Variability Of Stratospheric Water Vapor Research Articles

Responses of Lower Stratospheric water vapor to Regional Sea Surface Temperature Changes

Abstract The variability of stratospheric water vapor (SWV) plays a crucial role in stratospheric chemistry and the Earth's energy budget, strongly influenced by sea surface temperature (SST). In this study, we systematically investigate the response of low stratospheric water vapor (LSWV) to regional sea surface temperature changes using idealized SST patch experiments within a climate model. The results indicate that LSWV is most sensitive to tropical sea surface temperature, with the strongest response occurring in late autumn and early winter. Warming of the tropical Indian Ocean and western Pacific leads to stratospheric drying, while warming of the tropical Atlantic and eastern Pacific results in stratospheric moistening. The drying impact on LSWV due to warming in the western Pacific Ocean exceeds the wet effect in the Eastern Pacific Ocean by approximately 60%. The variations in tropical SST influence LSWV by modulating the temperature at the tropical tropopause layer especially over the Indo-Pacific Warm Pool through Matsuno-Gill responses. Furthermore, the response of LSWV to tropical SST changes exhibits nonnegligible nonlinearity, which indicates the importance of nonlinearity in determining the LSWV response to global surface warming.

Near‐Global Variability of Stratospheric Water Vapor Observed by SAGE III/ISS

AbstractThe Stratospheric Aerosol and Gas Experiment III instrument on the International Space Station (SAGE III/ISS) has been making high quality solar occultation measurements of stratospheric water vapor since June 2017. Here we evaluate the large‐scale geophysical variability of the SAGE III/ISS water vapor measurements for the first 3 years of observations (2017–2020) as part of data validation for retrieval version 5.1 (v5.1). Detailed comparisons of SAGE III/ISS v5.1 with the Aura Microwave Limb Sounder (MLS) version 5 retrievals show overall excellent agreement in terms of seasonal mean structure and large‐scale variability. SAGE III/ISS data capture the well‐known seasonal variations in water vapor including the vertically propagating “tape recorder” in the tropics and lower stratospheric maxima linked to the NH summer monsoons. The high vertical resolution (∼2 km) measurements from SAGE III/ISS demonstrate contributions of the monsoons to the wet phase of “tape recorder” during the Northern Hemisphere (NH) summer. Interannual variations over the short data record are also consistent between SAGE III/ISS and MLS in the stratosphere between 16 and 30 km. We furthermore evaluate large‐scale variations in relative humidity (RH) derived from the high vertical resolution SAGE III/ISS water vapor measurements, highlighting the detailed seasonal behavior and links to thermal structure near the tropical tropopause. Spatial distributions of RH at the cold point tropopause (CPT) show a close link between high RH and the minimum CPT temperature in both the NH winter and summer, consistent with temperature control of water vapor near the tropopause.

Multitimescale variations in modeled stratospheric water vapor derived from three modern reanalysis products

Abstract. Stratospheric water vapor (SWV) plays important roles in the radiation budget and ozone chemistry and is a valuable tracer for understanding stratospheric transport. Meteorological reanalyses provide variables necessary for simulating this transport; however, even recent reanalyses are subject to substantial uncertainties, especially in the stratosphere. It is therefore necessary to evaluate the consistency among SWV distributions simulated using different input reanalysis products. In this study, we evaluate the representation of SWV and its variations on multiple timescales using simulations over the period 1980–2013. Our simulations are based on the Chemical Lagrangian Model of the Stratosphere (CLaMS) driven by horizontal winds and diabatic heating rates from three recent reanalyses: ERA-Interim, JRA-55 and MERRA-2. We present an intercomparison among these model results and observationally based estimates using a multiple linear regression method to study the annual cycle (AC), the quasi-biennial oscillation (QBO), and longer-term variability in monthly zonal-mean H2O mixing ratios forced by variations in the El Niño–Southern Oscillation (ENSO) and the volcanic aerosol burden. We find reasonable consistency among simulations of the distribution and variability in SWV with respect to the AC and QBO. However, the amplitudes of both signals are systematically weaker in the lower and middle stratosphere when CLaMS is driven by MERRA-2 than when it is driven by ERA-Interim or JRA-55. This difference is primarily attributable to relatively slow tropical upwelling in the lower stratosphere in simulations based on MERRA-2. Two possible contributors to the slow tropical upwelling in the lower stratosphere are suggested to be the large long-wave cloud radiative effect and the unique assimilation process in MERRA-2. The impacts of ENSO and volcanic aerosol on H2O entry variability are qualitatively consistent among the three simulations despite differences of 50 %–100 % in the magnitudes. Trends show larger discrepancies among the three simulations. CLaMS driven by ERA-Interim produces a neutral to slightly positive trend in H2O entry values over 1980–2013 (+0.01 ppmv decade−1), while both CLaMS driven by JRA-55 and CLaMS driven by MERRA-2 produce negative trends but with significantly different magnitudes (−0.22 and −0.08 ppmv decade−1, respectively).

Impact of stratospheric major warmings and the quasi‐biennial oscillation on the variability of stratospheric water vapor

AbstractBased on simulations with the Chemical Lagrangian Model of the Stratosphere for the 1979–2013 period, driven by the European Centre for Medium‐Range Weather Forecasts ERA‐Interim reanalysis, we analyze the impact of the quasi‐biennial oscillation (QBO) and of Major Stratospheric Warmings (MWs) on the amount of water vapor entering the stratosphere during boreal winter. The amplitude of H2O variation related to the QBO amounts to 0.5 ppmv. The additional effect of MWs reaches its maximum about 2–4 weeks after the central date of the MW and strongly depends on the QBO phase. Whereas during the easterly QBO phase there is a pronounced drying of about 0.3 ppmv about 3 weeks after the MW, the impact of the MW during the westerly QBO phase is smaller (about 0.2 ppmv) and more diffusely spread over time. We suggest that the MW‐associated enhanced dehydration combined with a higher frequency of MWs after the year 2000 may have contributed to the lower stratospheric water vapor after 2000.

Morphology of Tropical Upwelling in the Lower Stratosphere

Abstract Sensitivity tests of a mechanistic model of the mean meridional circulation driven by specified eddy forcing are conducted to investigate how the morphology of tropical upwelling in the lower stratosphere is related to the structure of the forcing expected to be associated with the stratospheric surf zone. The basic morphology of tropical upwelling is found to be similar among the mechanistic model forced with reasonable eddy fluxes, the Geophysical Fluid Dynamics Laboratory (GFDL) SKYHI GCM, U.K. Met Office (UKMO) analyses, and other climate models, indicating the robustness of the upwelling features. Atmospheric data are analyzed to characterize the interannual variability of wave drag. The influence of such variations on the interannual variability of tropical upwelling in the lower stratosphere is explored, which may help explain the observed interannual variability of stratospheric water vapor.

Variability of Stratospheric Water Vapor Inferred from SAGE II, HALOE, and Boulder (Colorado) Balloon Measurements

Abstract The variability of stratospheric water vapor between 1996 and 2004 has been studied using multiyear measurements from the Stratospheric Aerosol and Gas Experiment II (SAGE II) version 6.2 dataset, the Halogen Occultation Experiment (HALOE) version 19 dataset, and the balloon-borne frost point hygrometer data record at Boulder, Colorado (40°N, 105°W). The features derived from SAGE II and HALOE for 20° latitudinal zones from 60°S to 60°N at various altitudes (16–34 km) show good quantitative agreement regarding the phases and magnitudes of annual, semiannual, and quasi-biennial oscillations (QBO). For the latitudinal zones 20°–40° and 40°–60°, the hemispheric asymmetry at 22 km with mainly QBO in the north and predominantly annual oscillations in the south has been revealed by both SAGE II and HALOE observations. Strong correlation exists between SAGE II and HALOE lower-stratospheric H2O anomalies over low latitudes and 100-hPa tropical zonal mean temperature anomalies. The correlation coefficients based on the 0°–20°S water vapor time series with H2O lagged by 2 months are 0.81 and 0.70 for HALOE and SAGE II, respectively. For 35°–45°N, SAGE II and HALOE show consistent trends generally varying from −0.05 to −0.02 ppmv yr−1 between 16 and 34 km. The corresponding analyses based on frost point measurements over Boulder show insignificant trends. These trends are not strongly dependent on the end points of the analysis and stand in contrast to the positive trends reported in previous studies that include data records prior to 1994. For the lower stratosphere, investigations of the entire balloon-borne dataset over Boulder indicate higher values of mixing ratios after 1992–93 compared to the period 1980–92. In contrast, SAGE II monthly zonal mean measurements for 35°–45°N show insignificant differences between the periods 1987–89 and 1996–2004.

Control of interannual and longer‐term variability of stratospheric water vapor

We use trajectory calculations based on 40‐year European Reanalysis (ERA‐40) data to predict the water mixing ratio of air entering the stratosphere in the tropics ([H2O]e) and thereby to examine interannual and longer‐term changes. [H2O]e is determined from the saturation mixing ratio of the coldest point during ascent from the troposphere to the stratosphere (the Lagrangian cold point). These model predictions for the time variation of [H2O]e agree very well with a broad range of measurements (Stratospheric Aerosol and Gas Experiment (SAGE) II, Halogen Occultation Experiment (HALOE), Microwave Limb Sounder (MLS), and Atmospheric Trace Molecule Spectroscopy (ATMOS)). During periods when measurements are consistent among various sensors and ERA‐40 temperatures show good agreement with radiosondes (1995–2002), the correlation between model predictions and HALOE water vapor anomalies in the tropical lower stratosphere is r = 0.81, as high as that between HALOE and SAGE II (r = 0.8). The model predictions suggest that the stratospheric quasi‐biennial oscillation, El Niño–Southern Oscillation, and possibly volcanic eruptions all play a significant role in modulating [H2O]e, leading to interannual anomalies of order 0.5 ppmv with timescales of several months to years. Although the Lagrangian calculations show substantial interannual variability of transport pathways into the stratosphere, the results show that the interannual anomalies of [H2O]e are dominated by anomalies of the zonal mean temperature rather than by transport changes or localized temperature anomalies. This reinforces the paradox of apparently increasing stratospheric water vapor concentrations alongside, if anything, slightly decreasing temperatures at the tropical tropopause. The combination of measurement uncertainties and relatively strong interannual variability with periods of several months to years, on the one hand, limits our ability to detect, attribute, and verify long‐term trends and, on the other hand, raises the question as to whether the previously published estimates of long‐term trends are too large.

Simulations of the Interannual Variability of Stratospheric Water Vapor

Abstract Observations and model results indicate that the quasi-biennial oscillation (QBO) modulation of stratospheric water vapor results from two causes. Dynamical redistribution of water vapor from the QBO-induced mean meridional circulation dominates the observed variability in the middle and upper stratosphere. In the lower stratosphere, the QBO water vapor variability is dominated by a “tape recorder” that results from the dehydration signal accompanying the QBO variation of the tropical cold point tropopause. It is suggested that another low frequency tape recorder exists due to ENSO modulations of the tropical tropopause, but insufficiently long observations of stratospheric water vapor exist to identify this in the observations.

In situ measurement of water vapor in the stratosphere with a cryogenically cooled Lyman‐alpha hygrometer

In situ measurements of water vapor in the stratosphere with a new instrument are reported. The instrument has been designed to observe daytime water vapor from a multi‐instrument balloon gondola that simultaneously measures free radicals such as OH, HO2, and O3 in the stratosphere up to 40 km. Lyman‐alpha photofragment fluorescence is used to measure water molecules in a flowing sample of ambient air. Outgassing from the interior walls of the instrument is avoided by cooling the walls with liquid nitrogen to a temperature near or below the dewpoint of the environment and by drawing air through the instrument with a fan. A brief description of the instrument is given, followed by the results of the first four balloon flights. Because frost formation in the scattering chamber resulted in a large and variable background, the data from July 15, 1987, have a relatively modest signal‐to‐noise ratio. The measured mixing ratio for this flight varies from 3.0–5.5 ppmv over the altitude range of 17–34 km. Adjustments in the cooling protocol for the flights of July 6, 1988, July 28, and August 25, 1989, result in a much higher signal‐to‐noise ratio. Profiles from these three flights are similar to, but somewhat higher, than the 1987 profile, The July 1988 and August 1989 profiles exhibit the highest mixing ratios, reaching peak values of about 6.5 ppmv near 35 km. Implications of these four measurements are discussed, as are the issues of short‐ and long‐term variability of stratospheric water vapor.