Long-term evolution of stratospheric water vapour using TIMED/ SABER measurements

Long-term evolution of stratospheric water vapour using TIMED/ SABER measurements | 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 Research Article Long-term evolution of stratospheric water vapour using TIMED/ SABER measurements Kandula V Subrahmanyam, Karnam Kishore Kumar, Dilna Damodaran, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7739072/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract One of the most prevalent greenhouse gases in the atmosphere is water vapour, which exhibits significant variability both in space and time. The water vapour warms the troposphere whereas it cools the stratosphere through radiative cooling and thus regulates the thermal structure of the stratosphere impacting the stratosphere-troposphere coupling process. The ground based measurements of stratospheric water vapour (SWV) are limited across the globe and the space based observations provide global distribution, which is very important for accessing it role in climate. The present study focuses on investigating the long-term evolution of SWV using Sounding Atmospheric Broadband Emission Radiometry (SABER) measurements onboard Thermosphere-Ionosphere‐Mesosphere Energetics and Dynamics (TIMED) satellite during January 2002 to December 2023. The results show that SWV magnitudes are extremely low in the lower stratosphere (3–5 ppmv) and relative high in the upper stratosphere (7–8 ppmv) owing to the methane oxidation in the upper stratosphere. The lower stratosphere exhibits relatively high variability as compared to upper stratosphere. The time series of water vapour shows robust increasing trend throughout the stratosphere. The analysis carried out separately for the period 2002–2012 and 2013–2023 show that the water vapour distribution peaks around 4.8 ppmv during 2002 to 2012 and around 5.0 ppmv during 2013 to 2023 at 30 km altitude. The frequency of distribution of SWV clearly shows a shift towards higher concentration. The long-term evolution of SWV shows an increasing trend in the entire stratosphere with a peak of 0.8ppmv/decade at 22 km altitude over the tropics. The analysis also has shown that the tropical latitude exhibits relatively large increasing trends as compared to global mean thus emphasizing the importance of focusing on tropical processes responsible for observed SWV enhancement and their impact on climate. stratosphere water vapour SABER TIMED distribution trends Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction One of the most important greenhouse gases in the atmosphere is water vapour, which varies significantly at spatial and temporal scales (Lambert et al., 2007). It has a profound impact on a wide range of atmospheric processes, including radiation, chemistry, microphysics, and dynamics (Kley et al., 2000). Clouds are almost non-existent, and water vapour concentrations are extremely low in the stratosphere as compared to troposphere. However, the stratospheric temperature structure is still largely regulated by local radiative cooling brought on by water vapour (Forster & Shine, 1999 & 1997; Charlesworth et al., 2023). The major processes responsible for water vapour in the stratosphere are oxidation of methane in the upper stratosphere, slow ascent from the troposphere, large-scale motion and turbulent diffusion through the tropopause and rapid ascent by convection or volcanic eruption (Danielsen, 1993; Dessler and Sherwood, 2004; Rohs et al., 2006; Brasseur et al., 1999). Water vapour plays a significant role in the heterogeneous ozone depletion observed in Polar Regions by favouring the production of polar stratospheric clouds (Drdla and Muller, 2012). The Quasi-biennial oscillation (QBO) and monsoon circulations are two of the primary dynamical atmospheric processes that affect the interannual and intra-seasonal variability of stratospheric water vapour, respectively(Diallo et al., 2018; Fueglistaler et al., 2005). The vertical transport of water vapour into the stratosphere is impacted by the QBO's alternating easterly and westerly wind regimes (Diallo et al., 2018). During the boreal summer, the Asian monsoon system plays a vital role in the variability of SWV (Ploeger et al., 2013). Moist air from the troposphere is lifted by convective activity associated with Asian summer monsoon into the upper troposphere and lower stratosphere (UTLS), where it is trapped within the monsoon anticyclone (Park et al., 2007; Gettelman et al., 2004). There is an increase in water vapour due to this trapping, which gradually rises and dehydrates when it crosses the cold point Tropopause (Randel et al., 2015). Seasonal and intra-seasonal distribution variations in stratospheric water vapour are also influenced by variations in the Brewer-Dobson circulation (Diallo et al., 2018). The interannual variability in water vapour can result from the interplay between the QBO and the El Niño–Southern Oscillation (ENSO) (Garfinkel and Hartmann, 2007; Taguchi, 2010). The QBO affects temperatures in the tropical tropopause region (TTL), through which water vapour enters the stratosphere (Reid and Gage, 1985; Zhou et al., 2001; Fujiwara et al., 2010). In particular, the stratosphere moistens during the westerly phase of QBO close to 50 hPa (wQBO) and dries out during the easterly phase of QBO close to 50 hPa (eQBO). Apart from the diurnal and seasonal cycles, the oscillation's regularity makes it the most well-known repeated mode of variability in the atmosphere (Osprey et al., 2016; Newman et al., 2016). Konopka et al. (2022) observed a moistening of the tropical stratosphere after 2000 during late boreal winter/spring, and it is partly attributed to the ENSO and volcanic eruptions. In the tropics and over the midlatitude regions, there is evidence of direct injection of ice crystals and subsequent sublimation in the lowest stratosphere by deep convection overshooting the tropopause (Lee et al., 2019; Schwartz et al., 2013; Dessler et al., 2016; Smith et al., 2022). Tropopause-overshooting convection simulated by several storm-resolving model simulations (Dauhut et al., 2018; Dauhut&Hohenegger, 2022; Hassim & Lane, 2010; Wang, 2003) show hydration of the lower stratosphere; however, it is less evident how this phenomenon affects the global stratospheric water vapour budget based on observations (Jensen et al., 2020). The convective process can have an indirect impact on the water vapour budget in the lower stratosphere by reducing the temperatures in the UTLS region (Ueyama et al., 2023; Randel et al., 2015). The study conducted by Salby and Callagha (2004) revealed that an increase or intensification of convection caused a cooling and rising of the tropical tropopause. It has been reported that variations in SWV either amplify or mitigate the consequences of global warming. Consequently, the accurate forecast of global climate change depends on an understanding of the changes and governing mechanisms of SWV (Solomon et al., 2010). Recently, Charlesworth et al. (2023) demonstrated that the presence of water vapour in the lower stratosphere causes various changes in local and regional climates. These changes include a strengthening of the stratospheric circulation, a poleward shift of tropospheric eddy-driven jet, and impacts on regional climates. Xia et al. (2021) also observed an increase in SWV, which radiatively cools the stratosphere, consequently leads to the widening of the Hadley cell in autumn and results in its poleward expansion. The changes in stratospheric water vapour are not uniform across the globe. The tropics exhibit the most substantial increases resulting in significant radiative effects, enhancing surface warming and altering stratospheric ozone concentrations (Hegglin et al., 2014). Randel et al. (2006) observed that after 2001, there was a decline in the near-global SWV (or persistently low values starting in 2001), which they attributed to the greater tropical upwelling. Hurst (2011) examined the SWV using balloon based measurements over Boulder, Colorado and investigated its multi-decadal variability. The SWV increased by an average of 1.0 ± 0.2 ppmv (27 ± 6%) between 1980 and 2010; however, between 2001 and 2005, it showed a trend that was different from other periods. Another substantial dip in the tropical SWV (10°S–10°N) was reported in 2011–2012. This decline was comparable to the one that was observed in the year 2000 (Hegglin et al., 2014). An alteration in atmospheric circulation that broadens the region of tropical rising motion across the tropopause can increase the amount of water entering the stratosphere (Zhou et al., 2001). Since stratospheric water vapour is a powerful greenhouse gas with a potential for warming in the troposphere and cooling in the stratosphere understanding their long-term evolution is essential for forecasting future climate scenarios (Forster & Shine, 2002). Therefore, the present study comprehensively analyzed these changes in SWV across the globe and quantified its trends using 23 years of SABER observations. Data and Methodology is presented in section 2 and Section 3 discusses the results. Summary and conclusions are presented in section 4 . 2. Data and Methodology Sounding of the Atmosphere using Broadband Emission Radiometry (SABER), is one of four instruments on the Thermosphere-Ionosphere‐Mesosphere Energetics and Dynamics (TIMED) satellite, which was launched on 7 December 2001. The SABER instrument was developed under a partnership between the NASA Langley Research Center, the Space Dynamics Laboratory (SDL) of Utah State University, Hampton University, and Global Atmospheric Technologies and Sciences (GATS). The TIMED satellite placed in a circular orbital of an altitude of 625 km with an inclination of 74.1°. The instrument's vertical instantaneous field of view at 60 km altitude is roughly 2.0 km, with a vertical sampling interval of approximately 0.4 km. It makes 15 orbits per day with a period of 1.6 hour and provides approximately 1400 profiles each of temperature and minor constituents (Russel et al., 1999). TIMED is a slowly precessing satellite; it takes; 60 days of twice-daily sampling to sample all 24-h local times (Zhao et al., 2021).It covers from 53°S to 83°N and switches to 83°S to 53°N every ~ 60 days, as it rotates 180° about its yaw axis (Russel et al., 1999). It has 10-channles broadband infrared limb sounding radiometer (1.27µm − 17µm), which provides the vertical measurements of H 2 O (6.8 µm), ozone (O 3 ) (9.6 µm and 1.27 µm), atomic oxygen ([O]) and hydrogen ([H]), and volume emission rates of nitric oxide (NO) (5.3 µm), hydroxyl (OH) (2.1 µm and 1.6 µm), and excited oxygen (O 2 (Δ)) (1.28 µm) with 2 km vertical resolution (Rong et al., 2019). The long‐standing positive bias caused by spectral out‐of‐band emission in the H 2 O channel due to O 3 has been corrected in version 2.07 data that are now publicly available. The data were also screened using a threshold of 12 ppmv in the 25–80 km range. Given that SABER sampling at high latitudes (52°N/S) only yields data for half the period, we concentrated on the latitude range between 50°N and 50°S in this study. The H 2 O measurements used in this analysis, which are TIMED/SABER version 2.0 (from http://saber.gats-inc.com ), span 264 continuous observation months, from January 2002 to December 2023. The profiles obtained in both the ascending and descending nodes of the TIMED orbit, corresponding to two local times every day are used. Single‐profile random errors are ~ 2–3% up to 60 km and increase rapidly to 30% at 80 km due to degraded signal‐to‐noise ratio in the radiance measurement. The total systematic error (10–20%) increases toward both the lower and upper ends of the altitude range caused by temperature and nonlocal thermal equilibrium effects, respectively (Rong et al., 2019). The observed positive bias caused by spectral out-of band emission in the H 2 channel due to O 3 has been corrected in version 2.07 (Esplin et al., 2023) and the same is utilized in the present study. Regular observations began from 22 January 2002 and are approved to further continue the regular operation through September 2026 (Mlynczak et al., 2022). Long-term evolution of SWV is carried in the present study using linear trend analysis at global scale. 3. Results and Discussion Figure 1 depicts the zonal mean water vapour (H 2 O) mixing ratio as a function of height and latitude, averaged over a span of two decades, from 2002 to 2023. The figure shows well known climatological features of SWV distribution reported earlier using other spaced based measurements thus validating relatively new retrievals of TIMED/SABER (e.g., Hegglin et al., 2014; Konopka et al., 2022). In the lower stratosphere, relatively higher water vapor concentration can be noticed within the ± 20° latitudes as compared to other latitudes in the 15–20 km altitude region. The mid-latitude lower stratosphere is very dry with water vapor mixing ratio less than 3 ppmv. As altitude increases from 20 km to about 30 km, there is a striking rise in the amount of water vapor, especially in regions closer to the Southern high latitudes compared their Northern counter parts. At approximately 35 km in altitude, a symmetric pattern in water vapour content is observed relative to the equator, indicating higher water vapour concentrations at higher latitudes (both north and south) and a relatively minimum near the equator. This pattern is a characteristic feature of stratospheric circulation, influenced by processes such as the Brewer-Dobson circulation, which transports water vapour from the tropics to higher latitudes. Furthermore, the tropical regions close to the equator create a "dry zone" due to lower water vapour amounts, especially in the lower stratosphere at 20–25 km altitudes. High water vapor concentrations are periodically blended from the upper tropical troposphere into the lowest stratosphere just above the extratropical tropopause, resulting in excessive moisture relative to stratospheric background values (Charlesworth et al., 2023). It is evident form the Fig. 1 that the observed water vapor show relatively large values in the upper stratosphere across all latitudes. Methane oxidation is an important source of water vapour, especially in the mid to upper stratosphere, where the cold trap effect limits direct transport from the troposphere (e.g., Noël et al., 2018). This complex interplay of atmospheric processes governing water vapour distribution is reflective of the intricate dynamics at play in the Earth's atmosphere (Randel et al., 2013). The direct transport of water vapor from upper troposphere to lower stratosphere, re-distribution by Brewer-Dobson circulation and methane oxidation governs the latitude-altitude distribution of water vapor shown in figure. Figure 2 (a) shows a time series of global mean water vapor from 2002 to 2024 for altitudes between 15 km and 50 km over the latitude range of 15°S to 15°N. The figure readily revels the well know ‘Tape Recorder’ phenomenon reported by several researchers in the past (e.g., Mote et al., 1995). This effect describes the upward propagation of water vapor from the tropical tropopause into the stratosphere, creating a "tape" of moisture that moves slowly upward over time. This is driven by the seasonal cycle of tropical convection, where moist air is lofted into the stratosphere during periods of intense tropical upwelling (e.g., Mote et al., 1995). The water vapor enters into the lower stratosphere during the summer months owing to the warm tropical tropopause and slowly ascends in response to Brewer-Dobson circulation triggered by the high-latitude planetary wave activity. These observations help in understanding the interplay between various atmospheric processes and their impact on global distribution of water vapor in the stratosphere. Over the past two decades, the water vapor mixing ratios displays consistent seasonal and annual cycles, characterized by regular fluctuations with peaks typically corresponding to the summer months. These cycles are especially prominent at lower altitudes (15–25 km), where the water vapour concentration ranges between 3 and 6 ppmv. The large amount of water vapour seen during the year 2022 is associated with the underwater volcano eruption of Hunga Tonga- Hunga Ha’apai. This volcanic event dumped large amount of water vapor in the lower stratosphere and there are numerous investigations on the impact of enhanced water vapor in the lower stratosphere in terms of radiation, chemistry and dynamics (e.g., Vömel et al., 2022; Xu et al., 2022; Asher et la., 2023; Fleming et al., 2024). Figure 2 (b) show the water vapor anomalies corresponding to the time series shown in Fig. 2 (a). The Hunga Tonga- Hunga Ha’apai event is excluded while estimating the anomalies. The periodic changes in water vapor anomalies show the cyclic patterns. These cycles could be linked to natural climatic oscillations such as the El Niño-Southern Oscillation (ENSO) and the Quasi-Biennial Oscillation (QBO), which affect atmospheric moisture distribution (e.g., Liang et al., 2023). From this figure it can be noted that 2004–2006 shows strong negative anomalies, indicating lower than average water vapor concentrations. In contrast, 2010–2012 and 2015–2017 display strong positive anomalies in the lower stratosphere, signifying higher than average concentrations. These anomalies might be associated with events such as volcanic eruptions (e.g., the 2010 eruption of Eyjafjallajökull) that inject water vapour and aerosols into the stratosphere. After 2016, there is a noticeable shift towards more consistent positive anomalies, suggesting a general increase in stratospheric water vapour levels. Figure 3 shows time series of SWV anomalies at three specific altitudes 25, 30 and 35 km averaged over latitude range 15°S to 15°N. From this figure, it can be noted that the most pronounced short-term variability occurs at 25 km altitude, with anomalies ranging from ~ -0.5 to 0.5 ppmv. Post-2014, there is a notable increase in positive anomalies, indicating higher water vapour concentrations likely influenced by rising global temperatures and associated increase in atmospheric convection, especially over shooting convection. A pronounced positive anomaly spike at 30 km during 2002–2003 is followed by a period of predominantly negative anomalies until about 2010. In contrast, at the 35 km altitude SWV shows the least variability among the three altitude regions, with anomalies ranging from − 0.5 ppmv to 0.3 ppmv. The reduced variability at 35 km suggests that the higher altitudes of stratosphere are less influenced by the rapid changes and more affected by slower, long-term processes such as vertical transport and photochemical reactions. This consistent rise in positive anomalies across all altitudes post-2016 suggests a general increase in stratospheric water vapour, potentially driven by long-term climatic trend like global warming, which enhances atmospheric moisture levels due to increased evaporation rates as well as convection (Reference). Top and bottom panels of Fig. 4 show the average spatial distribution of SWV at 25 km during 2002–2012 & 2012–2023 periods, respectively. There is an apparent increase in mean water vapour concentration at 30 km from 2002–2012 to 2013–2023 time period. The equatorial regions show a substantial increase in water vapour concentration, rising from about 3.5-4.0 ppmv in the first period to above 4.0 ppmv and often exceeding 5.0 ppmv in the second period. The mid-latitude regions also exhibit higher concentrations in the latter period. The variations observed could be associated with the increased convection impact on atmospheric dynamics and chemistry in the stratosphere (e.g., Patel, V.K. and Kuttippurath, 2023; Nowack et al., 2023). Increased stratospheric water vapor can impact the radiation budget, contributing to warming at the surface and in the troposphere and cooling in the stratosphere (e.g., Nowack et al., 2023). This positive feedback loop exacerbates global warming (e.g., Nowack et al., 2023). Further, the distribution of water vapour concentrations at 30 km altitude for two different 11-year periods: 2002–2012 and 2013–2023, is shown in Fig. 5 over the latitude range of 15°S to 15°N. It is observed that peak of SWV distribution occurs at ~ 4.8 ppmv in 2002–2012 (blue colour curve), whereas the peak shifts to ~ 5 ppmv in 2013–2023 (red colour). After the peak, the density of water vapour concentration decreases rapidly, showing a more limited range of values. The distribution's tail, representing the lower-density values, extends up to around 8 ppmv but with very few occurrences at those higher concentrations. In recent decade (2013–2023), the tail of the distribution extends beyond 8 ppmv, suggesting higher concentrations of water vapour are more common in this period compared to the previous one. From this figure, it is evident that there has been a clear change in the higher levels of water vapour concentration in the lower stratosphere in recent years. After investigating the climatology and distribution of SWV, the long-term trends in the stratosphere are estimated. Figure 6 shows the linear trend of water vapour mixing ratio per decade as a function of altitude (ranging from 15 km to 50 km) for two distinct datasets: global mean (50°S to 50°N) (shown in blue) and tropical mean (15°S to 15°N) (shown in red). The trends are measured in parts per million by volume per decade (ppmv/decade). Error bars are included to show the uncertainty associated with each trend. The striking feature of the figure is the positive trends in SWV throughout the stratosphere right from 15 to 50 km. The tropical trend generally shows relatively large increasing trends in water vapour mixing ratios compared to the global trend, particularly between 20 and 25 km. This indicates a rapid rise in stratospheric water vapour within the tropics during the observational period. Both profiles exhibit peaks at different altitudes. The global trend (blue line) starts at approximately 0.4(around 0.6) ppmv/decade and remains relatively constant around this value from 15 to 20 km (20 to 35 km) height. The tropical trend (red line) also starts around 0.4 ppmv/decade but shows slightly more variation within this altitude range, fluctuating between 0.3 and 0.6 ppmv/decade. From 20 to 35 km altitude region, the tropical trend exhibits a notable increase, peaking at around 0.8ppmv/decade at approximately 22 km before decreasing to about 0.6ppmv/decade by 25 km. Whereas in 35 to 50 km region, the global trend continues to increase, reaching its maximum value of approximately 0.4 ppmv/decade around 45 km. The tropical trend shows significant variation, peaking at approximately 0.6 (0.3) ppmv/decade around 40 km before decreasing to around 0.4 (0.2) ppmv/decade by 45 (50) km. The error bars, representing uncertainties, are wider in the tropical trend, indicating greater variability in the trend estimates for the tropical region compared to the global men. The higher and large variability of trends in the tropical region possibly linked to increased convective activity and changes in stratospheric-tropospheric exchange processes in response to climate change. The steady global increase reflects the overall trend of rising stratospheric water vapour, likely driven by global warming and increased water vapour transport from the tropics. At 45 km, the convergence indicates that photochemical reactions and stratospheric dynamics are consistent globally and in the tropics, reflecting similar rates of water vapour production and distribution due to photodissociation and circulation patterns. Summary and conclusion Stratospheric water vapor plays a crucial role in Earth's climate system and has significant implications for climate change. Given its importance, several studies made significant effort in understanding and quantifying the changes in SWV seasonality and intensity. In the present study, long-term evolution of SWV is investigated using observations of TIMED/SABER during 2002 to 2023 time period. The results showed that the equatorial regions consistently show high water vapor levels due to strong convection, while mid-latitudes have relatively lower concentrations. Seasonal variations show peaks in water vapour during summer months. A significant spike in the water vapour is observed during 2022 following the Hunga Tonga eruption. The time series of water vapour shows cyclical anomalies, potentially linked to oscillations like ENSO and QBO, indicating the complex interactions in the stratosphere. The decadal distribution of SWV is observed to be peaking about 4.8 ppmv during 2002 to 2012, rising to approximately 5.0 ppmv during 2013 to 2023. This indicates an increase in water vapor concentration at an altitude of 30 km, with the data suggesting that higher concentrations have become more frequent. The analysis of water vapour trends revealed a robust increase in both global and tropical mean. Global trends show a steady rise, peaking at around 0.5 ppmv/decade at higher altitudes, while tropical trends show larger variability, peaking at 0.8 ppmv/decade at 22 km. In contrast, in the region from 35 to 50 km, the global trend continues to rise, reaching its maximum value of approximately 0.4 ppmv per decade around 45 km. Thus, the present results reveal long-term changes in SWV and its trends at global and tropical scales, which have profound implications for Earth's climate. Declarations Acknowledgement: The work was carried out under NICES programme of ISRO. Authors sincerely thank NRSC, ISRO for providing the required support. Authors acknowledge the TIMED/SABER team for making the data publically. Author Contributions: Kandula V Subrahmanyam and Karanam Kishore Kumar contributed the study conception and carried out the analyses. Dilna Damodaran contributed to the analysis. The first draft of the manuscript was written by Kandula V Subrahmanyam. 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Large amounts of water vapor were injected into the stratosphere by the Hunga Tonga–Hunga Ha’apai volcano eruption. Atmosphere , 13 (6), 912. Yue, J., Russell III, J., Gan, Q., Wang, T., Rong, P., Garcia, R. and Mlynczak, M. (2019). Increasing water vapor in the stratosphere and mesosphere after 2002. Geophysical Research Letters , 46 (22), 13452-13460. Zhao, X.R., Sheng, Z., Shi, H.Q., Weng, L.B. and He, Y. (2021). Middle atmosphere temperature changes derived from SABER observations during 2002–20. Journal of Climate , 34 (19), 7995-8012. Zhou, X.L., Geller, M.A. and Zhang, M.H. (2001). Tropical cold point tropopause characteristics derived from ECMWF reanalyses and soundings. Journal of climate , 14 (8), 1823-1838. Additional Declarations No competing interests reported. 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19:40:53","extension":"html","order_by":16,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":71638,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7739072/v1/46011297dc425001bde59cb4.html"},{"id":95242371,"identity":"240c338c-e31f-4284-9c1c-39b3c05d9149","added_by":"auto","created_at":"2025-11-05 19:40:53","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":119582,"visible":true,"origin":"","legend":"\u003cp\u003eZonal mean water vapour mixing ratio as a function of latitude and height from 2002 to 2023 derived from TIMED/SABER measurements\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-7739072/v1/6d3c4c6a13429eef063c8ecc.png"},{"id":95242372,"identity":"0d4d1eae-8c45-4274-8cc9-6988aa5e2fed","added_by":"auto","created_at":"2025-11-05 19:40:53","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":829158,"visible":true,"origin":"","legend":"\u003cp\u003eTime series of global (a) average water vapour content and (b) anomalies from 2002 to 2024 between 15 km and 50 km over the latitude range 15°S to 15°N.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7739072/v1/585f0eb5c0c6f5619c8981fe.jpeg"},{"id":95312983,"identity":"fa42fe28-5d2f-4631-bc69-341b48bb6564","added_by":"auto","created_at":"2025-11-06 15:50:41","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":39762,"visible":true,"origin":"","legend":"\u003cp\u003eTime series of SWV anomalies at 25, 30 and 35 km altitudes averaged over the latitude range 15°S to 15°N.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-7739072/v1/cff967cf05ac3816d3fd2f42.png"},{"id":95312530,"identity":"582369b6-966b-4aba-ad09-c5469982c158","added_by":"auto","created_at":"2025-11-06 15:49:40","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":101149,"visible":true,"origin":"","legend":"\u003cp\u003eMean spatial distribution of SWV at 30 km during (a) 2002-2012 \u0026amp; (b) 2012-2023 time period\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-7739072/v1/f0497acc83a8db26e4c47fe0.png"},{"id":95312450,"identity":"58c04622-1911-4ce1-a2cb-b5105c18f88e","added_by":"auto","created_at":"2025-11-06 15:49:25","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":32394,"visible":true,"origin":"","legend":"\u003cp\u003eDensity distribution of water vapour concentration at 30 km over the tropics.\u003c/p\u003e","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-7739072/v1/d3b581a25209f722c64396dd.png"},{"id":95242379,"identity":"42c44971-6fa9-4716-b105-43fb2af6c9eb","added_by":"auto","created_at":"2025-11-05 19:40:53","extension":"jpeg","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":337229,"visible":true,"origin":"","legend":"\u003cp\u003eLinear trend of water vapour mixing ratio per decade as a function of altitude for global mean(50°S to 50°N) \u0026nbsp;(blue) and tropical mean (15°S to 15°N) (red).\u003c/p\u003e","description":"","filename":"floatimage6.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-7739072/v1/6c138d7719ebb05ee669eb27.jpeg"},{"id":96597815,"identity":"aa8110bc-335d-4493-bc80-db4370eebf28","added_by":"auto","created_at":"2025-11-24 07:54:16","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1922211,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7739072/v1/a8f8aade-d6ea-493d-af0d-f34cae2fe158.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Long-term evolution of stratospheric water vapour using TIMED/ SABER measurements","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eOne of the most important greenhouse gases in the atmosphere is water vapour, which varies significantly at spatial and temporal scales (Lambert et al., 2007). It has a profound impact on a wide range of atmospheric processes, including radiation, chemistry, microphysics, and dynamics (Kley et al., 2000). Clouds are almost non-existent, and water vapour concentrations are extremely low in the stratosphere as compared to troposphere. However, the stratospheric temperature structure is still largely regulated by local radiative cooling brought on by water vapour (Forster \u0026amp; Shine, 1999 \u0026amp; 1997; Charlesworth et al., 2023). The major processes responsible for water vapour in the stratosphere are oxidation of methane in the upper stratosphere, slow ascent from the troposphere, large-scale motion and turbulent diffusion through the tropopause and rapid ascent by convection or volcanic eruption (Danielsen, 1993; Dessler and Sherwood, 2004; Rohs et al., 2006; Brasseur et al., 1999). Water vapour plays a significant role in the heterogeneous ozone depletion observed in Polar Regions by favouring the production of polar stratospheric clouds (Drdla and Muller, 2012). The Quasi-biennial oscillation (QBO) and monsoon circulations are two of the primary dynamical atmospheric processes that affect the interannual and intra-seasonal variability of stratospheric water vapour, respectively(Diallo et al., 2018; Fueglistaler et al., 2005). The vertical transport of water vapour into the stratosphere is impacted by the QBO's alternating easterly and westerly wind regimes (Diallo et al., 2018). During the boreal summer, the Asian monsoon system plays a vital role in the variability of SWV (Ploeger et al., 2013). Moist air from the troposphere is lifted by convective activity associated with Asian summer monsoon into the upper troposphere and lower stratosphere (UTLS), where it is trapped within the monsoon anticyclone (Park et al., 2007; Gettelman et al., 2004). There is an increase in water vapour due to this trapping, which gradually rises and dehydrates when it crosses the cold point Tropopause (Randel et al., 2015). Seasonal and intra-seasonal distribution variations in stratospheric water vapour are also influenced by variations in the Brewer-Dobson circulation (Diallo et al., 2018). The interannual variability in water vapour can result from the interplay between the QBO and the El Ni\u0026ntilde;o\u0026ndash;Southern Oscillation (ENSO) (Garfinkel and Hartmann, 2007; Taguchi, 2010). The QBO affects temperatures in the tropical tropopause region (TTL), through which water vapour enters the stratosphere (Reid and Gage, 1985; Zhou et al., 2001; Fujiwara et al., 2010). In particular, the stratosphere moistens during the westerly phase of QBO close to 50 hPa (wQBO) and dries out during the easterly phase of QBO close to 50 hPa (eQBO). Apart from the diurnal and seasonal cycles, the oscillation's regularity makes it the most well-known repeated mode of variability in the atmosphere (Osprey et al., 2016; Newman et al., 2016). Konopka et al. (2022) observed a moistening of the tropical stratosphere after 2000 during late boreal winter/spring, and it is partly attributed to the ENSO and volcanic eruptions.\u003c/p\u003e\u003cp\u003eIn the tropics and over the midlatitude regions, there is evidence of direct injection of ice crystals and subsequent sublimation in the lowest stratosphere by deep convection overshooting the tropopause (Lee et al., 2019; Schwartz et al., 2013; Dessler et al., 2016; Smith et al., 2022). Tropopause-overshooting convection simulated by several storm-resolving model simulations (Dauhut et al., 2018; Dauhut\u0026amp;Hohenegger, 2022; Hassim \u0026amp; Lane, 2010; Wang, 2003) show hydration of the lower stratosphere; however, it is less evident how this phenomenon affects the global stratospheric water vapour budget based on observations (Jensen et al., 2020). The convective process can have an indirect impact on the water vapour budget in the lower stratosphere by reducing the temperatures in the UTLS region (Ueyama et al., 2023; Randel et al., 2015). The study conducted by Salby and Callagha (2004) revealed that an increase or intensification of convection caused a cooling and rising of the tropical tropopause. It has been reported that variations in SWV either amplify or mitigate the consequences of global warming. Consequently, the accurate forecast of global climate change depends on an understanding of the changes and governing mechanisms of SWV (Solomon et al., 2010). Recently, Charlesworth et al. (2023) demonstrated that the presence of water vapour in the lower stratosphere causes various changes in local and regional climates. These changes include a strengthening of the stratospheric circulation, a poleward shift of tropospheric eddy-driven jet, and impacts on regional climates. Xia et al. (2021) also observed an increase in SWV, which radiatively cools the stratosphere, consequently leads to the widening of the Hadley cell in autumn and results in its poleward expansion.\u003c/p\u003e\u003cp\u003eThe changes in stratospheric water vapour are not uniform across the globe. The tropics exhibit the most substantial increases resulting in significant radiative effects, enhancing surface warming and altering stratospheric ozone concentrations (Hegglin et al., 2014). Randel et al. (2006) observed that after 2001, there was a decline in the near-global SWV (or persistently low values starting in 2001), which they attributed to the greater tropical upwelling. Hurst (2011) examined the SWV using balloon based measurements over Boulder, Colorado and investigated its multi-decadal variability. The SWV increased by an average of 1.0\u0026thinsp;\u0026plusmn;\u0026thinsp;0.2 ppmv (27\u0026thinsp;\u0026plusmn;\u0026thinsp;6%) between 1980 and 2010; however, between 2001 and 2005, it showed a trend that was different from other periods. Another substantial dip in the tropical SWV (10\u0026deg;S\u0026ndash;10\u0026deg;N) was reported in 2011\u0026ndash;2012. This decline was comparable to the one that was observed in the year 2000 (Hegglin et al., 2014). An alteration in atmospheric circulation that broadens the region of tropical rising motion across the tropopause can increase the amount of water entering the stratosphere (Zhou et al., 2001). Since stratospheric water vapour is a powerful greenhouse gas with a potential for warming in the troposphere and cooling in the stratosphere understanding their long-term evolution is essential for forecasting future climate scenarios (Forster \u0026amp; Shine, 2002). Therefore, the present study comprehensively analyzed these changes in SWV across the globe and quantified its trends using 23 years of SABER observations. Data and Methodology is presented in section \u003cspan refid=\"Sec2\" class=\"InternalRef\"\u003e2\u003c/span\u003e and Section \u003cspan refid=\"Sec3\" class=\"InternalRef\"\u003e3\u003c/span\u003e discusses the results. Summary and conclusions are presented in section \u003cspan refid=\"Sec4\" class=\"InternalRef\"\u003e4\u003c/span\u003e.\u003c/p\u003e"},{"header":"2. Data and Methodology","content":"\u003cp\u003eSounding of the Atmosphere using Broadband Emission Radiometry (SABER), is one of four instruments on the Thermosphere-Ionosphere‐Mesosphere Energetics and Dynamics (TIMED) satellite, which was launched on 7 December 2001. The SABER instrument was developed under a partnership between the NASA Langley Research Center, the Space Dynamics Laboratory (SDL) of Utah State University, Hampton University, and Global Atmospheric Technologies and Sciences (GATS). The TIMED satellite placed in a circular orbital of an altitude of 625 km with an inclination of 74.1\u0026deg;. The instrument's vertical instantaneous field of view at 60 km altitude is roughly 2.0 km, with a vertical sampling interval of approximately 0.4 km. It makes 15 orbits per day with a period of 1.6 hour and provides approximately 1400 profiles each of temperature and minor constituents (Russel et al., 1999). TIMED is a slowly precessing satellite; it takes; 60 days of twice-daily sampling to sample all 24-h local times (Zhao et al., 2021).It covers from 53\u0026deg;S to 83\u0026deg;N and switches to 83\u0026deg;S to 53\u0026deg;N every\u0026thinsp;~\u0026thinsp;60 days, as it rotates 180\u0026deg; about its yaw axis (Russel et al., 1999). It has 10-channles broadband infrared limb sounding radiometer (1.27\u0026micro;m \u0026minus;\u0026thinsp;17\u0026micro;m), which provides the vertical measurements of H\u003csub\u003e2\u003c/sub\u003eO (6.8 \u0026micro;m), ozone (O\u003csub\u003e3\u003c/sub\u003e) (9.6 \u0026micro;m and 1.27 \u0026micro;m), atomic oxygen ([O]) and hydrogen ([H]), and volume emission rates of nitric oxide (NO) (5.3 \u0026micro;m), hydroxyl (OH) (2.1 \u0026micro;m and 1.6 \u0026micro;m), and excited oxygen (O\u003csub\u003e2\u003c/sub\u003e(Δ)) (1.28 \u0026micro;m) with 2 km vertical resolution (Rong et al., 2019). The long‐standing positive bias caused by spectral out‐of‐band emission in the H\u003csub\u003e2\u003c/sub\u003eO channel due to O\u003csub\u003e3\u003c/sub\u003e has been corrected in version 2.07 data that are now publicly available. The data were also screened using a threshold of 12 ppmv in the 25\u0026ndash;80 km range. Given that SABER sampling at high latitudes (52\u0026deg;N/S) only yields data for half the period, we concentrated on the latitude range between 50\u0026deg;N and 50\u0026deg;S in this study. The H\u003csub\u003e2\u003c/sub\u003eO measurements used in this analysis, which are TIMED/SABER version 2.0 (from \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://saber.gats-inc.com\u003c/span\u003e\u003cspan address=\"http://saber.gats-inc.com\" targettype=\"URL\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e), span 264 continuous observation months, from January 2002 to December 2023. The profiles obtained in both the ascending and descending nodes of the TIMED orbit, corresponding to two local times every day are used. Single‐profile random errors are ~\u0026thinsp;2\u0026ndash;3% up to 60 km and increase rapidly to 30% at 80 km due to degraded signal‐to‐noise ratio in the radiance measurement. The total systematic error (10\u0026ndash;20%) increases toward both the lower and upper ends of the altitude range caused by temperature and nonlocal thermal equilibrium effects, respectively (Rong et al., 2019). The observed positive bias caused by spectral out-of band emission in the H\u003csub\u003e2\u003c/sub\u003e channel due to O\u003csub\u003e3\u003c/sub\u003e has been corrected in version 2.07 (Esplin et al., 2023) and the same is utilized in the present study. Regular observations began from 22 January 2002 and are approved to further continue the regular operation through September 2026 (Mlynczak et al., 2022). Long-term evolution of SWV is carried in the present study using linear trend analysis at global scale.\u003c/p\u003e"},{"header":"3. Results and Discussion","content":"\u003cp\u003eFigure \u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e depicts the zonal mean water vapour (H\u003csub\u003e2\u003c/sub\u003eO) mixing ratio as a function of height and latitude, averaged over a span of two decades, from 2002 to 2023. The figure shows well known climatological features of SWV distribution reported earlier using other spaced based measurements thus validating relatively new retrievals of TIMED/SABER (e.g., Hegglin et al., 2014; Konopka et al., 2022). In the lower stratosphere, relatively higher water vapor concentration can be noticed within the \u0026plusmn;\u0026thinsp;20\u0026deg; latitudes as compared to other latitudes in the 15\u0026ndash;20 km altitude region. The mid-latitude lower stratosphere is very dry with water vapor mixing ratio less than 3 ppmv. As altitude increases from 20 km to about 30 km, there is a striking rise in the amount of water vapor, especially in regions closer to the Southern high latitudes compared their Northern counter parts. At approximately 35 km in altitude, a symmetric pattern in water vapour content is observed relative to the equator, indicating higher water vapour concentrations at higher latitudes (both north and south) and a relatively minimum near the equator. This pattern is a characteristic feature of stratospheric circulation, influenced by processes such as the Brewer-Dobson circulation, which transports water vapour from the tropics to higher latitudes. Furthermore, the tropical regions close to the equator create a \"dry zone\" due to lower water vapour amounts, especially in the lower stratosphere at 20\u0026ndash;25 km altitudes. High water vapor concentrations are periodically blended from the upper tropical troposphere into the lowest stratosphere just above the extratropical tropopause, resulting in excessive moisture relative to stratospheric background values (Charlesworth et al., 2023).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eIt is evident form the Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e that the observed water vapor show relatively large values in the upper stratosphere across all latitudes. Methane oxidation is an important source of water vapour, especially in the mid to upper stratosphere, where the cold trap effect limits direct transport from the troposphere (e.g., No\u0026euml;l et al., 2018). This complex interplay of atmospheric processes governing water vapour distribution is reflective of the intricate dynamics at play in the Earth's atmosphere (Randel et al., 2013). The direct transport of water vapor from upper troposphere to lower stratosphere, re-distribution by Brewer-Dobson circulation and methane oxidation governs the latitude-altitude distribution of water vapor shown in figure.\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a) shows a time series of global mean water vapor from 2002 to 2024 for altitudes between 15 km and 50 km over the latitude range of 15\u0026deg;S to 15\u0026deg;N. The figure readily revels the well know \u0026lsquo;Tape Recorder\u0026rsquo; phenomenon reported by several researchers in the past (e.g., Mote et al., 1995). This effect describes the upward propagation of water vapor from the tropical tropopause into the stratosphere, creating a \"tape\" of moisture that moves slowly upward over time. This is driven by the seasonal cycle of tropical convection, where moist air is lofted into the stratosphere during periods of intense tropical upwelling (e.g., Mote et al., 1995). The water vapor enters into the lower stratosphere during the summer months owing to the warm tropical tropopause and slowly ascends in response to Brewer-Dobson circulation triggered by the high-latitude planetary wave activity. These observations help in understanding the interplay between various atmospheric processes and their impact on global distribution of water vapor in the stratosphere. Over the past two decades, the water vapor mixing ratios displays consistent seasonal and annual cycles, characterized by regular fluctuations with peaks typically corresponding to the summer months. These cycles are especially prominent at lower altitudes (15\u0026ndash;25 km), where the water vapour concentration ranges between 3 and 6 ppmv. The large amount of water vapour seen during the year 2022 is associated with the underwater volcano eruption of Hunga Tonga- Hunga Ha\u0026rsquo;apai. This volcanic event dumped large amount of water vapor in the lower stratosphere and there are numerous investigations on the impact of enhanced water vapor in the lower stratosphere in terms of radiation, chemistry and dynamics (e.g., V\u0026ouml;mel et al., 2022; Xu et al., 2022; Asher et la., 2023; Fleming et al., 2024).\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(b) show the water vapor anomalies corresponding to the time series shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e(a). The Hunga Tonga- Hunga Ha\u0026rsquo;apai event is excluded while estimating the anomalies. The periodic changes in water vapor anomalies show the cyclic patterns. These cycles could be linked to natural climatic oscillations such as the El Ni\u0026ntilde;o-Southern Oscillation (ENSO) and the Quasi-Biennial Oscillation (QBO), which affect atmospheric moisture distribution (e.g., Liang et al., 2023). From this figure it can be noted that 2004\u0026ndash;2006 shows strong negative anomalies, indicating lower than average water vapor concentrations. In contrast, 2010\u0026ndash;2012 and 2015\u0026ndash;2017 display strong positive anomalies in the lower stratosphere, signifying higher than average concentrations. These anomalies might be associated with events such as volcanic eruptions (e.g., the 2010 eruption of Eyjafjallaj\u0026ouml;kull) that inject water vapour and aerosols into the stratosphere. After 2016, there is a noticeable shift towards more consistent positive anomalies, suggesting a general increase in stratospheric water vapour levels.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFigure \u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e shows time series of SWV anomalies at three specific altitudes 25, 30 and 35 km averaged over latitude range 15\u0026deg;S to 15\u0026deg;N. From this figure, it can be noted that the most pronounced short-term variability occurs at 25 km altitude, with anomalies ranging from ~ -0.5 to 0.5 ppmv. Post-2014, there is a notable increase in positive anomalies, indicating higher water vapour concentrations likely influenced by rising global temperatures and associated increase in atmospheric convection, especially over shooting convection. A pronounced positive anomaly spike at 30 km during 2002\u0026ndash;2003 is followed by a period of predominantly negative anomalies until about 2010. In contrast, at the 35 km altitude SWV shows the least variability among the three altitude regions, with anomalies ranging from \u0026minus;\u0026thinsp;0.5 ppmv to 0.3 ppmv. The reduced variability at 35 km suggests that the higher altitudes of stratosphere are less influenced by the rapid changes and more affected by slower, long-term processes such as vertical transport and photochemical reactions. This consistent rise in positive anomalies across all altitudes post-2016 suggests a general increase in stratospheric water vapour, potentially driven by long-term climatic trend like global warming, which enhances atmospheric moisture levels due to increased evaporation rates as well as convection (Reference). Top and bottom panels of Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e show the average spatial distribution of SWV at 25 km during 2002\u0026ndash;2012 \u0026amp; 2012\u0026ndash;2023 periods, respectively. There is an apparent increase in mean water vapour concentration at 30 km from 2002\u0026ndash;2012 to 2013\u0026ndash;2023 time period. The equatorial regions show a substantial increase in water vapour concentration, rising from about 3.5-4.0 ppmv in the first period to above 4.0 ppmv and often exceeding 5.0 ppmv in the second period. The mid-latitude regions also exhibit higher concentrations in the latter period. The variations observed could be associated with the increased convection impact on atmospheric dynamics and chemistry in the stratosphere (e.g., Patel, V.K. and Kuttippurath, 2023; Nowack et al., 2023). Increased stratospheric water vapor can impact the radiation budget, contributing to warming at the surface and in the troposphere and cooling in the stratosphere (e.g., Nowack et al., 2023). This positive feedback loop exacerbates global warming (e.g., Nowack et al., 2023).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eFurther, the distribution of water vapour concentrations at 30 km altitude for two different 11-year periods: 2002\u0026ndash;2012 and 2013\u0026ndash;2023, is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e over the latitude range of 15\u0026deg;S to 15\u0026deg;N. It is observed that peak of SWV distribution occurs at ~\u0026thinsp;4.8 ppmv in 2002\u0026ndash;2012 (blue colour curve), whereas the peak shifts to ~\u0026thinsp;5 ppmv in 2013\u0026ndash;2023 (red colour). After the peak, the density of water vapour concentration decreases rapidly, showing a more limited range of values. The distribution's tail, representing the lower-density values, extends up to around 8 ppmv but with very few occurrences at those higher concentrations. In recent decade (2013\u0026ndash;2023), the tail of the distribution extends beyond 8 ppmv, suggesting higher concentrations of water vapour are more common in this period compared to the previous one. From this figure, it is evident that there has been a clear change in the higher levels of water vapour concentration in the lower stratosphere in recent years.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAfter investigating the climatology and distribution of SWV, the long-term trends in the stratosphere are estimated. Figure\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e shows the linear trend of water vapour mixing ratio per decade as a function of altitude (ranging from 15 km to 50 km) for two distinct datasets: global mean (50\u0026deg;S to 50\u0026deg;N) (shown in blue) and tropical mean (15\u0026deg;S to 15\u0026deg;N) (shown in red). The trends are measured in parts per million by volume per decade (ppmv/decade). Error bars are included to show the uncertainty associated with each trend. The striking feature of the figure is the positive trends in SWV throughout the stratosphere right from 15 to 50 km. The tropical trend generally shows relatively large increasing trends in water vapour mixing ratios compared to the global trend, particularly between 20 and 25 km. This indicates a rapid rise in stratospheric water vapour within the tropics during the observational period. Both profiles exhibit peaks at different altitudes. The global trend (blue line) starts at approximately 0.4(around 0.6) ppmv/decade and remains relatively constant around this value from 15 to 20 km (20 to 35 km) height. The tropical trend (red line) also starts around 0.4 ppmv/decade but shows slightly more variation within this altitude range, fluctuating between 0.3 and 0.6 ppmv/decade. From 20 to 35 km altitude region, the tropical trend exhibits a notable increase, peaking at around 0.8ppmv/decade at approximately 22 km before decreasing to about 0.6ppmv/decade by 25 km. Whereas in 35 to 50 km region, the global trend continues to increase, reaching its maximum value of approximately 0.4 ppmv/decade around 45 km. The tropical trend shows significant variation, peaking at approximately 0.6 (0.3) ppmv/decade around 40 km before decreasing to around 0.4 (0.2) ppmv/decade by 45 (50) km. The error bars, representing uncertainties, are wider in the tropical trend, indicating greater variability in the trend estimates for the tropical region compared to the global men. The higher and large variability of trends in the tropical region possibly linked to increased convective activity and changes in stratospheric-tropospheric exchange processes in response to climate change. The steady global increase reflects the overall trend of rising stratospheric water vapour, likely driven by global warming and increased water vapour transport from the tropics. At 45 km, the convergence indicates that photochemical reactions and stratospheric dynamics are consistent globally and in the tropics, reflecting similar rates of water vapour production and distribution due to photodissociation and circulation patterns.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e"},{"header":"Summary and conclusion","content":"\u003cp\u003eStratospheric water vapor plays a crucial role in Earth's climate system and has significant implications for climate change. Given its importance, several studies made significant effort in understanding and quantifying the changes in SWV seasonality and intensity. In the present study, long-term evolution of SWV is investigated using observations of TIMED/SABER during 2002 to 2023 time period. The results showed that the equatorial regions consistently show high water vapor levels due to strong convection, while mid-latitudes have relatively lower concentrations. Seasonal variations show peaks in water vapour during summer months. A significant spike in the water vapour is observed during 2022 following the Hunga Tonga eruption. The time series of water vapour shows cyclical anomalies, potentially linked to oscillations like ENSO and QBO, indicating the complex interactions in the stratosphere. The decadal distribution of SWV is observed to be peaking about 4.8 ppmv during 2002 to 2012, rising to approximately 5.0 ppmv during 2013 to 2023. This indicates an increase in water vapor concentration at an altitude of 30 km, with the data suggesting that higher concentrations have become more frequent. The analysis of water vapour trends revealed a robust increase in both global and tropical mean. Global trends show a steady rise, peaking at around 0.5 ppmv/decade at higher altitudes, while tropical trends show larger variability, peaking at 0.8 ppmv/decade at 22 km. In contrast, in the region from 35 to 50 km, the global trend continues to rise, reaching its maximum value of approximately 0.4 ppmv per decade around 45 km. Thus, the present results reveal long-term changes in SWV and its trends at global and tropical scales, which have profound implications for Earth's climate.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgement:\u0026nbsp;\u003c/strong\u003eThe work was carried out under NICES programme of ISRO. Authors sincerely thank NRSC, ISRO for providing the required support. Authors acknowledge the TIMED/SABER team for making the data publically.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions:\u003c/strong\u003e Kandula V Subrahmanyam and Karanam Kishore Kumar contributed the study conception and carried out the analyses. Dilna Damodaran contributed to the analysis. The first draft of the manuscript was written by Kandula V Subrahmanyam. All authors contributed to the improvement of the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding\u003c/strong\u003e: This research received no external funding\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData Availability:\u0026nbsp;\u003c/strong\u003e The TIMED/SABER version 2.0 data is downloaded from http://saber.gats-inc.com.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflicts of Interest\u003c/strong\u003e: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAsher, E., Todt, M., Rosenlof, K., Thornberry, T., Gao, R.S., Taha, G., Walter, P., Alvarez, S., Flynn, J., Davis, S.M. and Evan, S. (2023). Unexpectedly rapid aerosol formation in the Hunga Tonga plume. \u003cem\u003eProceedings of the National Academy of Sciences\u003c/em\u003e, \u003cem\u003e120\u003c/em\u003e(46), p.e2219547120.\u003c/li\u003e\n\u003cli\u003eBrasseur, G.P., Orlando, J.J. and Tyndall, G.S. eds. (1999). \u003cem\u003eAtmospheric chemistry and global change\u003c/em\u003e (Vol. 654). New York: Oxford university press.\u003c/li\u003e\n\u003cli\u003eBrewer, A.W. (1949). Evidence for a world circulation provided by the measurements of helium and water vapour distribution in the stratosphere. \u003cem\u003eQuarterly Journal of the Royal Meteorological Society\u003c/em\u003e, \u003cem\u003e75\u003c/em\u003e(326), 351-363. \u003c/li\u003e\n\u003cli\u003eCharlesworth, E., Pl\u0026ouml;ger, F., Birner, T., Baikhadzhaev, R., Abalos, M., Abraham, N.L., Akiyoshi, H., Bekki, S., Dennison, F., J\u0026ouml;ckel, P. and Keeble, J. (2023). 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Tropical cold point tropopause characteristics derived from ECMWF reanalyses and soundings. \u003cem\u003eJournal of climate\u003c/em\u003e, \u003cem\u003e14\u003c/em\u003e(8), 1823-1838. \u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"stratosphere, water vapour, SABER, TIMED, distribution, trends","lastPublishedDoi":"10.21203/rs.3.rs-7739072/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7739072/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eOne of the most prevalent greenhouse gases in the atmosphere is water vapour, which exhibits significant variability both in space and time. The water vapour warms the troposphere whereas it cools the stratosphere through radiative cooling and thus regulates the thermal structure of the stratosphere impacting the stratosphere-troposphere coupling process. The ground based measurements of stratospheric water vapour (SWV) are limited across the globe and the space based observations provide global distribution, which is very important for accessing it role in climate. The present study focuses on investigating the long-term evolution of SWV using Sounding Atmospheric Broadband Emission Radiometry (SABER) measurements onboard Thermosphere-Ionosphere‐Mesosphere Energetics and Dynamics (TIMED) satellite during January 2002 to December 2023. The results show that SWV magnitudes are extremely low in the lower stratosphere (3\u0026ndash;5 ppmv) and relative high in the upper stratosphere (7\u0026ndash;8 ppmv) owing to the methane oxidation in the upper stratosphere. The lower stratosphere exhibits relatively high variability as compared to upper stratosphere. The time series of water vapour shows robust increasing trend throughout the stratosphere. The analysis carried out separately for the period 2002\u0026ndash;2012 and 2013\u0026ndash;2023 show that the water vapour distribution peaks around 4.8 ppmv during 2002 to 2012 and around 5.0 ppmv during 2013 to 2023 at 30 km altitude. The frequency of distribution of SWV clearly shows a shift towards higher concentration. The long-term evolution of SWV shows an increasing trend in the entire stratosphere with a peak of 0.8ppmv/decade at 22 km altitude over the tropics. The analysis also has shown that the tropical latitude exhibits relatively large increasing trends as compared to global mean thus emphasizing the importance of focusing on tropical processes responsible for observed SWV enhancement and their impact on climate.\u003c/p\u003e","manuscriptTitle":"Long-term evolution of stratospheric water vapour using TIMED/ SABER measurements","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-05 19:40:48","doi":"10.21203/rs.3.rs-7739072/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"a780d2cd-aa49-4591-9c64-4665d64c5f84","owner":[],"postedDate":"November 5th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-11-24T07:54:04+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-05 19:40:48","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7739072","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7739072","identity":"rs-7739072","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}