\u003cp\u003eFirst, to examine how the tropical EP cooling response develops, we consider a Hovm\u0026ouml;ller diagram of the SST response to ozone depletion averaged over the eastern Pacific basin (see box in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e and Methods). A 15-year running average is applied to isolate the low-frequency (decadal to multidecadal) SST response from potential high-frequency variabilities. Focusing on the eastern Pacific allows us to capture the maximum cooling response in both the SO and the tropical Pacific shown in the trend maps (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eStarting from the annual-mean (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea), the Hovm\u0026ouml;ller diagram shows no evidence of a connection between the tropical and SO SST response, as there is no robust SST response in the SO on the annual mean. Yet, significant cooling emerges in the subtropical eastern Pacific in the late 1990s, consistent with the trend map over recent decades (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eg). Turning to DJF, however, the Hovm\u0026ouml;ller diagram (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb) reveals an interesting communication of cold SST anomalies from the SO to lower latitudes. The SO (in the eastern Pacific basin) experiences cooling throughout the second half of the 20th century, starting from the 1970s along with the onset of the ozone hole. The negative SST anomalies are mostly confined to the high latitudes until the 1990s; after that, the SO SST anomalies expand to lower latitudes, driving a maximum cooling trend in the subtropical and tropical EP between the 1990s and the 2010s. This time-evolving response thus suggests that the tropical cooling response could be in part initiated by the SO changes in DJF. Once the SST cooling anomalies reach the subtropics in the late 1990s, they appear in all seasons and therefore stay in the annual mean (c.f. Figure\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea and b), suggesting that there are other local processes further amplify the subtropical cooling initiated from the high latitudes.\u003c/p\u003e \u003cp\u003eTo further understand the processes contributing to the significant annual-mean subtropical cooling, we perform a mixed-layer heat budget analysis (see Methods) for the SST trends over recent decades (1980\u0026ndash;2014, as in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e), following previous studies\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e. The heat budget analysis shows that latent heat flux driven by surface wind changes in the central and eastern Pacific (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb) and shortwave radiative flux off the coast of South America (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec) are the two leading contributors to the simulated subtropical EP cooling response (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea). The wind-driven latent heat term represents a coupled mechanism, where increases in surface wind stress cause surface cooling via evaporation, posing an anomalous SST meridional gradient that is conducive to further strengthening surface winds and amplifies surface cooling \u0026ndash; the process known as the wind-evaporation-SST (WES) feedback\u003csup\u003e\u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e. The shortwave radiative flux term reflects subtropical low-cloud feedback, where surface cooling in the eastern Pacific atmospheric subsidence regions increases low cloud cover via strengthening lower tropospheric stability\u003csup\u003e\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e,\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e,\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e. The increased low-cloud cover enhances the reflection of incoming shortwave radiation, further amplifying surface cooling. Both processes exert strong positive feedback for enhancing the magnitude of the subtropical EP cooling, consistent with other modeling studies\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e. Furthermore, these feedbacks are not directly ozone-forced and less season-dependent, thus playing a key role in maintaining the subtropical cooling response in all seasons (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea and Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eWhile the WES feedback and subtropical cloud feedback appear to make major contributions to the \u003cem\u003emagnitude\u003c/em\u003e of the simulated tropical SST response, it remains unclear what sets the \u003cem\u003etimescale\u003c/em\u003e of the tropical SST cooling response following the onset of the ozone hole. The eastern Pacific SST Hovm\u0026ouml;ller diagrams (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb) show that the tropical SST cooling occurs after the 1990s and is pronounced only after the 2000s, while the SO SST cooling begins decades earlier in about 1970s, following the maximum ozone depletion and its significant surface wind response (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). This \u003cem\u003edelayed\u003c/em\u003e timescale thus suggests that \u0026ndash; in addition to the previously established \u003cem\u003eatmospheric\u003c/em\u003e pathways (e.g., surface mean-wind advection in the southeast Pacific\u003csup\u003e\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e,\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e) that operate on timescales of several years \u0026ndash; \u003cem\u003eoceanic\u003c/em\u003e pathways are also crucial to the teleconnection in our fully-coupled simulations. Indeed, the surface heat budget analysis (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) confirms that oceanic processes, following the WES feedback and cloud feedback, make additional positive contributions to the equatorial and mid-latitude SST cooling trends (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed). Our further analysis finds that ocean Ekman advection only explains the tropical part (not shown), implying that the midlatitude oceanic process is likely associated with ocean upwelling. Although this ocean dynamic contribution is overall smaller in magnitude (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ed) compared to the other two leading terms, it may shed light on the critical timescale of the communication between high-latitude and low-latitude oceans. Next we will focus on the midlatitude oceanic process and its potential role in the delayed tropical response to ozone depletion.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTo examine the oceanic process in midlatitudes, we show four time-slices of 15-year-mean DJF ocean potential temperature and meridional overturning circulation (MOC) response in the Pacific sector, evolving from 1980 to 2010 (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ea-d). Throughout the period, a positive (clockwise) MOC response is found in the SO centered at 60\u0026deg;S, driven by the anomalous westerlies associated with the positive SAM response (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef). This SO MOC consists of northward (cold) advection at the surface causing surface cooling, and upwelling of subsurface warmer waters (Fig. S4) on the polar branch causing subsurface warming\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. Interestingly, accompanied with this positive MOC response is an opposite MOC response centered around 45\u0026deg;S, which has been reported also in other modeling studies\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e,\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e, \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e. This counter-clockwise MOC is driven by the anomalous easterlies in midlatitude arising from the poleward shift of the jet (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef). Although this mid-latitude MOC is weaker than the high-latitude MOC, it appears to also affect SST via two distinct stages, similar to the two-time-scale mechanism proposed for the SO MOC\u003csup\u003e\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e\u003c/sup\u003e. First, on a fast timescale, its surface branch yields surface \u003cem\u003ewarming\u003c/em\u003e, via a poleward Ekman advection from warmer low latitudes to colder high latitudes. Such a warming effect is most evident in the 15-yr average centered in 1990 (note the surface warm blob at 50\u0026deg;S in Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb). As time evolves, this fast surface wind-driven advection is overwhelmed by an opposite cooling effect, as the slow upwelling becomes more dominant in the second stage (at 40\u0026deg;S). The upwelling branch (at ~\u0026thinsp;40\u0026deg;S) brings colder subsurface water to the surface (Fig. S4), driving slow surface cooling at the equatorward edge after the 2000s (note the surface cooling blob at 40\u0026deg;S in Figs.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ec and d). Once the surface cooling emerges in the mid-latitudes after the 2000s, it propagates to the tropics quickly (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb), enhanced by atmospheric positive feedbacks from latent heat changes and shortwave cloud radiative effect (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn summary: by investigating the surface energy budget and time-evolving ocean response, we find that the remote SST response to ozone depletion results from several processes. The low-latitude SST cooling, occurring decades after the emergence of the SO response, is likely initiated by the midlatitude ocean circulation adjustment, rather than the fast advection of SO SST anomalies as previously proposed. Specifically, the ozone hole causes a poleward shift in midlatitude jet; the resulting surface wind anomalies, consisting of westerlies on the poleward side and easterlies on the equatorward side, drive a dipole pattern of MOC in the Pacific (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ef). The midlatitude (counter-clockwise) MOC affects SST via two stages: on the fast timescale, southward warm advection associated with easterlies yields midlatitude surface warming (pronounced in ~\u0026thinsp;1990s); on the slow timescale, the equatorward branch of upwelling brings subsurface colder waters to surface, yielding subtropical surface cooling a decade after the onset of the ozone hole (pronounced after 2000s). Although the wind-driven MOC response is stronger in DJF (c.f. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ee and f), its resulting low-latitude SST changes are persistent in the annual mean, as they are further amplified by other positive feedbacks in the tropics, including the WES feedback and cloud feedback (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb, c).\u003c/p\u003e"},{"header":"Discussions","content":"\u003cp\u003eIn this study, we have provided new modeling evidence that stratospheric ozone depletion can produce remote impacts on tropical oceans, causing SST cooling trends in the tropical East Pacific. Our all-but-stratospheric ozone (\u0026ldquo;xO3S\u0026rdquo;) experiments unequivocally isolate the surface cooling effect of stratospheric ozone depletion from all other radiative forcing agents. The results imply that stratospheric ozone depletion is a plausible contributor to the formation of the recently observed La Ni\u0026ntilde;a-like tropical warming pattern.\u003c/p\u003e \u003cp\u003eThe main conclusion of our study broadly confirms the hypothesized impact of the ozone hole on tropical SST trends in ref \u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e. However, using specifically designed model simulations, we have been able to elucidate the mechanisms whereby the Antarctic ozone hole causes remote tropical SST cooling trends. Specifically, we emphasize the following points and address the three questions posed at the beginning of this paper:\u003c/p\u003e \u003cp\u003e \u003cul\u003e \u003cli\u003e \u003cp\u003eThe lagged timescales between the ozone hole (maximum between the 1970s \u0026minus;\u0026thinsp;2000s) and the tropical SST cooling (pronounced over 1980\u0026ndash;2015 in observations): While ozone depletion in our model produces a remote impact, the tropical SST response emerges several decades after the onset of the ozone hole in 1970s. This delayed response suggests that oceanic processes play a critical role in initializing the low-latitude SST response. Our results show that midlatitude upwelling is a plausible mechanism, which itself results from ozone-induced surface wind changes.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eThe distinct seasonality of the ozone hole and its response: While ozone depletion and its extratropical impacts are limited to austral summer (DJF), the tropical SST response persists across all seasons. This is because atmosphere-ocean coupling in the tropical Pacific provides positive feedbacks (e.g. shortwave cloud feedback and WES feedback) that maintain and enhance the seasonal SST changes.\u003c/p\u003e \u003c/li\u003e \u003cli\u003e \u003cp\u003eThe associated atmospheric and oceanic processes: while the simulated magnitude of the subtropical and tropical SST cooling response to ozone is largely determined by shortwave cloud feedback and latent heat flux associated with wind speed changes (the WES feedback), other oceanic processes also make a contribution, particularly the midlatitude upwelling. This ocean circulation response is driven by the shift of midlatitude jets forced by ozone depletion and appears to be critical in setting the timescale of the teleconnection from high latitudes to low latitudes.\u003c/p\u003e \u003c/li\u003e \u003c/ul\u003e \u003c/p\u003e \u003cp\u003eOur findings support the argument that tropical SST patterns could be influenced by remote forcing from the polar regions. Unlike previous modeling studies that used heat flux anomalies to force the SO SST to change\u003csup\u003e\u003cspan additionalcitationids=\"CR18 CR19\" citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e, our simulations employ realistic stratospheric ozone concentrations and allow the atmosphere and oceans to freely respond, providing independent evidence for the teleconnections in the coupled system. Furthermore, while the SH extratropical impact of stratospheric ozone depletion has long been reported, investigations on its remote impact on lower latitudes have been limited to subtropical precipitation\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e. Our study establishes a remote impact of stratospheric ozone depletion (mostly over the South Pole) on the tropical oceans, highlighting a novel long-range impact of the ozone hole on tropical and global climate change.\u003c/p\u003e \u003cp\u003eWhile the goal of our study has been to bring out the specific role of stratospheric ozone depletion, our results in no way exclude the possibility that other processes may also have contributed to the observed tropical Pacific cooling\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. These may include, and are not limited to, anthropogenic aerosol emissions in the northern hemisphere\u003csup\u003e\u003cspan additionalcitationids=\"CR13\" citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e\u003c/sup\u003e, transient response to CO\u003csub\u003e2\u003c/sub\u003e forcing\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e, natural variability intrinsic to the Pacific Ocean\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u003c/sup\u003e, the Atlantic Ocean\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e or the Southern Ocean\u003csup\u003e\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e\u003c/sup\u003e, as well as Antarctic ice-sheet melting\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e. Model biases in tropical SST trend patterns may stem in part from these additional processes. For example, the transient cooling response to CO\u003csub\u003e2\u003c/sub\u003e forcing may be muted due to mode biases in their tropical ocean mean states\u003csup\u003e\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e\u003c/sup\u003e; additional cooling from Antarctic ice-sheet meltwater is not accounted for in current models, due to the lack of interactive ice-sheet coupling\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e,\u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e,\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e\u003c/sup\u003e. Our study suggests that the failure to simulate the observed tropical cooling trends may also be associated with remote response to ozone depletion. Models might underestimate the tropical cooling response to ozone depletion due to their too-weak subtropical low cloud feedback\u003csup\u003e\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e\u003c/sup\u003e\u0026ndash; a common bias in CMIP models\u003csup\u003e\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e \u0026ndash; thus biasing their simulated historical trends.\u003c/p\u003e \u003cp\u003eOne caveat of this study is that our results are based on a single model. A recent study\u003csup\u003e\u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e54\u003c/span\u003e\u003c/sup\u003e reported a similar finding using CMIP models, where a La Ni\u0026ntilde;a-like SST trend response to ozone is found in CMIP historical ozone-only simulations. Although this provides additional evidence to support our results, we note that those CMIP ozone-only simulations differ from our simulations in several aspects. For example, most CMIP6 models do not have a large ensemble (\u0026gt;\u0026thinsp;10 members) of ozone single-forcing simulations available; some models confound stratospheric and tropospheric ozone, considering the total column of ozone in their simulations. In contrast, our study provides the first set of large-ensemble all-but-stratospheric ozone simulations (to our knowledge) that allow us to robustly separate the forced response to stratospheric ozone depletion. Future confirmation from other models with different physics or different spatial resolutions will be valuable to establish the robustness of our findings.\u003c/p\u003e \u003cp\u003eFinally, and most importantly, we remind the reader that, unlike other GHGs which have continuously increased for many decades and are projected to keep increasing in the coming decades, the Antarctic ozone layer \u0026ndash; which was rapidly thinning out in the late 20th century\u003csup\u003e\u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e \u0026ndash; has started to heal\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e\u003c/sup\u003e and is projected to recover in the late 21st century as a consequence to the Montreal Protocol\u003csup\u003e\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e. If, as our findings suggest, the ozone hole has indeed been a key driver of the recent La Nina-like tropical SST trends, its closing in the coming decades would contribute to the projected weakening of this trend pattern. It may further drive a reversal towards a more El Ni\u0026ntilde;o-like pattern, alongside many other mechanisms that favor this long-time reversal\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e. In that case, the ozone-driven tropical El Ni\u0026ntilde;o-like pattern would influence future climate change in an opposite way than the past several decades, including a potentially higher global effective climate sensitivity and more precipitation in the southeast US. Understanding and quantifying how stratospheric ozone, together with other anthropogenic forcings and natural variability, shape future warming patterns is thus critical for accurately constraining future climate change.\u003c/p\u003e"},{"header":"Methods","content":"\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003eModel and simulation setup\u003c/h2\u003e \u003cp\u003eAll simulations analyzed in this study are carried out by the Community Earth System Model (CESM1), with the Community Atmospheric Model version 5 (CAM5), at the 1-degree horizontal resolution. The \u0026ldquo;ALL\u0026rdquo; ensemble, consisting of 20 members, is taken from the CESM1 Large Ensemble project\u003csup\u003e\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e\u003c/sup\u003e. All ensemble members are forced by the same historical forcings for 1955\u0026ndash;2005 and RCP8.5 forcings for 2006\u0026ndash;2020, except their initial conditions differ slightly. The \u0026ldquo;xO3S\u0026rdquo; ensemble (20 members) is identical to ALL, except their \u003cem\u003estratospheric\u003c/em\u003e ozone is held fixed at the 1955 values. This is accomplished as follows: given the vertical distribution of ozone, at each grid box we fix ozone only at levels where its concentration exceeds 150 ppbv\u003csup\u003e\u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e57\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e58\u003c/span\u003e\u003c/sup\u003e. This simple procedure avoids having to define a tropopause and leaves tropospheric ozone trends (which cause a substantial radiative forcing and are unrelated to the ozone hole) identical to the one in the historical runs. Note that the large seasonal cycle of stratospheric ozone is retained in the xO3S runs.\u003c/p\u003e \u003cp\u003eRecall that CESM1 has no interactive ozone chemistry, and thus stratospheric ozone and ozone-depleting substances (ODS) are prescribed independently in the model. In the \u0026ldquo;ALL\u0026rdquo; and \u0026ldquo;xO3S\u0026rdquo; ensembles, ODS concentrations are identical and time-dependent, such that the difference between the two ensembles reflects the impact of stratospheric ozone depletion \u003cem\u003ealone\u003c/em\u003e, without including the radiative effect of ODS as greenhouse gases, which is large and can cause considerable tropical warming\u003csup\u003e\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e59\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e\u003c/sup\u003e.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eAnalysis\u003c/h3\u003e\n\u003cp\u003eThe forced response to stratospheric ozone depletion is obtained by taking the ensemble mean difference between ALL and xO3S. Linear trends in any variable of interest are computed using the ordinary least squares (OLS) regression method. The significance of trends is evaluated using the student t-test at the 95% confidence level at each grid box. The significance of responses (used in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e and Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e) is evaluated as whether the mean of the response from all 20 ensemble members at each grid box is statistically different from zero at the 95% confidence level.\u003c/p\u003e\n\u003ch3\u003eIndices used for regional analysis\u003c/h3\u003e\n\u003cp\u003eThe tropical SST zonal gradient (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea) is computed as the difference between SST averaged over 110\u0026deg;E \u0026minus;\u0026thinsp;180\u0026deg;E, 10\u0026deg;S \u0026minus;\u0026thinsp;10\u0026deg;N and 210\u0026deg;E \u0026minus;\u0026thinsp;280\u0026deg;E, 10\u0026deg;S \u0026minus;\u0026thinsp;10\u0026deg;N (West minus East). A positive (negative) SST gradient refers to a La Ni\u0026ntilde;a-like (El Ni\u0026ntilde;o-like) SST pattern. The tropical SLP zonal gradient (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eb), i.e., the Pacific Walker Circulation index\u003csup\u003e\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e\u003c/sup\u003e, is computed as the difference between SLP averaged over 180E \u0026minus;\u0026thinsp;280\u0026deg;E, 5\u0026deg;S \u0026minus;\u0026thinsp;5\u0026deg;N and 80\u0026deg;E \u0026minus;\u0026thinsp;160\u0026deg;E, 5\u0026deg;S \u0026minus;\u0026thinsp;5\u0026deg;N (East minus West). A positive (negative) SLP gradient refers to a strengthened (weakened) Pacific Walker circulation. The SAM index (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb) is computed as the difference between zonal-mean SLP at 45\u0026deg;S and 60\u0026deg;S\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e. The SST Hovm\u0026ouml;ller diagram (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e) is computed for SST averaged over the eastern Pacific basin defined as 220\u0026deg;E \u0026minus;\u0026thinsp;290\u0026deg;E (box in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e panels g-i).\u003c/p\u003e \u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003eObservations\u003c/h2\u003e \u003cp\u003eIn Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e, we used SST observation from Extended Reconstructed SST data version 5 (ERSSTv5)\u003csup\u003e\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e\u003c/sup\u003e. In Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, we used SST observations from ERSSTv5 and the COBE SST dataset\u003csup\u003e\u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e\u003c/sup\u003e, and SLP observations from the ERA5\u003csup\u003e64\u003c/sup\u003e and JRA-55\u003csup\u003e65\u003c/sup\u003e reanalysis data. In Fig. S2, we used SLP and 850hPa winds from ERA5\u003csup\u003e64\u003c/sup\u003e reanalysis data.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003eMixed-layer heat budget analysis\u003c/h2\u003e \u003cp\u003eWe perform a mixed-layer heat budget analysis to decompose the contributions to SST trends, following the method widely used in previous studies\u003csup\u003e\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e,\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e,\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e,\u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e\u003c/sup\u003e. We consider the surface energy budget as:\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equa\" name=\"EquationSource\"\u003e\n$$\\:\\rho\\:{C}_{P}H\\frac{\\partial\\:T}{\\partial\\:t}=\\:{SW}^{{\\prime\\:}}+\\:{LW}^{{\\prime\\:}}+\\:{LH}^{{\\prime\\:}}+\\:{SH}^{{\\prime\\:}}+\\:{OD}^{{\\prime\\:}}\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\left(1\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere the left hand side represents the mixed-layer heat storage, with \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\rho\\:\\)\u003c/span\u003e\u003c/span\u003e being the density of ocean, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{C}_{P}\\)\u003c/span\u003e\u003c/span\u003e the specific heat of the ocean, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:H\\)\u003c/span\u003e\u003c/span\u003e the ocean mixed-layer depth and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:T\\)\u003c/span\u003e\u003c/span\u003e is surface temperature. The right hand side represents the mixed-layer heat budget terms, with \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:SW\\)\u003c/span\u003e\u003c/span\u003e surface shortwave flux, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:LW\\)\u003c/span\u003e\u003c/span\u003e surface longwave flux, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:LH\\)\u003c/span\u003e\u003c/span\u003e latent heat flux, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:SH\\)\u003c/span\u003e\u003c/span\u003e sensible heat flux, and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:OD\\)\u003c/span\u003e\u003c/span\u003e ocean heat transport convergence due to ocean dynamics (all heat fluxes are defined as positive downward). \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{\\prime\\:}\\)\u003c/span\u003e\u003c/span\u003e represents the trend between 1980\u0026ndash;2014. At the multi-decadal timescale that we consider in this study, the left hand side (the tendency term) is close to zero. Based on the linearized bulk formula for evaporation, latent heat changes associated with Newtonian cooling can be formulized as\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\:\\alpha\\:\\stackrel{-}{LH}{T}^{{\\prime\\:}}\\)\u003c/span\u003e\u003c/span\u003e, where \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\alpha\\:\\equiv\\:\\frac{{L}_{v}}{{R}_{v}{T}^{2}}\\:\\approx\\:\\:0.06/K\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{L}_{v}\\)\u003c/span\u003e\u003c/span\u003e is the latent heat of vaporization, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{R}_{v}\\)\u003c/span\u003e\u003c/span\u003e is the gas constant for moist air, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\stackrel{-}{LH}\\)\u003c/span\u003e\u003c/span\u003e is the climatological mean \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:LH\\)\u003c/span\u003e\u003c/span\u003e. Such that Eq.\u0026nbsp;1 can be rewritten into a \u003cem\u003ediagnostic\u003c/em\u003e equation for the SST trend:\u003cdiv id=\"Equb\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equb\" name=\"EquationSource\"\u003e\n$$\\:{T}^{{\\prime\\:}}=-\\frac{{SW}^{{\\prime\\:}}+\\:{LW}^{{\\prime\\:}}+\\:{LH}_{W}^{{\\prime\\:}}+{LH}_{RH}^{{\\prime\\:}}+{LH}_{T}^{{\\prime\\:}}+\\:{SH}^{{\\prime\\:}}+\\:{OD}^{{\\prime\\:}}}{\\alpha\\:\\stackrel{-}{LH}}\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\left(2\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ewhere \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{LH}_{W}^{{\\prime\\:}}\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{LH}_{RH}^{{\\prime\\:}}\\)\u003c/span\u003e\u003c/span\u003e, \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{LH}_{\\varDelta\\:T}^{{\\prime\\:}}\\)\u003c/span\u003e\u003c/span\u003e represent latent heat trend changes due to changes in surface wind speed (W), changes in surface relative humidity (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{RH}_{0}\\)\u003c/span\u003e\u003c/span\u003e) and changes in air-sea temperature gradient (\u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:\\varDelta\\:T\\:=\\:{T}_{a}\\:-\\:{T}_{s}\\:\\)\u003c/span\u003e\u003c/span\u003e with \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{T}_{a}\\)\u003c/span\u003e\u003c/span\u003e being the surface air temperature and \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\(\\:{T}_{s}\\:\\)\u003c/span\u003e\u003c/span\u003e the surface skin temperature), respectively:\u003cdiv id=\"Equc\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equc\" name=\"EquationSource\"\u003e\n$$\\:{LH}_{W}^{{\\prime\\:}}\\:=\\:\\stackrel{-}{LH}\\frac{W{\\prime\\:}}{\\stackrel{-}{W}}\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\left(3\\right)$$\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Equd\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equd\" name=\"EquationSource\"\u003e\n$$\\:{LH}_{RH}^{{\\prime\\:}}\\:=-\\frac{\\:\\stackrel{-}{LH}{RH}_{0}^{{\\prime\\:}}}{{e}^{\\alpha\\:\\stackrel{-}{\\varDelta\\:T}}-\\stackrel{-}{{RH}_{0}}}\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\left(4\\right)$$\u003c/div\u003e\u003c/div\u003e\u003cdiv id=\"Eque\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Eque\" name=\"EquationSource\"\u003e\n$$\\:{LH}_{\\varDelta\\:T}^{{\\prime\\:}}\\:=\\frac{\\:\\alpha\\:\\stackrel{-}{LH}\\:\\stackrel{-}{{RH}_{0}}\\:{{\\Delta\\:}T}^{{\\prime\\:}}}{{e}^{\\alpha\\:\\stackrel{-}{\\varDelta\\:T}}-\\stackrel{-}{{RH}_{0}}}\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\:\\left(5\\right)$$\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eOverbar denotes the climatological mean values.\u003c/p\u003e \u003c/div\u003e "},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData Availability\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe “ALL” ensemble is taken from the CESM1 LENS project\u003csup\u003e33\u003c/sup\u003e, available at\u0026nbsp;https://www.cesm.ucar.edu/community-projects/lens/data-sets. The “xO3S” dataset is available upon request from the corresponding author. ERSSTv5\u003csup\u003e30\u003c/sup\u003e and COBE4\u003csup\u003e8\u003c/sup\u003e SST observations are available from NOAA PSL at https://psl.noaa.gov/data/gridded/data.noaa.ersst.v5.html and https://psl.noaa.gov/data/gridded/data.cobe2.html, respectively. ERA5\u003csup\u003e49\u003c/sup\u003e SLP data is available from the Copernicus Climate Data Store at https://cds.climate.copernicus.eu/#!/search?text=ERA5%20monthly%20single%20levels%26type%3Ddataset. JRA55\u003csup\u003e50\u003c/sup\u003e SLP data is obtained from the NCAR Data Archive at https://rda.ucar.edu/datasets/d628001/.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCode Availability\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe Python code used to generate figures is available upon request from the corresponding author.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eYD is supported by the NOAA Climate and Global Change Postdoctoral Fellowship Program, administered by UCAR's Cooperative Programs for the Advancement of Earth System Science (CPAESS) under award NA210AR4310383. LMP is supported, in part, by an award from the US National Science Foundation to Columbia University. YTH is supported by National Science and Technology Council (NSTC 112-2111-M-002-016-MY4). MRE is supported by Royal Commission for the Exhibition of 1851. We would like to acknowledge high-performance computing support from Cheyenne (doi:10.5065/D6RX99HX) provided by NCAR's Computational and Information Systems Laboratory, sponsored by the National Science Foundation.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting Interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no financial or non-financial competing interests.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eYD designed and performed the research; MRE and YD conducted the xO3S simulations. YD analyzed data and wrote the initial draft. All authors contributed to the result interpretation and the final draft.\u0026nbsp;\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eWills, R. C., Dong, Y., Proistosecu, C., Armour, K. C., \u0026amp; Battisti, D. S. (2022). 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P., Shin, Y., Kim, H., Hwang, Y. T., Stuecker, M. F., \u0026hellip; Hawcroft,M. (2020). Walker circulation response to extratropical radiative forcing. \u003cem\u003eScience advances\u003c/em\u003e, \u003cem\u003e6\u003c/em\u003e(47), eabd3021.\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"npj-climate-and-atmospheric-science","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npjclimatsci","sideBox":"Learn more about [npj Climate and Atmospheric Science](http://www.nature.com/npjclimatsci/)","snPcode":"41612","submissionUrl":"https://submission.springernature.com/new-submission/41612/3","title":"npj Climate and Atmospheric Science","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-5073590/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-5073590/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eDespite the continuous global warming, over the past several decades, the tropical East Pacific has experienced a cooling trend whose origin remains an area of active research. Mounting evidence has linked tropical sea-surface temperature (SST) patterns to changes in the Southern Ocean via remote teleconnections. Using a fully-coupled global climate model, we demonstrate that stratospheric ozone depletion can produce a La Ni\u0026ntilde;a-like tropical SST trend pattern resembling recent observations. This tropical response initially arises from mid-latitude ocean adjustments to ozone-driven surface wind anomalies, which then enhance in the tropics via positive cloud feedback and wind-evaporation-SST feedback. Our finding suggests that the observed La Ni\u0026ntilde;a-like tropical SST trend pattern may have been, in part, caused by the formation of the ozone hole in the late 20th century. It also implies that ozone recovery in the coming decades will likely contribute to a future weakening or reversal of the observed tropical SST trends.\u003c/p\u003e","manuscriptTitle":"Stratospheric ozone depletion has contributed to the recent tropical La Niña-like cooling pattern","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-03-24 08:40:17","doi":"10.21203/rs.3.rs-5073590/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Accepted","date":"2025-03-22T10:46:26+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2025-03-22T00:22:10+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"241179188272550173992542823151875125051","date":"2025-03-22T00:10:34+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2025-03-21T12:39:17+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2025-03-21T12:36:47+00:00","index":"","fulltext":""},{"type":"submitted","content":"npj Climate and Atmospheric Science","date":"2025-03-20T21:00:02+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"npj-climate-and-atmospheric-science","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"npjclimatsci","sideBox":"Learn more about [npj Climate and Atmospheric Science](http://www.nature.com/npjclimatsci/)","snPcode":"41612","submissionUrl":"https://submission.springernature.com/new-submission/41612/3","title":"npj Climate and Atmospheric Science","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"NPJ","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"43166501-3b66-4a87-a693-3d9774a2847d","owner":[],"postedDate":"March 24th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":46060005,"name":"Earth and environmental sciences/Climate sciences/Atmospheric science"},{"id":46060006,"name":"Earth and environmental sciences/Climate sciences/Climate change"}],"tags":[],"updatedAt":"2025-04-28T16:07:59+00:00","versionOfRecord":{"articleIdentity":"rs-5073590","link":"https://doi.org/10.1038/s41612-025-01020-0","journal":{"identity":"npj-climate-and-atmospheric-science","isVorOnly":false,"title":"npj Climate and Atmospheric Science"},"publishedOn":"2025-04-22 15:57:23","publishedOnDateReadable":"April 22nd, 2025"},"versionCreatedAt":"2025-03-24 08:40:17","video":"","vorDoi":"10.1038/s41612-025-01020-0","vorDoiUrl":"https://doi.org/10.1038/s41612-025-01020-0","workflowStages":[]},"version":"v1","identity":"rs-5073590","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-5073590","identity":"rs-5073590","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}