{"paper_id":"44181810-2ead-4154-b572-2fa0fae53909","body_text":"Wind-driven upwelling sustains weakened Atlantic overturning under climate extremes | 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 Physical Sciences - Article Wind-driven upwelling sustains weakened Atlantic overturning under climate extremes Jonathan Baker, Michael Bell, Laura Jackson, Geoffrey K Vallis, and 2 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4319650/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 26 Feb, 2025 Read the published version in Nature → Version 1 posted You are reading this latest preprint version Abstract The Atlantic Meridional Overturning Circulation (AMOC), vital for northward heat transport across the Atlantic Ocean, is projected to weaken due to global warming 1 , with significant global climate impacts 2,3 . However, the extent of AMOC weakening is uncertain with wide variation across climate models 1,4,5 , while some statistical indicators suggest an imminent collapse 6 . Here, we evaluate the AMOC’s resilience to extreme greenhouse gas and North Atlantic freshwater forcings in 34 climate models by analysing its upwelling pathways that return AMOC deep waters to the surface. We find that upwelling in the Southern Ocean (SO), driven by persistent SO winds, sustains a weakened AMOC in all cases, preventing its complete collapse. Since SO upwelling must be balanced by downwelling in the Atlantic or Pacific, the AMOC can only collapse if a compensating Pacific overturning (PMOC) develops. Remarkably, a PMOC does emerge in almost all models, but it is too weak to balance all of the SO upwelling, suggesting an AMOC collapse is unlikely this century. Our findings reveal novel AMOC stabilising mechanisms with implications for past and future AMOC changes, and hence for ecosystems and ocean biogeochemistry. They suggest that better understanding and estimates of the SO and Indo-Pacific circulations are urgently needed to accurately predict future AMOC change. Earth and environmental sciences/Ocean sciences/Physical oceanography Earth and environmental sciences/Climate sciences/Climate change/Projection and prediction Earth and environmental sciences/Climate sciences/Ocean sciences/Physical oceanography Earth and environmental sciences/Climate sciences/Climate change/Climate and Earth system modelling Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction The future state of the Atlantic Meridional Overturning Circulation (AMOC) is critical for global and regional climate change 2,3 through its role in heat transport 7 and carbon uptake 8 . The AMOC is predicted to weaken during the 21 st century 1,9 driven by increased greenhouse gas (GHG) concentrations and freshwater input to the North Atlantic due to precipitation changes and Greenland ice sheet melt 10,11 . However, climate model projections from the Coupled Model Intercomparison Project Phase 6 (CMIP6) vary widely 1,4,5 , so its future evolution is uncertain 12 . Furthermore, there is a risk the AMOC could collapse, causing abrupt changes in climate 13,14 . Some statistical indicators suggest it is approaching a tipping point 6,15,16 , indicating climate models may be overstable 17 . Since AMOC changes will impact many aspects of the climate 2,3 , accurately predicting its future change is vital for planning adaptation and mitigation strategies. Here, we examine the AMOC’s response to extreme GHG and North Atlantic freshwater forcings in CMIP6 models. Although simpler models 14,18 and a few global climate models (GCMs) suggest the AMOC could collapse (i.e., weaken to zero or reverse) under such forcings 16,19 , it does not collapse in the model experiments considered here. Instead, the AMOC stabilises at a weaker strength that varies widely across models 2,20 . To understand how the AMOC is sustained under extreme climate forcing, and what causes differences in its weakened state across models and forcings, we quantify the AMOC’s upwelling pathways 21,22 that return deep North Atlantic waters to the surface either through Southern Ocean (SO) wind-driven upwelling 23,24 or diffusion in the Atlantic and Indo-Pacific Oceans 25 . The SO wind-driven upwelling rate depends on the SO westerly wind strength that drives the SO upwelling and on the opposing SO mesoscale eddy response, referred to as eddy compensation 26,27 . Our approach, focusing on AMOC upwelling pathways rather than the mean state of the North Atlantic 5,28 offers new perspectives on the AMOC response. While the AMOC’s Indo-Pacific upwelling pathway largely controls inter-model variations in AMOC decline in realistic scenarios 4 , the impact of extreme forcing on upwelling pathway changes and thereby potentially AMOC stability has not been explored. Here, we assess how ocean circulation changes remote from the Atlantic affect the AMOC and its upwelling pathways in climate models. Our analysis reveals that SO wind-driven upwelling maintains a weakened AMOC under extreme forcing, overcoming the destabilising effects of a developing Pacific MOC (PMOC), with implications for understanding both past, and future risks of, AMOC collapse. Experiments and AMOC upwelling pathways We examine the AMOC in 34 CMIP6 models from the pre-industrial control simulation 29 and their response under two extreme forcing scenarios (Methods): an abrupt quadrupling of atmospheric CO 2 (“4xCO2” 29 ) and a 0.3 Sv North Atlantic freshwater forcing (“u03_hos” 20 ). Only 7 of the models are available in u03_hos (Extended Data Table 1), herein named the u03_hos model subset. We apply the method of Baker et al., 2023 to calculate the AMOC’s upwelling pathways (Fig. 1a,b, and Methods), which represent the time-mean area-integrated volume transports that return deep waters from the AMOC’s southward branch into its shallower northward branch (Fig. 1c). We use the Atlantic, Indo-Pacific and global overturning streamfunctions to quantify these volume transports (Methods). We define the AMOC strength as the maximum strength of the Atlantic mid-depth overturning cell (purple circle in Fig. 1a). The AMOC’s upwelling pathways are defined by the regions where they upwell before rejoining the AMOC’s northward branch - the Atlantic Ocean (“Atlantic_Up”), the Indo-Pacific Ocean (“IndoPac_ResidualUp”), and the Southern Ocean (“SouthernOcean_Up”). AMOC deep waters that upwell in the Indo-Pacific Ocean and subsequently upwell in the SO are accounted for by SouthernOcean_Up, not IndoPac_ResidualUp (Methods). In the control simulation, SouthernOcean_Up is similar to the SO upper cell strength at 34.5°S (red circle in Fig. 1a), with adjustments for a localised South Atlantic circulation (Methods and Extended Data Fig. 8). Together the three upwelling pathways are equal to the AMOC strength (Fig. 1), ensuring volume is conserved in the ocean 30 . Changes in the overturing circulation In the pre-industrial control, all models have a clockwise, mid-depth overturning cell (i.e., the AMOC) in the Atlantic, an anti-clockwise cell in the Indo-Pacific, and a clockwise wind-driven upper cell in the SO (Fig. 1a-c and Extended Data Fig. 1). However, models vary widely in AMOC strength and their upwelling pathways (year 0 in Fig. 2a-d). Under GHG and freshwater forcings, all models show AMOC weakening with a wide spread (Fig. 2a,e), stabilising within 90 years (the “future” state herein). The AMOC weakens by 20%–81% (mean of 54%) 90 years after 4xCO2 forcing. Considering only the u03_hos model subset, the weakening ranges from 50%–80% in both 4xCO2 (mean of 56%) and u03_hos (mean of 61%). Thus, the future AMOC strength differs across the models, leading to radically different climate impacts 2 . In the Indo-Pacific, the anti-clockwise overturning circulation not only weakens 4,31 , but reverses rapidly (Fig. 1d and Extended Data Fig. 2,3), indicating the presence of an Atlantic-Pacific seesaw 32 in CMIP6 models. The resulting clockwise Pacific MOC (PMOC), analogous to the AMOC, is found in 91% (86%) of models in the future state of 4xCO2 (u03_hos), with variable strength (Extended Data Fig. 4a,c). Its mean strength at 34.5°S is 4.9 Sv in 4xCO2 (6.4 Sv for u03_hos subset), but only 2.3 Sv in u03_hos. The SO upper cell at 34.5°S strengthens in 4xCO2 (Extended Data Fig. 5a) due to enhanced SO westerly winds 33,34 that more than compensates the weakening from their poleward shift (Extended Data Fig. 5b). Conversely, the SO upper cell weakens in u03_hos (Extended Data Fig. 4d), likely due to a poleward shift in the SO westerly winds, characterised by a dipole in SO overturning changes (Chen et al., 2019; Extended Data Fig. 6). Since changes in SO wind stress in 4xCO2 are relatively similar across the models (Extended Data Fig. 5), we attribute inter-model spread in SO upper cell strength changes (~4 Sv) to differences in eddy compensation, which depends on model resolution and eddy parameterisation 26,27 . Notably, in 4xCO2, lower resolution models from the HadGEM3, MPI, CNRM, and EC-Earth3-Veg model groups have greater strengthening of the SO upper cell (Extended Data Fig. 5), implying they have a weaker eddy compensation. AMOC resilience under extreme forcing Under the extreme climate forcings, all AMOC upwelling pathways ultimately weaken to conserve volume as the AMOC weakens, ensuring upwelling balances downwelling, but there is large inter-model spread (Fig. 2b-d,f-h). The Atlantic (Fig. 2b,f) and Indo-Pacific residual (Fig. 2d,h) upwelling pathways weaken to approximately zero in most models after 90 years of forcing due to AMOC shoaling, which reduces “cell overlap” between the AMOC and the SO lower cell (Fig. 1a, Extended Data Fig. 2,3), cutting off its main pathway into the Indo-Pacific Ocean 36–38 (Fig. 1d). Rapid changes in the Indo-Pacific MOC triggered by wave processes as the AMOC weakens 31 also reduce the Indo-Pacific residual upwelling pathway. The future Atlantic upwelling pathway is greater in u03_hos than in 4xCO2, but it is diminished in both scenarios (<2 Sv in most models) (Fig. 2b,f). In 4xCO2, the SO upwelling pathway initially increases due to SO upper cell strengthening, before it decreases (Fig. 2c). The future SO upwelling pathway is therefore greater in 4xCO2 than in u03_hos (Extended Data Fig. 7c,g), but it is the dominant upwelling pathway (2-13 Sv) in both scenarios (Fig. 2c,g), sustaining the future AMOC. This suggests that without SO upwelling, the AMOC would collapse under these forcings since the Atlantic and Indo-Pacific residual upwelling pathways would still diminish due to AMOC shoaling. Since the future SO upwelling pathway primarily sustains the future AMOC, their future magnitudes are directly proportional in both 4xCO2 (r=0.99; Fig. 3a) and u03_hos (r=0.93; Fig. 3d), excluding four outlying models in 4xCO2 (shaded orange in Fig. 3a). These four models have anomalously strong future AMOCs due to notable Atlantic or Indo-Pacific residual upwelling pathways (Fig. 2a-d). These upwelling pathways are unrealistically large in the control simulations of these models in comparison to the observed AMOC strength (Fig. 2b,d). If SO upwelling pathway changes were consistent across models and thus the control SO upwelling pathway was directly proportional to its future magnitude, it would also correlate strongly with the future AMOC strength. Excluding the four outlying models, correlation between these variables is statistically significant but weak in 4xCO2 (Fig. 3b; r=0.38; p<0.05 (Methods)), whereas it is insignificant in u03_hos (Fig. 3e; r=0.65, p>0.05). Thus, while a stronger SO upwelling pathway (and SO upper cell at 34.5°S) in the control is associated with a stronger future AMOC, the relationship is weak. Indo-Pacific and Southern Ocean impacts on future AMOC strength In 4xCO2, the SO upwelling pathway weakens (Extended Data Fig. 7c), despite a strengthening of the SO upper cell at 34.5°S in 86% of models (Extended Data Fig. 5). We attribute this weakening to the PMOC that emerges in the Indo-Pacific (Extended Data Fig. 4a), possibly instigated by North Atlantic forcing. The PMOC upwells via the SO upper cell, thereby reducing the volume of AMOC origin waters that can upwell in the SO, leading to a weakening of the AMOC’s SO upwelling pathway (Fig. 4a). Variability in PMOC strength across models largely explains why seven models in 4xCO2 (besides the four prementioned unrealistic models) have enhanced (shaded red) or greatly weakened (shaded green) future SO upwelling pathways (Fig. 4a). These differences result in anomalously strong or weak AMOCs, respectively, relative to their control SO upwelling pathways (Fig. 3b). Thus, models with relatively strong future AMOCs (shaded red; Fig. 3b) have weak future PMOCs (<2.5 Sv; Fig. 4a), stabilising the AMOC (Extended Data Fig. 9a). Conversely, models with relatively weak future AMOCs (shaded green; Fig. 3b) tend to have strong future PMOCs (~8 Sv; Fig. 4a) that enabled the AMOC to weaken further (Extended Data Fig. 9b). However, not all models with strong future PMOCs (>5 Sv; Fig. 4a) have anomalously weak future AMOCs relative to their control SO upwelling pathways because some models are compensated by strong increases in SO upper cell strength (Extended Data Fig. 5). Therefore, changes in the AMOC’s SO upwelling pathway depend largely on both changes in SO upper cell strength and the future PMOC strength (Fig. 4b; r=0.95). Exceptions are models with a deep northward PMOC branch that allows AMOC deep waters to zonally enter the PMOC via the SO, before returning to the Atlantic via SO upwelling, thereby unexpectedly stabilising the AMOC. This is small in most models (<1 Sv) except CESM2-based models (purple shading in Fig. 4b,d) due to their anomalously deep South Pacific subtropical gyre cells (Extended Data Figs. 2a,3a). Unlike in 4xCO2, the SO upper cell at 34.5°S weakens in u03_hos (Extended Data Fig. 4d), so the SO upwelling pathway weakens further, despite a weaker future PMOC (cf. mutual models in Fig. 4a,c). Hence, both a weakening SO upper cell and an emerging PMOC reduce the AMOC’s SO upwelling pathway in u03_hos (Fig. 4d). Future fate of the AMOC We have shown that SO wind-driven upwelling prevents an AMOC collapse under extreme climate forcing in CMIP6 models. The AMOC’s future strength depends on the future SO upper cell strength (which depends on its present-day strength and change), but also on the future PMOC strength (if it forms) (Fig. 3c,e; r=0.90 (r=0.98) for 4xCO2 (u03_hos), excluding anomalous models shaded in orange and purple). Previous studies 23,39 highlighted SO wind-driven upwelling’s role in sustaining an AMOC in simple models, but its importance in comprehensive GCMs under extreme forcing was unexplored. GHG forcing strengthens the SO westerly winds 33 , strengthening the SO upper cell 34 . Thus, for an AMOC collapse or substantial AMOC weakening to occur, a strong PMOC that upwells in the SO (Fig. 1d) is essential because SO wind-driven upwelling must be balanced by downwelling in either the Atlantic or Pacific Oceans to conserve volume 30,40 . The emergence of a PMOC as the AMOC weakens has previously been found in GCM extreme forcing experiments 32,41,42 , but Indo-Pacific overturning changes are generally overlooked. The consistency of PMOC formation under extreme climate forcing across CMIP6 models highlights the importance of the Atlantic-Pacific seesaw 32 in extreme AMOC weakening. Our findings suggest a PMOC can develop without closure of the Bering Strait, in contrast to 43 . However, North Atlantic freshwater forcing in u03_hos may reduce freshwater export from the Pacific Ocean through the Bering Strait 43 , limiting the PMOC strength. In 4xCO2, Pacific freshwater export continues, contributing to AMOC weakening 44 and potentially enabling a stronger PMOC. Although the PMOC tends to form in the North Pacific subtropical gyre rather than at higher latitudes (Extended Data Figs. 2,3), its strength is probably sensitive to salinity changes in the Pacific due to changes in the ocean’s freshwater transports or atmospheric freshwater fluxes 45,46 . Further research is needed on how changes in these processes affect PMOC formation and AMOC decline. An active PMOC associated with a weakened or collapsed AMOC was evident in past climates, including during the Last Glacial Maximum and its termination 46,47 and the warm Pliocene when atmospheric CO2 levels were similar to present-day levels 48 . Our findings suggest PMOC formation, by reducing the SO upwelling of AMOC deep waters, may have facilitated AMOC collapses in past climates. The PMOC transports heat northwards 49 , and affects ocean biogeochemistry and carbon uptake 47,50 , highlighting the need to assess its impact on future climate in CMIP6 models. While SO upwelling sustains the AMOC under the extreme forcings considered here, increased forcings may further weaken the AMOC, but only if the SO upper cell weakens or the PMOC strengthens. AMOC weakening is therefore resisted through maintenance of the SO winds and freshwater input to the Pacific Ocean, implying a large forcing is required for an AMOC collapse 13,16 . Hence, we argue that changes in freshwater transports into the Pacific Ocean, as well as those into the Atlantic Ocean 16 , or changes in the hydrological cycle, may be crucial for AMOC tipping by enabling formation of a strong PMOC. If models underestimate the ability of the PMOC to strengthen, they may also underestimate the risk of a future AMOC collapse, so understanding model biases is essential. Our findings highlight the need for improved observational estimates of the SO and Indo-Pacific overturning and their heat and freshwater transports, similar to efforts in the South Atlantic 51,52 . This would allow us to identify climate models with realistic present-day SO upper cell strength, and to detect changes in the AMOC’s upwelling pathways and transports due to global warming. Constraining the AMOC’s upwelling pathways would also improve predictions of AMOC weakening under realistic forcing scenarios 4 . The future extent of AMOC decline remains uncertain 1,6,12,15 , despite its critical impact on heat, carbon and nutrient transports 7,8 , and thus on global climate 2,3 and ecosystems. We have shown that SO wind-driven upwelling prevents an AMOC collapse in CMIP6 models under extreme GHG and North Atlantic freshwater forcings. With the present-day Bering Strait open and predicted stronger SO winds, we conclude that a 21 st Century AMOC collapse is unlikely. However, to refine AMOC projections, a greater focus on ocean circulation changes beyond the North Atlantic and their driving mechanisms is essential. Future changes in both the Atlantic and Pacific overturning would impact regional weather, climate, ecosystems and agriculture, so accurate projections of both are required to inform adaptation and resilience to climate change. Methods Models and experiments We analyse the AMOC upwelling pathways in 34 CMIP6 models (Extended Data Table 1) from the pre-industrial control (piControl) simulation 29 and their response under two extreme forcing scenarios: the “abrupt-4xCO2” experiment 29 (“4xCO2” herein) and the “u03_hos” experiment from the North Atlantic Hosing Model Intercomparison Project 20 (NAHosMIP). In 4xCO2, atmospheric CO 2 concentrations are instantaneously quadrupled from piControl levels and maintained for 150 years. In u03_hos, a uniform freshwater forcing of 0.3 Sv is applied to the North Atlantic between 50°N and the Bering Strait for at least 100 years. We examine 7 CMIP6 models in u03_hos. We use a single ensemble member from each run (Extended Data Table 1). All available models are included in our analysis to ensure a wide range in the AMOC’s upwelling pathways enabling robust relationships to be inferred. Variables We analyse the monthly-mean overturning mass streamfunction, including both Eulerian-mean and parameterised eddy components 34 , in depth space (variables, “msftmz” or “msftyz”). We average the overturning streamfunction over the first 50 years of the piControl simulation, and over the 20-year period centred on 90 years into the 4xCO2 and u03_hos experiments to obtain the “future” state. We focus on the period 90 years into the extreme forcing experiments because it is available in all models and the AMOC has generally stabilised. We calculate the AMOC strength from the maximum Atlantic streamfunction value north of the equator and below 500 m depth. We calculate PMOC strength from the maximum Indo-Pacific streamfunction value at 34.5°S, between 500 m and 4000 m depth, to exclude wind-driven gyres and unrelated deep overturning circulations. Calculating the AMOC’s upwelling pathways Upwelling pathway definitions We apply the method of Baker et al. (2023), adapted from Baker et al. (2020), to calculate the AMOC’s upwelling pathways (Fig. 1a), which quantify the time-mean area-integrated volume transports that return deep waters from the AMOC’s southward branch into its shallower northward branch (Fig. 1c). We define the AMOC strength as its maximum below 500 m depth in the North Atlantic (Fig. 1a, purple box). The upwelling pathways – Atlantic (“Atlantic_Up”), Indo-Pacific residual (“IndoPac_ResidualUp”), and Southern Ocean (“SouthernOcean_Up”) – define the regions where AMOC origin waters upwell before rejoining the AMOC’s northward branch. Each upwelling pathway is greater than or equal to zero, and collectively match the AMOC strength (Eq. S4; Fig. 1) since the global overturning circulation conserves volume. We determine the three main upwelling pathways by analysing the zonally-integrated meridional overturning streamfunction in the Atlantic and Indo-Pacific Oceans, and a globally-integrated streamfunction in the Southern Ocean (SO), defined as latitudes south of 34.5°S (Fig. 1). These have units of Sverdrups [10 6 m 3 s -1 ]. The Atlantic upwelling pathway (Atlantic_Up, blue box) corresponds to the upwelling rate of AMOC deep waters in the Atlantic Ocean that return northward nearer the surface, inferred from the closed streamlines of the AMOC (Fig. 1a). The SO upwelling pathway (SouthernOcean_Up, orange box) quantifies total upwelling of North Atlantic (i.e., AMOC) origin waters by the SO upper cell, including those that first upwell in the Indo-Pacific (Fig. 1). The Indo-Pacific residual upwelling pathway (IndoPac_ResidualUp, green box in Fig. 1b) quantifies the AMOC’s upwelling pathway in the Indo-Pacific Ocean that does not later upwell in the SO (the latter is accounted for by SouthernOcean_Up ­ ). We calculate the Indo-Pacific residual upwelling pathway as a residual using Eq. S4 that ensures volume conservation in the ocean. In Baker et al. (2023), we showed that in a transient state, changes in AMOC strength are balanced by changes in the AMOC’s upwelling pathways, and vice-versa. These changes are communicated rapidly by non-advective wave processes 31 , ensuring a global upwelling-downwelling balance to conserve volume. Thus, on a decadal or longer timescale, the time-mean AMOC upwelling pathways are equal to the time-mean AMOC strength (Fig. 2). Changes in the Meridional Overturning Circulation (MOC) remote from, but connected to, the North Atlantic through the overturning streamfunction, therefore modulate the AMOC strength, even if the AMOC weakening is instigated by changes in North Atlantic forcing. Equations The equations used to calculate the upwelling pathways (Eq. S4-S4) are: Atlantic_Up = AMOC max – AMOC min (Eq. S1) South_Atlantic_local = AMOC_34S – AMOC min (Eq. S2) SouthernOcean_Up = min( ψ max │ ϕ=34.5°S – South_Atlantic_local – PMOC│ z_AMOC_34S , AMOC min ) (Eq. S3) IndoPac_ResidualUp = AMOC max – Atlantic_Up – SouthernOcean_Up (Eq. S4) where AMOC max is the maximum AMOC strength north of the equator (referred to as the AMOC strength in the main text; Fig. 1a), AMOC min is the minimum AMOC strength between 34.5°S and the equator (Extended Data Fig. 8), AMOC_34S is the maximum AMOC strength at 34.5°S, PMOC│ z_AMOC_34S is the PMOC strength (≥0 Sv) at 34.5°S at the depth of AMOC_34S, ψ max │ ϕ=34.5°S is the globally-integrated SO upper cell strength at 34.5°S. South Atlantic and Pacific overturning cells In some models, a weak, localised South Atlantic circulation at 34.5°S, isolated from the North Atlantic, upwells in the SO (Extended Data Fig. 8). This circulation, denoted “South_Atlantic_local” (Eq. S2, Extended Data Fig. 8), is accounted for when calculating both the Atlantic upwelling pathway (Eq. S1) and the SO upwelling pathway (Eq. S3), since it reduces the SO upwelling (i.e., the globally-integrated SO upper cell strength at 34.5°S, ψ max │ ϕ=34.5°S ­ (red circle in Fig. 1a)) available for upwelling AMOC deep waters. “South_Atlantic_local”, determined from Eq. S2, is reduced if the South Atlantic waters enter an anti-clockwise overturning cell in the Indo-Pacific Ocean, upwell, and rejoin the localised South Atlantic circulations northward branch via the SO. This is because these waters do not upwell in the SO, and thus do not reduce SouthernOcean_Up. Under extreme forcing, most models develop a PMOC that upwells in the SO, further reducing the SO upwelling available to upwell AMOC deep waters. We therefore modify the method of Baker et al., (2023) to account for the presence of a PMOC. A latitudinally expansive PMOC at the depth of the AMOC maximum at 34.5°S, z_AMOC_34S, indicates that all AMOC waters that enter and upwell in the Indo-Pacific Ocean must later upwell in the SO to rejoin the AMOC’s northward branch, since they cannot bypass the PMOC. This scenario occurs in the future state of all models with a PMOC (Extended Data Figs. 2 and 3; Figs. S2 and S3), ensuring IndoPac_ResidualUp is zero (Fig. 2d,h). If SouthernOcean_Up, calculated from the left-hand sum in Eq. S3, exceeds the AMOC pathway into the SO (AMOC min ; grey circle in Extended Data Fig. 8), we adjust SouthernOcean_Up to match AMOC min (Eq. S3). This ensures the SO upwelling pathway is not stronger than the AMOC transport into the SO, as required by conservation of volume. We therefore implicitly account for the PMOC’s impact on the AMOC’s SO upwelling pathway in this case, based on conservation of volume. We account for the PMOC implicitly rather than explicitly to prevent inaccuracies in the upwelling pathways that would otherwise occur in the following scenarios: If the PMOC maximum is below z_AMOC_35S, then AMOC deep waters can enter the PMOC’s northward branch before upwelling in the SO (most notable in CESM2-based models). If a localised clockwise overturning cell at 34.5°S is present in the upper Pacific Ocean (found in the control state of a few models; e.g., Extended Data Fig. 1f,g), then SO upper cell waters enter the northward near-surface branch of these Pacific cells, sink, and later enter the AMOC’s northward branch via the SO. In these scenarios, not all southward PMOC transports at 34.5°S reduce SouthernOcean_Up, so explicitly accounting for the maximum PMOC strength at 34.5°S would underestimate the magnitude of the AMOC’s SO upwelling pathway. We therefore explicitly account for PMOC upwelling in the SO in Eq. S3 only if it cannot be connected to the AMOC, unlike in the scenarios above. Thus, we subtract the PMOC strength at the depth of the AMOC maximum at 34.5°S, z_AMOC_34S, from ψ max │ ϕ=34.5°S , when calculating SouthernOcean_Up in Eq. S3. Hence, if the PMOC maximum is shallower (deeper) than z_AMOC_34S, we subtract the magnitude of the southward (northward) PMOC transport below (above) z_AMOC_34S to calculate SouthernOcean_Up. This results in a small decrease in SouthernOcean_Up during the initial 30 years of the forcing experiments, relative to not explicitly accounting for these PMOC transports. Our approach is validated by IndoPac_ResidualUp tending rapidly (over several decades) towards zero (Fig. 2d,h) before we constrain it to zero by setting SouthernOcean_Up to AMOC min (Eq. S3). This change in IndoPac_ResidualUp is expected because the PMOC rapidly expands, preventing upwelling above the PMOC’s base. Our approach is further validated by the strong correlation between the inverse future PMOC strength and the change in the AMOC’s SO upwelling pathway (Fig 4a,c), despite our approach not explicitly accounting for the PMOC in the future state calculations (when SouthernOcean_Up is set to AMOC min ). The outlying CESM2-based models in Fig. 3c,f and Fig. 4b,d (purple shading) emphasise the importance of implicitly accounting for the PMOC in the future state to prevent inaccuracies in the magnitude of SouthernOcean_Up (see scenario 1 above). We further validated our method by examining overturning streamfunctions across models and experiments for inconsistencies with their calculated upwelling pathways. Declarations Significance tests We conduct a two-tailed Student’s t-test to assess significance of correlations between variables. A P value below 0.05 is considered significant, indicating a 95% confidence level. Data availability The pre-industrial control and 4xCO2 experiment CMIP6 data used in this study is available at https://esgf-index1.ceda.ac.uk/search/cmip6-ceda/. The NAHosMIP u03_hos experiment data 53 used in this study is available at https://doi.org/10.5281/zenodo.7643437. Code availability The code used to calculate the AMOC’s upwelling pathways (Methods) will be made available in a Zenodo repository. References 53. Jackson, L. et al. NAHosMIP experimental protocol. Zenodo https://doi.org/10.5281/zenodo.7225014 (2022). Acknowledgements This is ClimTip contribution #[XXXX]; the ClimTip project has received funding from the European Union's Horizon Europe research and innovation programme under grant agreement No. 101137601. JAB, LCJ, MJB and RAW were supported by the Met Office Hadley Centre Climate Programme funded by BEIS. Author Contributions J.A.B conceived the study, performed the analysis, and wrote the original draft manuscript. J.A.B and A.J.W generated the figures. All authors discussed and interpreted the results and commented on the draft manuscript. Competing interests The authors declare no competing interests. Additional information Correspondence and requests for materials should be addressed to Jonathan A. Baker ( [email protected] ) Reprints and permissions information is available at www.nature.com/reprints. References Weijer, W., Cheng, W., Garuba, O. A., Hu, A. & Nadiga, B. T. CMIP6 Models Predict Significant 21st Century Decline of the Atlantic Meridional Overturning Circulation. Geophysical Research Letters 47 , e2019GL086075 (2020). Bellomo, K., Angeloni, M., Corti, S. & von Hardenberg, J. Future climate change shaped by inter-model differences in Atlantic meridional overturning circulation response. Nature Communications 2021 12:1 12 , 1–10 (2021). Liu, W., Fedorov, A. V., Xie, S. P. & Hu, S. Climate impacts of a weakened Atlantic meridional overturning circulation in a warming climate. Science Advances 6 , (2020). Baker, J. 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W., Behrens, E., Biastoch, A., Getzlaff, K. & Bamber, J. L. Emerging impact of Greenland meltwater on deepwater formation in the North Atlantic Ocean. Nature Geosci 9 , 523–527 (2016). Stocker, T. F. & Wright, D. G. Rapid transitions of the ocean’s deep circulation induced by changes in surface water fluxes. Nature 351 , 729–732 (1991). Intergovernmental Panel on Climate Change (IPCC). Climate Change 2021 – The Physical Science Basis: Working Group I Contribution to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change . (Cambridge University Press, Cambridge, 2021). doi:10.1017/9781009157896. Rahmstorf, S. et al. Thermohaline circulation hysteresis: A model intercomparison. Geophysical Research Letters 32 , 1–5 (2005). Stommel, H. Thermohaline Convection with Two Stable Regimes of Flow. Tellus 13 , 224–230 (1961). Boers, N. Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation. Nature Climate Change 2021 11:8 11 , 680–688 (2021). Westen, R. M. van, Kliphuis, M. & Dijkstra, H. A. Physics-based early warning signal shows that AMOC is on tipping course. Science Advances 10 , 1189 (2024). Hofmann, M. & Rahmstorf, S. On the stability of the Atlantic meridional overturning circulation. Proceedings of the National Academy of Sciences of the United States of America 106 , 20584–20589 (2009). Wood, R. A., Rodríguez, J. M., Smith, R. S., Jackson, L. C. & Hawkins, E. Observable, low-order dynamical controls on thresholds of the Atlantic meridional overturning circulation. Climate Dynamics 53 , 6815–6834 (2019). Hawkins, E. et al. Bistability of the Atlantic overturning circulation in a global climate model and links to ocean freshwater transport. Geophysical Research Letters 38 , (2011). Jackson, L. C. et al. Understanding AMOC stability: the North Atlantic Hosing Model Intercomparison Project. Geosci. Model Dev 16 , (2023). Lumpkin, R. & Speer, K. Global Ocean Meridional Overturning. Journal of Physical Oceanography 37 , 2550–2562 (2007). Talley, L. D. Closure of the global overturning circulation through the Indian, Pacific, and southern oceans. Oceanography 26 , 80–97 (2013). Nikurashin, M. & Vallis, G. A Theory of the Interhemispheric Meridional Overturning Circulation and Associated Stratification. Journal of Physical Oceanography 42 , 1652–1667 (2012). Toggweiler, J. R. & Samuels, B. On the Ocean’s Large-Scale Circulation near the Limit of No Vertical Mixing. Journal of Physical Oceanography 28 , 1832–1852 (1998). Munk, W. & Wunsch, C. Abyssal recipes II: energetics of tidal and wind mixing. Deep Sea Research Part I: Oceanographic Research Papers 45 , 1977–2010 (1998). Downes, S. M. & Hogg, A. M. C. C. Southern Ocean Circulation and Eddy Compensation in CMIP5 Models. Journal of Climate 26 , 7198–7220 (2013). Viebahn, J. & Eden, C. Towards the impact of eddies on the response of the Southern Ocean to climate change. Ocean Modelling 34 , 150–165 (2010). Jackson, L. C. et al. Impact of ocean resolution and mean state on the rate of AMOC weakening. Climate Dynamics 55 , 1711–1732 (2020). Eyring, V. et al. Overview of the Coupled Model Intercomparison Project Phase 6 (CMIP6) experimental design and organization. Geoscientific Model Development 9 , 1937–1958 (2016). Gnanadesikan, A. A simple predictive model for the structure of the oceanic pycnocline. Science 283 , 2077–2079 (1999). Sun, S., Thompson, A. F. & Eisenman, I. Transient Overturning Compensation between Atlantic and Indo-Pacific Basins. Journal of Physical Oceanography 50 , 2151–2172 (2020). Saenko, O. A., Schmittner, A. & Weaver, A. J. The Atlantic–Pacific Seesaw. J. Climate 17 , 2033–2038 (2004). Deng, K. et al. Changes of Southern Hemisphere westerlies in the future warming climate. Atmospheric Research 270 , 106040 (2022). Lee, S. K. et al. Human-induced changes in the global meridional overturning circulation are emerging from the Southern Ocean. Communications Earth & Environment 2023 4:1 4 , 1–12 (2023). Chen, C., Liu, W. & Wang, G. Understanding the Uncertainty in the 21st Century Dynamic Sea Level Projections: The Role of the AMOC. Geophysical Research Letters 46 , 210–217 (2019). Baker, J. A., Watson, A. J. & Vallis, G. K. Meridional overturning circulation in a multibasin model. Part i: Dependence on southern ocean buoyancy forcing. Journal of Physical Oceanography 50 , 1159–1178 (2020). Baker, J. A., Watson, A. J. & Vallis, G. K. Meridional Overturning Circulation in a Multibasin Model. Part II: Sensitivity to Diffusivity and Wind in Warm and Cool Climates. Journal of Physical Oceanography 51 , 1813–1828 (2021). Nadeau, L. P. & Jansen, M. F. Overturning Circulation Pathways in a Two-Basin Ocean Model. Journal of Physical Oceanography 50 , 2105–2122 (2020). Wolfe, C. L. & Cessi, P. The Adiabatic Pole-to-Pole Overturning Circulation. Journal of Physical Oceanography 41 , 1795–1810 (2011). Johnson, H. L., Marshall, D. P. & Sproson, D. A. J. Reconciling theories of a mechanically driven meridional overturning circulation with thermohaline forcing and multiple equilibria. Climate Dynamics 29 , 821–836 (2007). Curtis, P. E. & Fedorov, A. V. Spontaneous Activation of the Pacific Meridional Overturning Circulation (PMOC) in Long-Term Ocean Response to Greenhouse Forcing. Journal of Climate 37 , 1551–1565 (2024). Jackson, L. C., Smith, R. S. & Wood, R. A. Ocean and atmosphere feedbacks affecting AMOC hysteresis in a GCM. Climate Dynamics 49 , 173–191 (2016). Hu, A. et al. The Pacific-Atlantic seesaw and the Bering Strait. Geophysical Research Letters 39 , (2012). Hu, A. et al. Dichotomy between freshwater and heat flux effects on oceanic conveyor belt stability and global climate. Communications Earth & Environment 2023 4:1 4 , 1–15 (2023). Mikolajewicz, U., Crowley, T. J., Schiller, A. & Voss, R. Modelling teleconnections between the North Atlantic and North Pacific during the Younger Dryas. Nature 387 , 384–387 (1997). Okazaki, Y. et al. Deepwater Formation in the North Pacific During the Last Glacial Termination. Science 329 , 200–204 (2010). Rafter, P. A. et al. Global reorganization of deep-sea circulation and carbon storage after the last ice age. Science Advances 8 , eabq5434 (2022). Burls, N. J. et al. Active Pacific meridional overturning circulation (PMOC) during the warm Pliocene. Science Advances 3 , (2017). Liu, W. & Hu, A. The role of the PMOC in modulating the deglacial shift of the ITCZ. Climate Dynamics 45 , 3019–3034 (2015). Carter, B. R. et al. Pacific Anthropogenic Carbon Between 1991 and 2017. Global Biogeochemical Cycles 33 , 597–617 (2019). Dong, S. et al. Synergy of In Situ and Satellite Ocean Observations in Determining Meridional Heat Transport in the Atlantic Ocean. Journal of Geophysical Research: Oceans 126 , e2020JC017073 (2021). Kersalé, M. et al. Highly variable upper and abyssal overturning cells in the South Atlantic. Science Advances 6 , (2020). Additional Declarations There is NO Competing Interest. Supplementary Files ExtendedDataFig.docx Cite Share Download PDF Status: Published Journal Publication published 26 Feb, 2025 Read the published version in Nature → Version 1 posted 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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {\"props\":{\"pageProps\":{\"initialData\":{\"identity\":\"rs-4319650\",\"acceptedTermsAndConditions\":true,\"allowDirectSubmit\":false,\"archivedVersions\":[],\"articleType\":\"Physical Sciences - Article\",\"associatedPublications\":[],\"authors\":[{\"id\":295598962,\"identity\":\"25339636-a151-4772-8502-be633962a0af\",\"order_by\":0,\"name\":\"Jonathan Baker\",\"email\":\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABP0lEQVRIie2RMWuDQBSAnwhOgutBQv0LykHaofhHuiiCXbQUAiWhQwThXCxZ7dw/EOjSodAngl2sXQtZAoHMhiwZMvSwaVqqdS7Ub3gc793He+8OoKPjDyL4n4GAyONplS1H2uGG2agkX4oDIIEQ5y1KxTcl3SvwuyKGN8lm/WBcQC9YauvH1zNFDRIfL40jUO4yIjCnNlhU2CTJ7SH0M2rFq7l3yyTLR82mQFYOV9yaErsaSZho+cSkqYxzb5bJx8tSQ57JqSawUYNCtwmb8Avnm3SHhfeUyTrvgpMWZcC7pFxxqQ2I3kz6UExQIn3RNFiUD05e2LPF+vlQj9D24sypdtEZkayFWdTW18OIvo3ZtTXthfdki4Y3DVL+YjtDVZQUsbyya4q/P0g/KyARs/Ej1XrqgIItxY6Ojo5/xDvIc3tCzI+cIQAAAABJRU5ErkJggg==\",\"orcid\":\"https://orcid.org/0000-0001-5695-6341\",\"institution\":\"Met Office\",\"correspondingAuthor\":true,\"submittingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Jonathan\",\"middleName\":\"\",\"lastName\":\"Baker\",\"suffix\":\"\"},{\"id\":295598963,\"identity\":\"fcf92107-11da-4ba4-936f-40e1192b99f3\",\"order_by\":1,\"name\":\"Michael Bell\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Met Office\",\"correspondingAuthor\":false,\"submittingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Michael\",\"middleName\":\"\",\"lastName\":\"Bell\",\"suffix\":\"\"},{\"id\":295598964,\"identity\":\"d1cb6aa4-ce80-4bde-9c95-6d56d1032a45\",\"order_by\":2,\"name\":\"Laura Jackson\",\"email\":\"\",\"orcid\":\"https://orcid.org/0000-0003-2326-305X\",\"institution\":\"UK Met Office\",\"correspondingAuthor\":false,\"submittingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Laura\",\"middleName\":\"\",\"lastName\":\"Jackson\",\"suffix\":\"\"},{\"id\":295598965,\"identity\":\"6c167ca6-3606-4daa-8115-304626ae4067\",\"order_by\":3,\"name\":\"Geoffrey K Vallis\",\"email\":\"\",\"orcid\":\"https://orcid.org/0000-0002-5971-8995\",\"institution\":\"University of Exeter\",\"correspondingAuthor\":false,\"submittingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Geoffrey\",\"middleName\":\"K\",\"lastName\":\"Vallis\",\"suffix\":\"\"},{\"id\":295598966,\"identity\":\"6cf06c90-ba1d-40bf-81f6-9094743dcaa0\",\"order_by\":4,\"name\":\"Andrew Watson\",\"email\":\"\",\"orcid\":\"https://orcid.org/0000-0002-9654-8147\",\"institution\":\"University of Exeter\",\"correspondingAuthor\":false,\"submittingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Andrew\",\"middleName\":\"\",\"lastName\":\"Watson\",\"suffix\":\"\"},{\"id\":295598967,\"identity\":\"4d3a6c53-f211-42dd-9dde-060a9189490b\",\"order_by\":5,\"name\":\"Richard Wood\",\"email\":\"\",\"orcid\":\"https://orcid.org/0000-0002-3960-9513\",\"institution\":\"Hadley Centre\",\"correspondingAuthor\":false,\"submittingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Richard\",\"middleName\":\"\",\"lastName\":\"Wood\",\"suffix\":\"\"}],\"badges\":[],\"createdAt\":\"2024-04-24 16:55:23\",\"currentVersionCode\":1,\"declarations\":\"\",\"doi\":\"10.21203/rs.3.rs-4319650/v1\",\"doiUrl\":\"https://doi.org/10.21203/rs.3.rs-4319650/v1\",\"draftVersion\":[],\"editorialEvents\":[{\"content\":\"https://doi.org/10.1038/s41586-024-08544-0\",\"type\":\"published\",\"date\":\"2025-02-26T05:00:00+00:00\"}],\"editorialNote\":\"\",\"failedWorkflow\":false,\"files\":[{\"id\":55509303,\"identity\":\"61b9e124-e31b-4c54-a41c-811f48bceef6\",\"added_by\":\"auto\",\"created_at\":\"2024-04-29 12:22:34\",\"extension\":\"png\",\"order_by\":1,\"title\":\"Figure 1\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":498557,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eSchematic and analysis method for AMOC upwelling pathways. a,b,\\u003c/strong\\u003e Meridional overturning streamfunction in Sverdrups (Sv [10\\u003csup\\u003e6\\u003c/sup\\u003e m\\u003csup\\u003e3\\u003c/sup\\u003es\\u003csup\\u003e-1\\u003c/sup\\u003e], with 2 Sv contour intervals) from the CMCC-ESM2 pre-industrial control simulation, highlighting the methodology for separating the AMOC’s upwelling pathways. Streamfunctions are shown for the Atlantic (\\u003cstrong\\u003ea\\u003c/strong\\u003e) and Indo-Pacific (\\u003cstrong\\u003eb\\u003c/strong\\u003e) Oceans north of 34.5°S (indicated by vertical dashed lines) and globally within the Southern Ocean (“S.O.”). The 0-Sv streamline is marked by a solid black line, while the vertical green line at 34.5°S in (\\u003cstrong\\u003eb\\u003c/strong\\u003e) denotes the net volume transport from the Indo-Pacific to the Southern Ocean over the highlighted depth. Colored circles highlight the MOC strength at these locations. “IndoPac_ResidualUp” is calculated as a residual by rearranging Eq. 1. \\u003cstrong\\u003ec,d, \\u003c/strong\\u003eSchematic of the AMOC’s upwelling pathways in the present-day (\\u003cstrong\\u003ec\\u003c/strong\\u003e) and under scenarios of extreme greenhouse gas or North Atlantic freshwater forcing (\\u003cstrong\\u003ed\\u003c/strong\\u003e). Transport pathways are sketched, with increasing water mass density illustrated by a color gradient from yellow to dark green.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"1.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4319650/v1/6923a0b533a886c939c4e096.png\"},{\"id\":55508438,\"identity\":\"7500f920-c1d6-431f-a24f-6aaf14b77b60\",\"added_by\":\"auto\",\"created_at\":\"2024-04-29 12:14:30\",\"extension\":\"png\",\"order_by\":2,\"title\":\"Figure 2\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":470184,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eDecadal-mean evolution of AMOC strength and upwelling pathways under extreme forcing. a-d\\u003c/strong\\u003e Abrupt quadrupling of CO\\u003csub\\u003e2\\u003c/sub\\u003e (“4xCO2”) and \\u003cstrong\\u003ee-h\\u003c/strong\\u003e North Atlantic freshwater hosing (“u03_hos”) forcing scenarios. Variables plotted are (\\u003cstrong\\u003ea,e\\u003c/strong\\u003e) AMOC strength, and (\\u003cstrong\\u003eb,f\\u003c/strong\\u003e) Atlantic, (\\u003cstrong\\u003ec,g\\u003c/strong\\u003e) Southern Ocean, and (\\u003cstrong\\u003ed,h\\u003c/strong\\u003e) Indo-Pacific residual upwelling pathways of the AMOC. The control simulation averaged over the first 50 years is plotted at year 0, with magnitudes under the forcing scenarios calculated in 10-year intervals. Models used in both the 4xCO2 and u03_hos scenarios are labelled *.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"2.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4319650/v1/71b80cfad3763adab13e5f23.png\"},{\"id\":55509304,\"identity\":\"75fdbfbc-9fd6-4936-8146-b587e98ee4c1\",\"added_by\":\"auto\",\"created_at\":\"2024-04-29 12:22:35\",\"extension\":\"png\",\"order_by\":3,\"title\":\"Figure 3\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":324948,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eImpact of Southern Ocean upwelling and PMOC on future AMOC strength under extreme forcing. a,d\\u003c/strong\\u003e Future and \\u003cstrong\\u003eb,e\\u003c/strong\\u003e control Southern Ocean upwelling pathways of the AMOC, and \\u003cstrong\\u003ec,f \\u003c/strong\\u003ecombination of future SO upper cell strength at 34.5°S and inverted future PMOC strength at 34.5°S, against future AMOC strength. The future state is 90 years into the (\\u003cstrong\\u003ea-c\\u003c/strong\\u003e) 4xCO2 and (\\u003cstrong\\u003ed-f\\u003c/strong\\u003e) North Atlantic freshwater hosing (“u03_hos”) forcing scenarios. A line of best fit across the whole ensemble, excluding models shaded in orange (\\u003cstrong\\u003ea-f\\u003c/strong\\u003e) and purple (\\u003cstrong\\u003ec,f\\u003c/strong\\u003e) is shown, with blue shading indicating the 95% confidence interval. A line of equality (dashed line) is also shown.\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"3.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4319650/v1/baa948b4d10130f798e111a1.png\"},{\"id\":55508436,\"identity\":\"77170a14-c2de-4c92-89ea-fbb571ce69db\",\"added_by\":\"auto\",\"created_at\":\"2024-04-29 12:14:30\",\"extension\":\"png\",\"order_by\":4,\"title\":\"Figure 4\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":242109,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003e\\u003cstrong\\u003eMechanisms driving changes in the AMOC’s Southern Ocean upwelling pathway under extreme forcing.\\u003c/strong\\u003eCorrelation of changes in the AMOC’s Southern Ocean (SO) upwelling pathway with \\u003cstrong\\u003ea,c,\\u003c/strong\\u003e projected future PMOC strength at 34.5°S, and with \\u003cstrong\\u003eb,d,\\u003c/strong\\u003ecombined effect of changes in SO upper cell strength at 34.5°S and inverted future PMOC strength at 34.5°S. Changes are between the control simulation and the future state 90 years after applying (\\u003cstrong\\u003ea-c\\u003c/strong\\u003e) 4xCO2 or (\\u003cstrong\\u003ed-f\\u003c/strong\\u003e) North Atlantic freshwater hosing (“u03_hos”) forcing scenarios. A line of best fit across the whole ensemble (\\u003cstrong\\u003ea,c\\u003c/strong\\u003e), excluding models shaded in purple (\\u003cstrong\\u003eb,d\\u003c/strong\\u003e), is shown.\\u003cstrong\\u003e \\u003c/strong\\u003eA line of equality (dashed line) is shown in (\\u003cstrong\\u003ec\\u003c/strong\\u003e) and (\\u003cstrong\\u003ef\\u003c/strong\\u003e).\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"4.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4319650/v1/8e56446a4cea215050e396ec.png\"},{\"id\":77299012,\"identity\":\"3fb14e04-0f9d-4e6d-9fd6-cb24075a31af\",\"added_by\":\"auto\",\"created_at\":\"2025-02-27 08:05:38\",\"extension\":\"pdf\",\"order_by\":0,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"manuscript-pdf\",\"size\":2458118,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"manuscript.pdf\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4319650/v1/762da7a1-9470-4216-a59a-fc5ae0d1bf90.pdf\"},{\"id\":55508440,\"identity\":\"558297d9-0a02-43f1-97d3-4c15d836a3a0\",\"added_by\":\"auto\",\"created_at\":\"2024-04-29 12:14:30\",\"extension\":\"docx\",\"order_by\":1,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"supplement\",\"size\":18652728,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"ExtendedDataFig.docx\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-4319650/v1/8e84b4caa9117f172c7e4d5a.docx\"}],\"financialInterests\":\"There is \\u003cb\\u003eNO\\u003c/b\\u003e Competing Interest.\",\"formattedTitle\":\"Wind-driven upwelling sustains weakened Atlantic overturning under climate extremes\",\"fulltext\":[{\"header\":\"Introduction\",\"content\":\"\\u003cp\\u003eThe future state of the Atlantic Meridional Overturning Circulation (AMOC) is critical for global and regional climate change\\u003csup\\u003e2,3\\u003c/sup\\u003e through its role in heat transport\\u003csup\\u003e7\\u003c/sup\\u003e and carbon uptake\\u003csup\\u003e8\\u003c/sup\\u003e. The AMOC is predicted to weaken during the 21\\u003csup\\u003est\\u003c/sup\\u003e century\\u003csup\\u003e1,9\\u003c/sup\\u003e driven by increased greenhouse gas (GHG) concentrations and freshwater input to the North Atlantic due to precipitation changes and Greenland ice sheet melt\\u003csup\\u003e10,11\\u003c/sup\\u003e. However, climate model projections from the Coupled Model Intercomparison Project Phase 6 (CMIP6) vary widely\\u003csup\\u003e1,4,5\\u003c/sup\\u003e, so its future evolution is uncertain\\u003csup\\u003e12\\u003c/sup\\u003e. Furthermore,\\u0026nbsp;there is a risk the AMOC could collapse, causing abrupt changes in climate\\u003csup\\u003e13,14\\u003c/sup\\u003e.\\u0026nbsp;Some statistical indicators suggest it is approaching a tipping point\\u003csup\\u003e6,15,16\\u003c/sup\\u003e, indicating climate models may be overstable\\u003csup\\u003e17\\u003c/sup\\u003e. Since AMOC changes will impact many aspects of the climate\\u003csup\\u003e2,3\\u003c/sup\\u003e, accurately predicting its future change is vital for planning adaptation and mitigation strategies.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eHere, we examine the AMOC’s response to extreme GHG and North Atlantic freshwater forcings in CMIP6 models. Although\\u0026nbsp;simpler models\\u003csup\\u003e14,18\\u003c/sup\\u003e and a few global climate models (GCMs)\\u0026nbsp;suggest the AMOC could collapse (i.e., weaken to zero or reverse)\\u0026nbsp;under such forcings\\u003csup\\u003e16,19\\u003c/sup\\u003e, it does not collapse in the\\u0026nbsp;model experiments considered here. Instead, the AMOC stabilises at a weaker strength that varies widely across models\\u003csup\\u003e2,20\\u003c/sup\\u003e.\\u003c/p\\u003e\\n\\u003cp\\u003eTo understand how the AMOC is sustained under extreme climate forcing, and what causes differences in its weakened state across models and forcings, we quantify the AMOC’s upwelling pathways\\u003csup\\u003e21,22\\u003c/sup\\u003e that\\u0026nbsp;return deep North Atlantic waters to the surface\\u0026nbsp;either through Southern Ocean (SO) wind-driven upwelling\\u003csup\\u003e23,24\\u003c/sup\\u003e or diffusion in the Atlantic and Indo-Pacific Oceans\\u003csup\\u003e25\\u003c/sup\\u003e. The SO wind-driven upwelling rate depends on the SO westerly wind strength that drives the SO upwelling and on the opposing SO mesoscale eddy response, referred to as eddy compensation\\u003csup\\u003e26,27\\u003c/sup\\u003e. Our approach, focusing on AMOC upwelling pathways\\u0026nbsp;rather than the mean state of the North Atlantic\\u003csup\\u003e5,28\\u003c/sup\\u003e offers new perspectives on the AMOC response.\\u0026nbsp;While the AMOC’s Indo-Pacific upwelling pathway largely controls inter-model variations in AMOC decline in realistic scenarios\\u003csup\\u003e4\\u003c/sup\\u003e, the impact of extreme forcing on upwelling pathway changes and thereby potentially AMOC stability has not been explored. Here, we assess how ocean circulation changes remote from the Atlantic affect the AMOC and its upwelling pathways in climate models. Our analysis reveals that SO wind-driven upwelling maintains a weakened AMOC under extreme forcing, overcoming the destabilising effects of a developing Pacific MOC (PMOC), with implications for understanding both past, and future risks of, AMOC collapse.\\u003c/p\\u003e\\n\\n\\n\\n\\n\\n\\n\\n\\n\\n\\n\\n\\n\\n\\n\\n\\n\\n\\n\\n\\n\\n\\n\\n\"},{\"header\":\"Experiments and AMOC upwelling pathways\",\"content\":\"\\u003cp\\u003eWe examine the AMOC in 34 CMIP6 models from the pre-industrial control simulation\\u003csup\\u003e29\\u003c/sup\\u003e and their response under two extreme forcing scenarios (Methods): an abrupt quadrupling of atmospheric CO\\u003csub\\u003e2\\u003c/sub\\u003e (“4xCO2”\\u003csup\\u003e29\\u003c/sup\\u003e) and a 0.3 Sv North Atlantic freshwater forcing (“u03_hos”\\u003csup\\u003e20\\u003c/sup\\u003e). Only 7 of the models are available in u03_hos (Extended Data Table 1), herein named the u03_hos model subset.\\u0026nbsp;\\u003c/p\\u003e\\u003cp\\u003eWe apply the method of\\u0026nbsp;Baker et al., 2023\\u0026nbsp;to calculate the AMOC’s upwelling pathways (Fig. 1a,b, and Methods), which represent the time-mean area-integrated volume transports that return deep waters from the AMOC’s southward branch into its shallower northward branch (Fig. 1c). We\\u0026nbsp;use the Atlantic, Indo-Pacific and global overturning streamfunctions to quantify these volume transports (Methods). We define the AMOC strength as the maximum strength of the Atlantic mid-depth overturning cell (purple circle in Fig. 1a). The AMOC’s upwelling pathways are defined by the regions where they upwell before rejoining the AMOC’s northward branch - the Atlantic Ocean (“Atlantic_Up”), the Indo-Pacific Ocean (“IndoPac_ResidualUp”), and the Southern Ocean (“SouthernOcean_Up”). AMOC deep waters that upwell in the Indo-Pacific Ocean and subsequently upwell in the SO are accounted for by SouthernOcean_Up, not IndoPac_ResidualUp (Methods). In the control simulation, SouthernOcean_Up is similar to the SO upper cell strength at 34.5°S (red circle in Fig. 1a), with adjustments for a localised South Atlantic circulation (Methods and Extended Data Fig. 8). Together the three upwelling pathways are equal to the AMOC strength (Fig. 1), ensuring volume is conserved in the ocean\\u003csup\\u003e30\\u003c/sup\\u003e.\\u0026nbsp;\\u003c/p\\u003e\"},{\"header\":\"Changes in the overturing circulation\",\"content\":\"\\u003cp\\u003eIn the pre-industrial control, all models have a clockwise, mid-depth overturning cell (i.e., the AMOC) in the Atlantic, an anti-clockwise cell in the Indo-Pacific, and a clockwise wind-driven upper cell in the SO (Fig. 1a-c and Extended Data Fig. 1). However, models vary widely in AMOC strength and their upwelling pathways (year 0 in Fig. 2a-d).\\u003c/p\\u003e\\u003cp\\u003eUnder GHG and freshwater forcings, all models show AMOC weakening with a wide spread (Fig. 2a,e), stabilising within 90 years (the “future” state herein). The AMOC weakens by 20%–81% (mean of 54%) 90 years after 4xCO2 forcing. Considering only the u03_hos model subset, the weakening ranges from 50%–80% in both 4xCO2 (mean of 56%) and u03_hos (mean of 61%). Thus, the future AMOC strength differs across the models, leading to radically different climate impacts\\u003csup\\u003e2\\u003c/sup\\u003e.\\u003cstrong\\u003e\\u0026nbsp;\\u003c/strong\\u003e\\u003c/p\\u003e\\u003cp\\u003eIn the Indo-Pacific, the anti-clockwise overturning circulation not only weakens\\u003csup\\u003e4,31\\u003c/sup\\u003e, but reverses rapidly (Fig. 1d and Extended Data Fig. 2,3), indicating the presence of an Atlantic-Pacific seesaw\\u003csup\\u003e32\\u003c/sup\\u003e in CMIP6 models. The resulting clockwise Pacific MOC (PMOC), analogous to the AMOC, is found in 91% (86%) of models in the future state of 4xCO2 (u03_hos), with variable strength (Extended Data Fig. 4a,c). Its mean strength at 34.5°S is 4.9 Sv in 4xCO2 (6.4 Sv for u03_hos subset), but only 2.3 Sv in u03_hos.\\u003c/p\\u003e\\u003cp\\u003eThe SO upper cell at 34.5°S strengthens in 4xCO2 (Extended Data Fig. 5a) due to enhanced SO westerly winds\\u003csup\\u003e33,34\\u003c/sup\\u003e that more than compensates the weakening from their poleward shift (Extended Data Fig. 5b). Conversely, the SO upper cell weakens in u03_hos (Extended Data Fig. 4d), likely due to a poleward shift in the SO westerly winds, characterised by a dipole in SO overturning changes (Chen et al., 2019; Extended Data Fig. 6).\\u003c/p\\u003e\\u003cp\\u003eSince changes in SO wind stress in 4xCO2 are relatively similar across the models (Extended Data Fig. 5), we attribute inter-model spread in SO upper cell strength changes (~4 Sv) to differences in eddy compensation, which depends on model resolution and eddy parameterisation\\u003csup\\u003e26,27\\u003c/sup\\u003e. Notably, in 4xCO2, lower resolution models from the HadGEM3, MPI, CNRM, and EC-Earth3-Veg model groups have greater strengthening of the SO upper cell (Extended Data Fig. 5), implying they have a weaker eddy compensation.\\u003c/p\\u003e\"},{\"header\":\"AMOC resilience under extreme forcing\",\"content\":\"\\u003cp\\u003eUnder the extreme climate forcings, all AMOC upwelling pathways ultimately weaken to conserve volume as the AMOC weakens, ensuring upwelling balances downwelling, but there is large inter-model spread (Fig. 2b-d,f-h). The Atlantic (Fig. 2b,f) and Indo-Pacific residual (Fig. 2d,h) upwelling pathways weaken to approximately zero in most models after 90 years of forcing due to AMOC shoaling, which reduces “cell overlap” between the AMOC and the SO lower cell (Fig. 1a, Extended Data Fig. 2,3), cutting off its main pathway into the Indo-Pacific Ocean\\u003csup\\u003e36–38\\u003c/sup\\u003e (Fig. 1d).\\u003cstrong\\u003e\\u0026nbsp;\\u003c/strong\\u003eRapid changes in the Indo-Pacific MOC triggered by wave processes as the AMOC weakens\\u003csup\\u003e31\\u003c/sup\\u003e also reduce the Indo-Pacific residual upwelling pathway. The future Atlantic upwelling pathway is greater in u03_hos than in 4xCO2, but it is diminished in both scenarios (\\u0026lt;2 Sv in most models) (Fig. 2b,f). In 4xCO2, the SO upwelling pathway initially increases due to SO upper cell strengthening, before it decreases (Fig. 2c). The future SO upwelling pathway is therefore greater in 4xCO2 than in u03_hos (Extended Data Fig. 7c,g), but it is the dominant upwelling pathway (2-13 Sv) in both scenarios (Fig. 2c,g), sustaining the future AMOC. This suggests that without SO upwelling, the AMOC would collapse under these forcings since the Atlantic and Indo-Pacific residual upwelling pathways would still diminish due to AMOC shoaling.\\u003c/p\\u003e\\u003cp\\u003eSince the future SO upwelling pathway primarily sustains the future AMOC, their future magnitudes are directly proportional in both 4xCO2 (r=0.99; Fig. 3a) and u03_hos (r=0.93; Fig. 3d), excluding four outlying models in 4xCO2 (shaded orange in Fig. 3a). These four models have anomalously strong future AMOCs due to notable Atlantic or Indo-Pacific residual upwelling pathways (Fig. 2a-d). These upwelling pathways are unrealistically large in the control simulations of these models in comparison to the observed AMOC strength (Fig. 2b,d).\\u003c/p\\u003e\\u003cp\\u003eIf SO upwelling pathway changes were consistent across models and thus the control SO upwelling pathway was directly proportional to its future magnitude, it would also correlate strongly with the future AMOC strength. Excluding the four outlying models, correlation between these variables is statistically significant but weak in 4xCO2 (Fig. 3b; r=0.38; p\\u0026lt;0.05 (Methods)), whereas it is insignificant in u03_hos (Fig. 3e; r=0.65, p\\u0026gt;0.05). Thus, while a stronger SO upwelling pathway (and SO upper cell at 34.5°S) in the control is associated with a stronger future AMOC, the relationship is weak.\\u003c/p\\u003e\"},{\"header\":\"Indo-Pacific and Southern Ocean impacts on future AMOC strength\",\"content\":\"\\u003cp\\u003eIn 4xCO2, the SO upwelling pathway weakens (Extended Data Fig. 7c), despite a strengthening of the SO upper cell at 34.5°S in 86% of models (Extended Data Fig. 5). We attribute this weakening to the PMOC that emerges in the Indo-Pacific (Extended Data Fig. 4a), possibly instigated by North Atlantic forcing. The PMOC upwells via the SO upper cell, thereby reducing the volume of AMOC origin waters that can upwell in the SO, leading to a weakening of the AMOC’s SO upwelling pathway (Fig. 4a).\\u0026nbsp;\\u003c/p\\u003e\\u003cp\\u003eVariability in PMOC strength across models largely explains why seven models in 4xCO2 (besides the four prementioned unrealistic models) have enhanced (shaded red) or greatly weakened (shaded green) future SO upwelling pathways (Fig. 4a). These differences result in anomalously strong or weak AMOCs, respectively, relative to their control SO upwelling pathways (Fig. 3b).\\u003cstrong\\u003e\\u0026nbsp;\\u003c/strong\\u003eThus, models with relatively strong future AMOCs (shaded red; Fig. 3b) have weak future PMOCs (\\u0026lt;2.5 Sv; Fig. 4a), stabilising the AMOC (Extended Data Fig. 9a). Conversely, models with relatively weak future AMOCs (shaded green; Fig. 3b) tend to have strong future PMOCs (~8 Sv; Fig. 4a) that enabled the AMOC to weaken further\\u0026nbsp;(Extended Data Fig. 9b). However, not all models with strong future PMOCs (\\u0026gt;5 Sv; Fig. 4a) have anomalously weak future AMOCs relative to their control SO upwelling pathways because some models are compensated by strong increases in SO upper cell strength (Extended Data Fig. 5). Therefore, changes in the AMOC’s SO upwelling pathway depend largely on both changes in SO upper cell strength and the future PMOC strength (Fig. 4b; r=0.95). Exceptions are models with a deep northward PMOC branch that allows AMOC deep waters to zonally enter the PMOC via the SO, before returning to the Atlantic via SO upwelling, thereby unexpectedly stabilising the AMOC. This is small in most models (\\u0026lt;1 Sv) except CESM2-based models (purple shading in Fig. 4b,d) due to their\\u003cstrong\\u003e\\u0026nbsp;\\u003c/strong\\u003eanomalously deep South Pacific subtropical gyre cells (Extended Data Figs. 2a,3a).\\u003c/p\\u003e\\u003cp\\u003eUnlike in 4xCO2, the SO upper cell at 34.5°S weakens in u03_hos (Extended Data Fig. 4d),\\u003cstrong\\u003e\\u0026nbsp;\\u003c/strong\\u003eso the SO upwelling pathway weakens further, despite a weaker future PMOC (cf. mutual models in Fig. 4a,c). Hence, both a weakening SO upper cell and an emerging PMOC reduce the AMOC’s SO upwelling pathway in u03_hos (Fig. 4d).\\u0026nbsp;\\u003c/p\\u003e\"},{\"header\":\"Future fate of the AMOC\",\"content\":\"\\u003cp\\u003eWe have shown that SO wind-driven upwelling prevents an AMOC collapse under extreme climate forcing in CMIP6 models. The AMOC’s future strength depends on the future SO upper cell strength (which depends on its present-day strength and change), but also on the future PMOC strength (if it forms) (Fig. 3c,e; r=0.90 (r=0.98) for 4xCO2 (u03_hos), excluding anomalous models shaded in orange and purple). Previous studies\\u003csup\\u003e23,39\\u003c/sup\\u003e highlighted SO wind-driven upwelling’s role in sustaining an AMOC in simple models, but its importance in comprehensive GCMs under extreme forcing was unexplored. GHG forcing strengthens the SO westerly winds\\u003csup\\u003e33\\u003c/sup\\u003e, strengthening the SO upper cell\\u003csup\\u003e34\\u003c/sup\\u003e. Thus, for an AMOC collapse or substantial AMOC weakening to occur, a strong PMOC that upwells in the SO (Fig. 1d) is essential because SO wind-driven upwelling must be balanced by downwelling in either the Atlantic or Pacific Oceans to conserve volume\\u003csup\\u003e30,40\\u003c/sup\\u003e.\\u0026nbsp;\\u003c/p\\u003e\\u003cp\\u003eThe emergence of a PMOC as the AMOC weakens has previously been found in GCM extreme forcing experiments\\u003csup\\u003e32,41,42\\u003c/sup\\u003e, but Indo-Pacific overturning changes are generally overlooked. The consistency of PMOC formation under extreme\\u003cstrong\\u003e\\u0026nbsp;\\u003c/strong\\u003eclimate forcing across CMIP6 models highlights the importance of the Atlantic-Pacific seesaw\\u003csup\\u003e32\\u003c/sup\\u003e in extreme AMOC weakening. Our findings suggest a PMOC can develop without closure of the Bering Strait, in contrast to \\u003csup\\u003e43\\u003c/sup\\u003e. However, North Atlantic freshwater forcing in u03_hos may reduce freshwater export from the Pacific Ocean through the Bering Strait\\u003csup\\u003e43\\u003c/sup\\u003e, limiting the PMOC strength. In 4xCO2, Pacific freshwater export continues, contributing to AMOC\\u003cstrong\\u003e\\u0026nbsp;\\u003c/strong\\u003eweakening\\u003csup\\u003e44\\u003c/sup\\u003e and potentially enabling a stronger PMOC. Although the PMOC tends to form in the North Pacific subtropical gyre rather than at higher latitudes (Extended Data Figs. 2,3), its strength is probably sensitive to salinity changes in the Pacific due to changes in the ocean’s freshwater transports or atmospheric freshwater fluxes\\u003csup\\u003e45,46\\u003c/sup\\u003e. Further research is needed on how changes in these processes affect PMOC formation and AMOC decline.\\u003c/p\\u003e\\u003cp\\u003eAn active PMOC associated with a weakened or collapsed AMOC was evident in past climates, including during the Last Glacial Maximum and its termination\\u003csup\\u003e46,47\\u003c/sup\\u003e and the warm Pliocene when\\u0026nbsp;atmospheric CO2 levels were similar to present-day levels\\u003csup\\u003e48\\u003c/sup\\u003e. Our findings suggest PMOC formation, by reducing the SO upwelling of AMOC deep waters, may have facilitated AMOC collapses in past climates. The PMOC transports heat northwards\\u003csup\\u003e49\\u003c/sup\\u003e, and affects ocean biogeochemistry and carbon uptake\\u003csup\\u003e47,50\\u003c/sup\\u003e, highlighting the need to assess its impact on future climate in CMIP6 models.\\u0026nbsp;\\u003c/p\\u003e\\u003cp\\u003eWhile SO upwelling sustains the AMOC under the extreme forcings considered here, increased forcings may further weaken the AMOC, but only if the SO upper cell weakens or the PMOC strengthens. AMOC weakening is therefore resisted through maintenance of the SO winds and freshwater input to the Pacific Ocean, implying a large forcing is required for an AMOC collapse\\u003csup\\u003e13,16\\u003c/sup\\u003e. Hence, we argue that changes in freshwater transports into the Pacific Ocean, as well as those into the Atlantic Ocean\\u003csup\\u003e16\\u003c/sup\\u003e, or changes in the hydrological cycle, may be crucial for AMOC tipping by enabling formation of a strong PMOC. If models underestimate the ability of the PMOC to strengthen, they may also underestimate the risk of a future AMOC collapse, so understanding model biases is essential.\\u003c/p\\u003e\\u003cp\\u003eOur findings highlight the need for improved observational estimates of the SO and Indo-Pacific overturning and their heat and freshwater transports, similar to efforts in the South Atlantic\\u003csup\\u003e51,52\\u003c/sup\\u003e\\u003cstrong\\u003e.\\u0026nbsp;\\u003c/strong\\u003eThis\\u003cstrong\\u003e\\u0026nbsp;\\u003c/strong\\u003ewould allow us to identify climate models with realistic present-day SO upper cell strength, and to detect changes in the AMOC’s upwelling pathways and transports due to global warming.\\u003cstrong\\u003e\\u0026nbsp;\\u003c/strong\\u003eConstraining the AMOC’s upwelling pathways would also improve predictions of AMOC weakening under realistic forcing scenarios\\u003csup\\u003e4\\u003c/sup\\u003e.\\u003c/p\\u003e\\u003cp\\u003eThe future extent of AMOC decline remains uncertain\\u003csup\\u003e1,6,12,15\\u003c/sup\\u003e, despite its critical impact on heat, carbon and nutrient transports\\u003csup\\u003e7,8\\u003c/sup\\u003e, and thus on global climate\\u003csup\\u003e2,3\\u003c/sup\\u003e and ecosystems. We have shown that SO wind-driven upwelling prevents an AMOC collapse in CMIP6 models under extreme GHG and North Atlantic freshwater forcings. With the present-day Bering Strait open and predicted stronger SO winds, we conclude that a 21\\u003csup\\u003est\\u003c/sup\\u003e Century AMOC collapse is unlikely. However, to refine AMOC projections, a greater focus on ocean circulation changes beyond the North Atlantic and their driving mechanisms is essential. Future changes in both the Atlantic and Pacific overturning would impact regional weather, climate, ecosystems and agriculture, so accurate projections of both are required to inform adaptation and resilience to climate change.\\u0026nbsp;\\u003c/p\\u003e\"},{\"header\":\"Methods\",\"content\":\"\\u003ch2\\u003eModels and experiments\\u003c/h2\\u003e\\n\\u003cp\\u003eWe analyse the AMOC upwelling pathways in 34 CMIP6 models (Extended Data Table 1) from the pre-industrial control (piControl) simulation\\u003csup\\u003e29\\u003c/sup\\u003e and their response under two extreme forcing scenarios: the \\u0026ldquo;abrupt-4xCO2\\u0026rdquo; experiment\\u003csup\\u003e29\\u003c/sup\\u003e (\\u0026ldquo;4xCO2\\u0026rdquo; herein) and the \\u0026ldquo;u03_hos\\u0026rdquo; experiment from the North Atlantic Hosing Model Intercomparison Project\\u003csup\\u003e20\\u003c/sup\\u003e (NAHosMIP). In 4xCO2, atmospheric CO\\u003csub\\u003e2\\u003c/sub\\u003e concentrations are instantaneously quadrupled from piControl levels and maintained for 150 years. In u03_hos, a uniform freshwater forcing of 0.3 Sv is applied to the North Atlantic between 50\\u0026deg;N and the Bering Strait for at least 100 years. We examine 7 CMIP6 models in u03_hos. We use a single ensemble member from each run (Extended Data Table 1). All available models are included in our analysis to ensure a wide range in the AMOC\\u0026rsquo;s upwelling pathways enabling robust relationships to be inferred.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eVariables\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eWe analyse the monthly-mean overturning mass streamfunction, including both Eulerian-mean and parameterised eddy components\\u003csup\\u003e34\\u003c/sup\\u003e, in depth space (variables, \\u0026ldquo;msftmz\\u0026rdquo; or \\u0026ldquo;msftyz\\u0026rdquo;). We average the overturning streamfunction over the first 50 years of the piControl simulation, and over the 20-year period centred on 90 years into the 4xCO2 and u03_hos experiments to obtain the \\u0026ldquo;future\\u0026rdquo; state. We focus on the period 90 years into the extreme forcing experiments because it is available in all models and the AMOC has generally stabilised.\\u003cstrong\\u003e\\u0026nbsp;\\u003c/strong\\u003eWe calculate the AMOC strength from the maximum Atlantic streamfunction value north of the equator and below 500 m depth. We calculate PMOC strength from the maximum Indo-Pacific streamfunction value at 34.5\\u0026deg;S, between 500 m and 4000 m depth, to exclude wind-driven gyres and unrelated deep overturning circulations.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eCalculating the AMOC\\u0026rsquo;s upwelling pathways\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eUpwelling pathway definitions\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eWe apply the method of Baker et al. (2023), adapted from Baker et al. (2020), to calculate the AMOC\\u0026rsquo;s upwelling pathways (Fig. 1a), which quantify the time-mean area-integrated volume transports that return deep waters from the AMOC\\u0026rsquo;s southward branch into its shallower northward branch (Fig. 1c). We define the AMOC strength as its maximum below 500 m depth in the North Atlantic (Fig. 1a, purple box). The upwelling pathways \\u0026ndash; Atlantic (\\u0026ldquo;Atlantic_Up\\u0026rdquo;), Indo-Pacific residual (\\u0026ldquo;IndoPac_ResidualUp\\u0026rdquo;), and Southern Ocean (\\u0026ldquo;SouthernOcean_Up\\u0026rdquo;) \\u0026ndash; define the regions where AMOC origin waters upwell before rejoining the AMOC\\u0026rsquo;s northward branch. Each upwelling pathway is greater than or equal to zero, and collectively match the AMOC strength (Eq. S4; Fig. 1) since the global overturning circulation conserves volume.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eWe determine the three main upwelling pathways by analysing the zonally-integrated meridional overturning streamfunction in the Atlantic and Indo-Pacific Oceans, and a globally-integrated streamfunction in the Southern Ocean (SO), defined as latitudes south of 34.5\\u0026deg;S (Fig. 1). These have units of Sverdrups [10\\u003csup\\u003e6\\u003c/sup\\u003e m\\u003csup\\u003e3\\u003c/sup\\u003e s\\u003csup\\u003e-1\\u003c/sup\\u003e]. The Atlantic upwelling pathway (Atlantic_Up, blue box) corresponds to the upwelling rate of AMOC deep waters in the Atlantic Ocean that return northward nearer the surface, inferred from the closed streamlines of the AMOC (Fig. 1a). The SO upwelling pathway (SouthernOcean_Up, orange box) quantifies total upwelling of North Atlantic (i.e., AMOC) origin waters by the SO upper cell, including those that first upwell in the Indo-Pacific (Fig. 1). The Indo-Pacific residual upwelling pathway (IndoPac_ResidualUp, green box in Fig. 1b) quantifies the AMOC\\u0026rsquo;s upwelling pathway in the Indo-Pacific Ocean that does not later upwell in the SO (the latter is accounted for by SouthernOcean_Up\\u003csub\\u003e\\u0026shy;\\u003c/sub\\u003e). We calculate the Indo-Pacific residual upwelling pathway as a residual using Eq. S4 that ensures volume conservation in the ocean.\\u003c/p\\u003e\\n\\u003cp\\u003eIn\\u0026nbsp;Baker et al. (2023), we showed that in a transient state, changes in AMOC strength are balanced by changes in the AMOC\\u0026rsquo;s upwelling pathways, and vice-versa. These changes are communicated rapidly by non-advective wave processes\\u003csup\\u003e31\\u003c/sup\\u003e, ensuring a global upwelling-downwelling balance to conserve volume. Thus, on a decadal or longer timescale, the time-mean AMOC upwelling pathways are equal to the time-mean AMOC strength (Fig. 2). Changes in the Meridional Overturning Circulation (MOC) remote from, but connected to, the North Atlantic through the overturning streamfunction, therefore modulate the AMOC strength, even if the AMOC weakening is instigated by changes in North Atlantic forcing.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eEquations\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe equations used to calculate the upwelling pathways (Eq. S4-S4) are:\\u003c/p\\u003e\\n\\u003cp\\u003eAtlantic_Up \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; = AMOC\\u003csub\\u003emax\\u0026nbsp;\\u003c/sub\\u003e\\u0026ndash; AMOC\\u003csub\\u003emin \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;\\u003c/sub\\u003e(Eq. S1)\\u003c/p\\u003e\\n\\u003cp\\u003eSouth_Atlantic_local \\u0026nbsp; = AMOC_34S \\u0026ndash; AMOC\\u003csub\\u003emin \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;\\u003c/sub\\u003e(Eq. S2)\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eSouthernOcean_Up \\u0026nbsp; \\u0026nbsp; = \\u0026nbsp;min( \\u0026psi;\\u003csub\\u003emax\\u003c/sub\\u003e│\\u003csub\\u003eϕ=34.5\\u0026deg;S\\u003c/sub\\u003e \\u0026ndash; South_Atlantic_local \\u0026ndash; PMOC│\\u003csub\\u003ez_AMOC_34S\\u003c/sub\\u003e , AMOC\\u003csub\\u003emin\\u0026nbsp;\\u003c/sub\\u003e) \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; (Eq. S3)\\u003c/p\\u003e\\n\\u003cp\\u003eIndoPac_ResidualUp\\u003csub\\u003e\\u0026nbsp; \\u0026nbsp; \\u0026nbsp;\\u003c/sub\\u003e= AMOC\\u003csub\\u003emax\\u003c/sub\\u003e \\u0026ndash; Atlantic_Up \\u0026ndash; SouthernOcean_Up\\u003csub\\u003e\\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp; \\u0026nbsp;\\u0026nbsp;\\u003c/sub\\u003e(Eq. S4)\\u003c/p\\u003e\\n\\u003cp\\u003ewhere AMOC\\u003csub\\u003emax\\u003c/sub\\u003e is the maximum AMOC strength north of the equator (referred to as the AMOC strength in the main text; Fig. 1a), AMOC\\u003csub\\u003emin\\u0026nbsp;\\u003c/sub\\u003eis the minimum AMOC strength between 34.5\\u0026deg;S and the equator (Extended Data Fig. 8), AMOC_34S\\u003csub\\u003e\\u0026nbsp;\\u003c/sub\\u003eis the maximum AMOC strength at 34.5\\u0026deg;S, PMOC│\\u003csub\\u003ez_AMOC_34S\\u0026nbsp;\\u003c/sub\\u003eis the PMOC strength (\\u0026ge;0 Sv) at 34.5\\u0026deg;S at the depth of AMOC_34S,\\u0026nbsp;\\u0026psi;\\u003csub\\u003emax\\u003c/sub\\u003e│\\u003csub\\u003eϕ=34.5\\u0026deg;S\\u0026nbsp;\\u003c/sub\\u003eis the globally-integrated SO upper cell strength at 34.5\\u0026deg;S.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eSouth Atlantic and Pacific overturning cells\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eIn some models, a weak, localised South Atlantic circulation at 34.5\\u0026deg;S, isolated from the North Atlantic, upwells in the SO (Extended Data Fig. 8). This circulation, denoted \\u0026ldquo;South_Atlantic_local\\u0026rdquo; (Eq. S2, Extended Data Fig. 8), is accounted for when calculating both the Atlantic upwelling pathway (Eq. S1) and the SO upwelling pathway (Eq. S3), since it reduces the SO upwelling (i.e., the globally-integrated SO upper cell strength at 34.5\\u0026deg;S,\\u0026nbsp;\\u0026psi;\\u003csub\\u003emax\\u003c/sub\\u003e│\\u003csub\\u003eϕ=34.5\\u0026deg;S\\u003c/sub\\u003e\\u0026shy; (red circle in Fig. 1a)) available for upwelling AMOC deep waters. \\u0026ldquo;South_Atlantic_local\\u0026rdquo;, determined from Eq. S2, is reduced if the South Atlantic waters enter an anti-clockwise overturning cell in the Indo-Pacific Ocean, upwell, and rejoin the localised South Atlantic circulations northward branch via the SO. This is because these waters do not upwell in the SO, and thus do not reduce SouthernOcean_Up.\\u003c/p\\u003e\\n\\u003cp\\u003eUnder extreme forcing, most models develop a PMOC that upwells in the SO, further reducing the SO upwelling available to upwell AMOC deep waters. We therefore modify the method of Baker et al., (2023) to account for the presence of a PMOC. A latitudinally expansive PMOC at the depth of the AMOC maximum at 34.5\\u0026deg;S, z_AMOC_34S, indicates that all AMOC waters that enter and upwell in the Indo-Pacific Ocean must later upwell in the SO to rejoin the AMOC\\u0026rsquo;s northward branch, since they cannot bypass the PMOC. This scenario occurs in the future state of all models with a PMOC (Extended Data Figs. 2 and 3; Figs. S2 and S3), ensuring IndoPac_ResidualUp is zero (Fig. 2d,h). If SouthernOcean_Up, calculated from the left-hand sum in Eq. S3, exceeds the AMOC pathway into the SO (AMOC\\u003csub\\u003emin\\u003c/sub\\u003e; grey circle in Extended Data Fig. 8), we adjust SouthernOcean_Up to match AMOC\\u003csub\\u003emin\\u003c/sub\\u003e (Eq. S3). This ensures the SO upwelling pathway is not stronger than the AMOC transport into the SO, as required by conservation of volume. We therefore implicitly account for the PMOC\\u0026rsquo;s impact on the AMOC\\u0026rsquo;s SO upwelling pathway in this case, based on conservation of volume. We account for the PMOC implicitly rather than explicitly to prevent inaccuracies in the upwelling pathways that would otherwise occur in the following scenarios:\\u003c/p\\u003e\\n\\u003col class=\\\"decimal_type\\\"\\u003e\\n \\u003cli\\u003eIf the PMOC maximum is below z_AMOC_35S, then AMOC deep waters can enter the PMOC\\u0026rsquo;s northward branch before upwelling in the SO (most notable in CESM2-based models).\\u003c/li\\u003e\\n \\u003cli\\u003eIf a localised clockwise overturning cell at 34.5\\u0026deg;S is present in the upper Pacific Ocean (found in the control state of a few models; e.g., Extended Data Fig. 1f,g), then SO upper cell waters enter the northward near-surface branch of these Pacific cells, sink, and later enter the AMOC\\u0026rsquo;s northward branch via the SO.\\u0026nbsp;\\u003c/li\\u003e\\n\\u003c/ol\\u003e\\n\\u003cp\\u003eIn these scenarios, not all southward PMOC transports at 34.5\\u0026deg;S reduce SouthernOcean_Up, so explicitly accounting for the maximum PMOC strength at 34.5\\u0026deg;S would underestimate the magnitude of the AMOC\\u0026rsquo;s SO upwelling pathway.\\u003c/p\\u003e\\n\\u003cp\\u003eWe therefore explicitly account for PMOC upwelling in the SO in Eq. S3 only if it cannot be connected to the AMOC, unlike in the scenarios above. Thus, we subtract the PMOC strength at the depth of the AMOC maximum at 34.5\\u0026deg;S, z_AMOC_34S, from \\u0026psi;\\u003csub\\u003emax\\u003c/sub\\u003e│\\u003csub\\u003eϕ=34.5\\u0026deg;S\\u003c/sub\\u003e, when calculating SouthernOcean_Up in Eq. S3. Hence, if the PMOC maximum is shallower (deeper) than z_AMOC_34S, we subtract the magnitude of the southward (northward) PMOC transport below (above) z_AMOC_34S to calculate SouthernOcean_Up. This results in a small decrease in SouthernOcean_Up during the initial 30 years of the forcing experiments, relative to not explicitly accounting for these PMOC transports.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eOur approach is validated by\\u0026nbsp;IndoPac_ResidualUp tending rapidly (over several decades) towards zero (Fig. 2d,h) before we constrain it to zero by setting SouthernOcean_Up to AMOC\\u003csub\\u003emin\\u003c/sub\\u003e (Eq. S3). This change in IndoPac_ResidualUp is expected because the PMOC rapidly expands, preventing upwelling above the PMOC\\u0026rsquo;s base. Our approach is further validated by the strong correlation between the inverse future PMOC strength and the change in the AMOC\\u0026rsquo;s SO upwelling pathway (Fig 4a,c), despite our approach not explicitly accounting for the PMOC in the future state calculations (when SouthernOcean_Up is set to AMOC\\u003csub\\u003emin\\u003c/sub\\u003e). The outlying CESM2-based models in Fig. 3c,f and Fig. 4b,d (purple shading) emphasise the importance of implicitly accounting for the PMOC in the future state to prevent inaccuracies in the magnitude of SouthernOcean_Up (see scenario 1 above). We further validated our method by examining overturning streamfunctions across models and experiments for inconsistencies with their calculated upwelling pathways.\\u003c/p\\u003e\"},{\"header\":\"Declarations\",\"content\":\"\\u003cp\\u003e\\u003cstrong\\u003eSignificance tests\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eWe conduct a two-tailed Student\\u0026rsquo;s t-test to assess significance of correlations between variables. A P value below 0.05 is considered significant, indicating a 95% confidence level.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eData availability\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe pre-industrial control and 4xCO2 experiment CMIP6 data used in this study is available at https://esgf-index1.ceda.ac.uk/search/cmip6-ceda/. The NAHosMIP u03_hos experiment data\\u003csup\\u003e53\\u003c/sup\\u003e used in this study is available at https://doi.org/10.5281/zenodo.7643437.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eCode availability\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe code used to calculate the AMOC\\u0026rsquo;s upwelling pathways (Methods) will be made available in a Zenodo repository.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eReferences\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003e53. \\u0026nbsp;Jackson, L. \\u003cem\\u003eet al.\\u003c/em\\u003e NAHosMIP experimental protocol. Zenodo https://doi.org/10.5281/zenodo.7225014 (2022).\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eAcknowledgements\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThis is ClimTip contribution #[XXXX]; the ClimTip project has received funding from the European Union\\u0026apos;s Horizon Europe research and innovation programme under grant agreement No. 101137601.\\u0026nbsp;JAB, LCJ, MJB and RAW were supported by\\u0026nbsp;the Met Office Hadley Centre Climate Programme funded by BEIS.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eAuthor Contributions\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eJ.A.B conceived the study, performed the analysis, and wrote the original draft manuscript. J.A.B and A.J.W generated the figures. All authors discussed and interpreted the results and commented on the draft manuscript.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eCompeting interests\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe authors declare no competing interests.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eAdditional information\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eCorrespondence and requests for materials\\u0026nbsp;\\u003c/strong\\u003eshould be addressed to Jonathan A. Baker (jonathan.baker@metoffice.gov.uk)\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eReprints and permissions information\\u003c/strong\\u003e is available at www.nature.com/reprints.\\u003c/p\\u003e\"},{\"header\":\"References\",\"content\":\"\\u003col\\u003e\\n\\u003cli\\u003eWeijer, W., Cheng, W., Garuba, O. A., Hu, A. \\u0026amp; Nadiga, B. T. 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R. \\u003cem\\u003eet al.\\u003c/em\\u003e Pacific Anthropogenic Carbon Between 1991 and 2017. \\u003cem\\u003eGlobal Biogeochemical Cycles\\u003c/em\\u003e \\u003cstrong\\u003e33\\u003c/strong\\u003e, 597\\u0026ndash;617 (2019).\\u003c/li\\u003e\\n\\u003cli\\u003eDong, S. \\u003cem\\u003eet al.\\u003c/em\\u003e Synergy of In Situ and Satellite Ocean Observations in Determining Meridional Heat Transport in the Atlantic Ocean. \\u003cem\\u003eJournal of Geophysical Research: Oceans\\u003c/em\\u003e \\u003cstrong\\u003e126\\u003c/strong\\u003e, e2020JC017073 (2021).\\u003c/li\\u003e\\n\\u003cli\\u003eKersal\\u0026eacute;, M. \\u003cem\\u003eet al.\\u003c/em\\u003e Highly variable upper and abyssal overturning cells in the South Atlantic. \\u003cem\\u003eScience Advances\\u003c/em\\u003e \\u003cstrong\\u003e6\\u003c/strong\\u003e, (2020).\\u003c/li\\u003e\\n\\u003c/ol\\u003e\"}],\"fulltextSource\":\"\",\"fullText\":\"\",\"funders\":[],\"hasAdminPriorityOnWorkflow\":false,\"hasManuscriptDocX\":true,\"hasOptedInToPreprint\":true,\"hasPassedJournalQc\":\"\",\"hasAnyPriority\":true,\"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\":\"info@researchsquare.com\",\"identity\":\"nature-portfolio\",\"isNatureJournal\":true,\"hasQc\":false,\"allowDirectSubmit\":false,\"externalIdentity\":\"\",\"sideBox\":\"\",\"snPcode\":\"\",\"submissionUrl\":\"\",\"title\":\"Nature Portfolio\",\"twitterHandle\":\"\",\"acdcEnabled\":false,\"dfaEnabled\":false,\"editorialSystem\":\"ejp\",\"reportingPortfolio\":\"\",\"inReviewEnabled\":true,\"inReviewRevisionsEnabled\":false},\"keywords\":\"\",\"lastPublishedDoi\":\"10.21203/rs.3.rs-4319650/v1\",\"lastPublishedDoiUrl\":\"https://doi.org/10.21203/rs.3.rs-4319650/v1\",\"license\":{\"name\":\"CC BY 4.0\",\"url\":\"https://creativecommons.org/licenses/by/4.0/\"},\"manuscriptAbstract\":\"\\u003cp\\u003eThe Atlantic Meridional Overturning Circulation (AMOC), vital for northward heat transport across the Atlantic Ocean, is projected to weaken due to global warming\\u003csup\\u003e1\\u003c/sup\\u003e, with significant global climate impacts\\u003csup\\u003e2,3\\u003c/sup\\u003e. However, the extent of AMOC weakening is uncertain with wide variation across climate models\\u003csup\\u003e1,4,5\\u003c/sup\\u003e, while some statistical indicators suggest an imminent collapse\\u003csup\\u003e6\\u003c/sup\\u003e. Here, we evaluate the AMOC’s resilience to extreme greenhouse gas and North Atlantic freshwater forcings in 34 climate models by analysing its upwelling pathways that return AMOC deep waters to the surface. We find that upwelling in the Southern Ocean (SO), driven by persistent SO winds, sustains a weakened AMOC in all cases, preventing its complete collapse. Since SO upwelling must be balanced by downwelling in the Atlantic or Pacific, the AMOC can only collapse if a compensating Pacific overturning (PMOC) develops. Remarkably, a PMOC does emerge in almost all models, but it is too weak to balance all of the SO upwelling, suggesting an AMOC collapse is unlikely this century. Our findings reveal novel AMOC stabilising mechanisms with implications for past and future AMOC changes, and hence for ecosystems and ocean biogeochemistry. They suggest that better understanding and estimates of the SO and Indo-Pacific circulations are urgently needed to accurately predict future AMOC change.\\u003c/p\\u003e\",\"manuscriptTitle\":\"Wind-driven upwelling sustains weakened Atlantic overturning under climate extremes\",\"msid\":\"\",\"msnumber\":\"\",\"nonDraftVersions\":[{\"code\":1,\"date\":\"2024-04-29 12:14:25\",\"doi\":\"10.21203/rs.3.rs-4319650/v1\",\"editorialEvents\":[],\"status\":\"published\",\"journal\":{\"display\":false,\"email\":\"info@researchsquare.com\",\"identity\":\"nature\",\"isNatureJournal\":true,\"hasQc\":false,\"allowDirectSubmit\":false,\"externalIdentity\":\"nature\",\"sideBox\":\"Learn more about [Nature](http://www.nature.com/nature/)\",\"snPcode\":\"\",\"submissionUrl\":\"\",\"title\":\"Nature\",\"twitterHandle\":\"\",\"acdcEnabled\":true,\"dfaEnabled\":true,\"editorialSystem\":\"ejp\",\"reportingPortfolio\":\"Nature\",\"inReviewEnabled\":true,\"inReviewRevisionsEnabled\":false}}],\"origin\":\"\",\"ownerIdentity\":\"2ba509ff-2033-4375-a959-bda34b268b4c\",\"owner\":[],\"postedDate\":\"April 29th, 2024\",\"published\":true,\"recentEditorialEvents\":[],\"rejectedJournal\":[],\"revision\":\"\",\"amendment\":\"\",\"status\":\"published-in-journal\",\"subjectAreas\":[{\"id\":31173931,\"name\":\"Earth and environmental sciences/Ocean sciences/Physical oceanography\"},{\"id\":31173932,\"name\":\"Earth and environmental sciences/Climate sciences/Climate change/Projection and prediction\"},{\"id\":31173933,\"name\":\"Earth and environmental sciences/Climate sciences/Ocean sciences/Physical oceanography\"},{\"id\":31173934,\"name\":\"Earth and environmental sciences/Climate sciences/Climate change/Climate and Earth system modelling\"}],\"tags\":[],\"updatedAt\":\"2025-02-27T08:05:32+00:00\",\"versionOfRecord\":{\"articleIdentity\":\"rs-4319650\",\"link\":\"https://doi.org/10.1038/s41586-024-08544-0\",\"journal\":{\"identity\":\"nature\",\"isVorOnly\":false,\"title\":\"Nature\"},\"publishedOn\":\"2025-02-26 05:00:00\",\"publishedOnDateReadable\":\"February 26th, 2025\"},\"versionCreatedAt\":\"2024-04-29 12:14:25\",\"video\":\"\",\"vorDoi\":\"10.1038/s41586-024-08544-0\",\"vorDoiUrl\":\"https://doi.org/10.1038/s41586-024-08544-0\",\"workflowStages\":[]},\"version\":\"v1\",\"identity\":\"rs-4319650\",\"journalConfig\":\"researchsquare\"},\"__N_SSP\":true},\"page\":\"/article/[identity]/[[...version]]\",\"query\":{\"redirect\":\"/article/rs-4319650\",\"identity\":\"rs-4319650\",\"version\":[\"v1\"]},\"buildId\":\"WrCJVZZCHTDjtuVLN7oU0\",\"isFallback\":false,\"isExperimentalCompile\":false,\"dynamicIds\":[84888],\"gssp\":true,\"scriptLoader\":[]}","source_license":"CC-BY-4.0","license_restricted":false}