Stratospheric Water Vapor Research Articles

Potential of future stratospheric ozone loss in the midlatitudes under global warming and sulfate geoengineering

Abstract. The potential of heterogeneous chlorine activation in the midlatitude lowermost stratosphere during summer is a matter of debate. The occurrence of heterogeneous chlorine activation through the presence of aerosol particles could cause ozone destruction. This chemical process requires low temperatures and is accelerated by an enhancement of the stratospheric water vapour and sulfate amount. In particular, the conditions present in the lowermost stratosphere during the North American Summer Monsoon season (NAM) are expected to be cold and moist enough to cause the occurrence of heterogeneous chlorine activation. Furthermore, the temperatures, the water vapour mixing ratio and the sulfate aerosol abundance are affected by future global warming and by the potential application of sulfate geoengineering. Hence, both future scenarios could promote this ozone destruction process. We investigate the likelihood of the occurrence of heterogeneous chlorine activation and its impact on ozone in the lowermost-stratospheric mixing layer between tropospheric and stratospheric air above central North America (30.6–49.6∘ N, 72.25–124.75∘ W) in summer for conditions today, at the middle and at the end of the 21st century. Therefore, the results of the Geoengineering Large Ensemble Simulations (GLENS) for the lowermost-stratospheric mixing layer between tropospheric and stratospheric air are considered together with 10-day box-model simulations performed with the Chemical Lagrangian Model of the Stratosphere (CLaMS). In GLENS two future scenarios are simulated: the RCP8.5 global warming scenario and a geoengineering scenario, where sulfur is additionally injected into the stratosphere to keep the global mean surface temperature from changing. In the GLENS simulations, the mixing layer will warm and moisten in both future scenarios with a larger effect in the geoengineering scenario. The likelihood of chlorine activation occurring in the mixing layer is highest in the years 2040–2050 if geoengineering is applied, accounting for 3.3 %. In comparison, the likelihood of conditions today is 1.0 %. At the end of the 21st century, the likelihood of this ozone destruction process occurring decreases. We found that 0.1 % of the ozone mixing ratios in the mixing layer above central North America is destroyed for conditions today. A maximum ozone destruction of 0.3 % in the mixing layer occurs in the years 2040–2050 if geoengineering is applied. Comparing the southernmost latitude band (30–35∘ N) and the northernmost latitude band (44–49∘ N) of the considered region, we found a higher likelihood of the occurrence of heterogeneous chlorine activation in the southernmost latitude band, causing a higher impact on ozone as well. However, the ozone loss process is found to have a minor impact on the midlatitude ozone column.

Global modeling of hydrogen using GFDL-AM4.1: Sensitivity of soil removal and radiative forcing

Hydrogen (H2) has been proposed as an alternative energy carrier to reduce the carbon footprint and associated radiative forcing of the current energy system. Here, we describe the representation of H2 in the GFDL-AM4.1 model including updated emission inventories and improved representation of H2 soil removal, the dominant sink of H2. The model best captures the overall distribution of surface H2, including regional contrasts between climate zones, when vd(H2) is modulated by soil moisture, temperature, and soil carbon content. We estimate that the soil removal of H2 increases with warming (2–4% per K), with large uncertainties stemming from different regional response of soil moisture and soil carbon. We estimate that H2 causes an indirect radiative forcing of 0.84 mW m−2/(Tg(H2)yr−1) or 0.13 mW m−2 ppbv−1, primarily due to increasing CH4 lifetime and stratospheric water vapor production.

Methane chemistry in a nutshell – the new submodels CH4 (v1.0) and TRSYNC (v1.0) in MESSy (v2.54.0)

Abstract. Climate projections including chemical feedbacks rely on state-of-the-art chemistry–climate models (CCMs). Of particular importance is the role of methane (CH4) for the budget of stratospheric water vapour (SWV), which has an important climate impact. However, simulations with CCMs are, due to the large number of involved chemical species, computationally demanding, which limits the simulation of sensitivity studies. To allow for sensitivity studies and ensemble simulations with a reduced demand for computational resources, we introduce a simplified approach to simulate the core of methane chemistry in form of the new Modular Earth Submodel System (MESSy) submodel CH4. It involves an atmospheric chemistry mechanism reduced to the sink reactions of CH4 with predefined fields of the hydroxyl radical (OH), excited oxygen (O(1D)), and chlorine (Cl), as well as photolysis and the reaction products limited to water vapour (H2O). This chemical production of H2O is optionally fed back onto the specific humidity (q) of the connected general circulation model (GCM), to account for the impact onto SWV and its effect on radiation and stratospheric dynamics. The submodel CH4 is further capable of simulating the four most prevalent CH4 isotopologues for carbon and hydrogen (CH4 and CH3D, as well as 12CH4 and 13CH4). Furthermore, the production of deuterated water vapour (HDO) is, similar to the production of H2O in the CH4 oxidation, optionally passed back to the isotopological hydrological cycle simulated by the submodel H2OISO, using the newly developed auxiliary submodel TRSYNC. Moreover, the simulation of a user-defined number of diagnostic CH4 age and emission classes is possible, the output of which can be used for offline inverse optimization techniques. The presented approach combines the most important chemical hydrological feedback including the isotopic signatures with the advantages concerning the computational simplicity of a GCM, in comparison to a full-featured CCM.

Effective radiative forcing from emissions of reactive gases and aerosols – a multi-model comparison

Abstract. This paper quantifies the pre-industrial (1850) to present-day (2014) effective radiative forcing (ERF) of anthropogenic emissions of NOX, volatile organic compounds (VOCs; including CO), SO2, NH3, black carbon, organic carbon, and concentrations of methane, N2O and ozone-depleting halocarbons, using CMIP6 models. Concentration and emission changes of reactive species can cause multiple changes in the composition of radiatively active species: tropospheric ozone, stratospheric ozone, stratospheric water vapour, secondary inorganic and organic aerosol, and methane. Where possible we break down the ERFs from each emitted species into the contributions from the composition changes. The ERFs are calculated for each of the models that participated in the AerChemMIP experiments as part of the CMIP6 project, where the relevant model output was available. The 1850 to 2014 multi-model mean ERFs (± standard deviations) are −1.03 ± 0.37 W m−2 for SO2 emissions, −0.25 ± 0.09 W m−2 for organic carbon (OC), 0.15 ± 0.17 W m−2 for black carbon (BC) and −0.07 ± 0.01 W m−2 for NH3. For the combined aerosols (in the piClim-aer experiment) it is −1.01 ± 0.25 W m−2. The multi-model means for the reactive well-mixed greenhouse gases (including any effects on ozone and aerosol chemistry) are 0.67 ± 0.17 W m−2 for methane (CH4), 0.26 ± 0.07 W m−2 for nitrous oxide (N2O) and 0.12 ± 0.2 W m−2 for ozone-depleting halocarbons (HC). Emissions of the ozone precursors nitrogen oxides (NOx), volatile organic compounds and both together (O3) lead to ERFs of 0.14 ± 0.13, 0.09 ± 0.14 and 0.20 ± 0.07 W m−2 respectively. The differences in ERFs calculated for the different models reflect differences in the complexity of their aerosol and chemistry schemes, especially in the case of methane where tropospheric chemistry captures increased forcing from ozone production.

Slow feedbacks resulting from strongly enhanced atmospheric methane mixing ratios in a chemistry–climate model with mixed-layer ocean

Abstract. In a previous study the quasi-instantaneous chemical impacts (rapid adjustments) of strongly enhanced methane (CH4) mixing ratios have been analysed. However, to quantify the influence of the respective slow climate feedbacks on the chemical composition it is necessary to include the radiation-driven temperature feedback. Therefore, we perform sensitivity simulations with doubled and quintupled present-day (year 2010) CH4 mixing ratios with the chemistry–climate model EMAC (European Centre for Medium-Range Weather Forecasts, Hamburg version – Modular Earth Submodel System (ECHAM/MESSy) Atmospheric Chemistry) and include in a novel set-up a mixed-layer ocean model to account for tropospheric warming. Strong increases in CH4 lead to a reduction in the hydroxyl radical in the troposphere, thereby extending the CH4 lifetime. Slow climate feedbacks counteract this reduction in the hydroxyl radical through increases in tropospheric water vapour and ozone, thereby dampening the extension of CH4 lifetime in comparison with the quasi-instantaneous response. Changes in the stratospheric circulation evolve clearly with the warming of the troposphere. The Brewer–Dobson circulation strengthens, affecting the response of trace gases, such as ozone, water vapour and CH4 in the stratosphere, and also causing stratospheric temperature changes. In the middle and upper stratosphere, the increase in stratospheric water vapour is reduced with respect to the quasi-instantaneous response. We find that this difference cannot be explained by the response of the cold point and the associated water vapour entry values but by a weaker strengthening of the in situ source of water vapour through CH4 oxidation. However, in the lower stratosphere water vapour increases more strongly when tropospheric warming is accounted for, enlarging its overall radiative impact. The response of the stratosphere adjusted temperatures driven by slow climate feedbacks is dominated by these increases in stratospheric water vapour as well as strongly decreased ozone mixing ratios above the tropical tropopause, which result from enhanced tropical upwelling. While rapid radiative adjustments from ozone and stratospheric water vapour make an essential contribution to the effective CH4 radiative forcing, the radiative impact of the respective slow feedbacks is rather moderate. In line with this, the climate sensitivity from CH4 changes in this chemistry–climate model set-up is not significantly different from the climate sensitivity in carbon-dioxide-driven simulations, provided that the CH4 effective radiative forcing includes the rapid adjustments from ozone and stratospheric water vapour changes.

Validation of SAGE III/ISS Solar Water Vapor Data With Correlative Satellite and Balloon‐Borne Measurements

AbstractSince June 2017, the Stratospheric Aerosol and Gas Experiment III instrument on the International Space Station (SAGE III/ISS) has been providing vertical profiles of upper tropospheric to stratospheric water vapor (SWV) retrieved from solar occultation transmission measurements. The goal of this study is to evaluate the publicly released SAGE III/ISS beta version 5.1 WV retrieval through intercomparison with independent satellite‐ and balloon‐based measurements, and to present recommendations for SAGE III/ISS data quality screening criteria. Overall, we find that SAGE III/ISS provides high quality water vapor measurements. Low quality profiles are predominately due to retrieval instabilities in the upper stratosphere that cause step‐like changes in the profile, and aerosol/cloud‐related interferences (below ∼20 km). Above 35 km, the retrieved uncertainty and noise in the data rapidly grow with increasing altitude due to relatively low extinction signal from water vapor. Below the tropopause, retrieved uncertainty increases with decreasing altitude due to enhanced molecular scattering and aerosol extinction. After screening low‐quality data using the procedures described herein, SAGE III/ISS WV is shown to be in good agreement with independent satellite and balloon‐based measurements. From 20 to 40 km, SAGE III/ISS WV v5.1 data exhibit a bias of 0.0 to −0.5 ppmv (∼10%) relative to the independent data, depending on the instrument and altitude. Despite its status as a beta version, the level of SAGE III/ISS WV agreement with independent data is similar to previous SAGE instruments, and therefore the data are suitable for scientific studies of SWV.

Understanding balloon-borne frost point hygrometer measurements after contamination by mixed-phase clouds

Abstract. Balloon-borne water vapour measurements in the upper troposphere and lower stratosphere (UTLS) by means of frost point hygrometers provide important information on air chemistry and climate. However, the risk of contamination from sublimating hydrometeors collected by the intake tube may render these measurements unusable, particularly after crossing low clouds containing supercooled droplets. A large set of (sub)tropical measurements during the 2016–2017 StratoClim balloon campaigns at the southern slopes of the Himalayas allows us to perform an in-depth analysis of this type of contamination. We investigate the efficiency of wall contact and freezing of supercooled droplets in the intake tube and the subsequent sublimation in the UTLS using computational fluid dynamics (CFD). We find that the airflow can enter the intake tube with impact angles up to 60∘, owing to the pendulum motion of the payload. Supercooled droplets with radii > 70 µm, as they frequently occur in mid-tropospheric clouds, typically undergo contact freezing when entering the intake tube, whereas only about 50 % of droplets with 10 µm radius freeze, and droplets < 5 µm radius mostly avoid contact. According to CFD, sublimation of water from an icy intake can account for the occasionally observed unrealistically high water vapour mixing ratios (χH2O > 100 ppmv) in the stratosphere. Furthermore, we use CFD to differentiate between stratospheric water vapour contamination by an icy intake tube and contamination caused by outgassing from the balloon and payload, revealing that the latter starts playing a role only during ascent at high altitudes (p < 20 hPa).

Extreme Outliers in Lower Stratospheric Water Vapor Over North America Observed by MLS: Relation to Overshooting Convection Diagnosed From Colocated Aqua-MODIS Data.

Convectively injected water vapor (HO) in the North American (NA) summer lowermost stratosphere results in significant outliers in the 100‐hPa HO measurements from the Aura Microwave Limb Sounder (MLS). MLS statistics from 15 years confirm that the NA region contains over 60% of global 100‐hPa HO > 12 ppmv, despite having only 1.8% of all MLS observations. A profile sampled in August 2019 stands out, with H2O=26.3 ppmv, far exceeding the prior record and the median 4.5‐ppmv abundance in NA. This particular outlier is associated with a large overshooting convective event (OCE) that spanned multiple U.S. states and persisted for several hours. Colocation of the MLS data over NA with cloud observations from Aqua's Moderate Resolution Imaging Spectroradiometer (MODIS) reveals the unique character of this case, as only 2.3% of MLS profiles are as close to an OCE and only 0.024% of OCEs cover as large an area within a 500‐km perimeter of a profile.

Increase in Lower Stratospheric Water Vapor in the Past 100 Years Related to Tropical Atlantic Warming

AbstractLower Stratospheric water vapor (SWV) is one of important drivers of global climate change. Increases and decreases in lower SWV have been found to strengthen and offset global warming effects, respectively. Using several data sets, we find that sea surface temperature (SST) warming in the past 100 years has caused an increase in SWV. SST warming over the tropical Indian Ocean and the western Pacific has resulted in a drier stratosphere. However, tropical Atlantic Ocean warming has resulted in a significantly wetter stratosphere and is the main contributor to the increasing trend of SWV in the past 100 years. The responses of Rossby and Kelvin waves over the Indian Ocean and western Pacific to Atlantic warming have led to a warmer tropopause temperature, resulting in more water vapor entering the stratosphere. This study suggests that SWV trend may simply be the result of a game between warm pool SST and tropical Atlantic SST changes.

The response of stratospheric water vapor to climate change driven by different forcing agents

Abstract. We investigate the response of stratospheric water vapor (SWV) to different forcing agents within the Precipitation Driver and Response Model Intercomparison Project (PDRMIP) framework. For each model and forcing agent, we break down the SWV response into a slow response, which is coupled to surface temperature changes, and a fast response, which is the response to external forcing but before the sea surface temperatures have responded. Our results show that, for most climate perturbations, the slow SWV response dominates the fast response. The slow SWV response exhibits a similar sensitivity to surface temperature across all climate perturbations. Specifically, the sensitivity is 0.35 ppmv K−1 in the tropical lower stratosphere (TLS), 2.1 ppmv K−1 in the northern hemispheric lowermost stratosphere (LMS), and 0.97 ppmv K−1 in the southern hemispheric LMS. In the TLS, the fast SWV response only dominates the slow SWV response when the forcing agent radiatively heats the cold-point region – for example, black carbon, which directly heats the atmosphere by absorbing solar radiation. The fast SWV response in the TLS is primarily controlled by the fast adjustment of cold-point temperature across all climate perturbations. This control becomes weaker at higher altitudes in the tropics and altitudes below 150 hPa in the LMS.

Influence of convection on stratospheric water vapor in the North American monsoon region

Abstract. We quantify the connection between deep convective occurrence and summertime 100 hPa water vapor anomaly over the North American (NA) region and find substantial consistency between their interannual variations and also that the water vapor mixing ratio over the NA region is up to ∼1 ppmv higher when deep convection occurs. We use a Lagrangian trajectory model to demonstrate that the structure and the location of the NA anticyclone, as well as the tropical upper tropospheric temperature, mediate the moistening impact of convection. The deep convection mainly occurs over the Central Plains region. Most of the convectively moistened air is then transported to the center of the NA anticyclone, and the anticyclonic structure helps maintain high water vapor content there. This explains both the summer seasonal cycle and interannual variability of the convective moistening efficiency in the NA region and can provide valuable insight into modeling stratospheric water vapor.

Far‐Ranging Impact of Mountain Waves Excited Over Greenland on Stratospheric Dehydration and Rehydration

AbstractIn situ observations of reduced stratospheric water vapor combined with those of ice particle formation are rarely conducted. On the one hand, they are essential to broaden our knowledge about the formation of polar stratospheric clouds (PSCs). On the other hand, the observed profiles allow the comparison with global circulation models. Here we report about a balloon‐borne observation above Sodankylä, Finland on 26 January 2005. The frostpoint hygrometer detected layers of reduced water vapor by up to 2 ppmv from 18.5 to 23 km. Beneath, a 1‐km‐deep layer of increased water vapor was identified. An aerosol backscatter sonde measured the presence of stratospheric ice clouds. According to meteorological analysis the PSCs were formed upstream above the east coast of Greenland due to mountain wave‐induced cooling. The inertia‐gravity waves generated a large and persistent stratospheric wake far downstream of Greenland and led to the observed dehydration. Comparing the most recent ERA5 data with operational analyses from 2005, we find an improved representation of mesoscale internal gravity waves, dehydration and PSC formation for this particular event.

The Surface Warming Attributable to Stratospheric Water Vapor in CO2‐Caused Global Warming

AbstractStratospheric water vapor (SWV) is recognized as a potentially important positive feedback in global warming. The SWV change induces significant downward radiative flux perturbation at the tropopause and therefore is hypothesized to substantially amplify the surface warming. To test this hypothesis, we use a global climate model to quantify the surface warming contributed by the SWV change in the context of the quadrupled CO2. By prescribing the SWV increase as an external forcing, we find that SWV only accounts for 0.42 K surface warming, making up merely 5.4% of the total CO2‐caused surface warming (7.7 K). The efficacy of the stratosphere‐adjusted SWV forcing is small (38%), where the efficacy is defined as the ratio of the global temperature response per unit radiative forcing relative to that of the CO2 forcing. With the aid of a series of auxiliary experiments, we find that although the stratosphere‐adjusted SWV forcing at the top of atmosphere (TOA) is significant (1.13 W m−2), more than half of the forcing is offset by a high‐cloud decrease and an upper tropospheric warming in the tropospheric adjustment. The direct radiative impact of the SWV increase on surface temperature is negligible, and the SWV‐induced surface temperature change is a result of interactions between the radiative and nonradiative processes.

Analysis of factors influencing tropical lower stratospheric water vapor during 1980\u20132017

Tropical cold point tropopause temperature (CPT) anomalies determine lower stratospheric water vapor (LSWV) variations, leading to a high correlation between variations in tropical average CPT and changes in tropical average LSWV. However, this high correlation is only found in winter and spring. This work revisits the factors controlling LSWV variations using observations and simulations over the past ~40 years. It is found that the first and second empirical orthogonal function (EOF) modes of tropical CPT variations together explain the tropical average LSWV changes much better than the tropical average CPT variations. The high correlation between the first and second EOF modes of tropical CPT variations and tropical average LSWV changes holds in all four seasons. A further analysis shows that the first and second EOF modes of tropical CPT variations are related to canonical El Niño–Southern Oscillation (ENSO) activity and sea surface temperature (SST) variations in the central Pacific Ocean, respectively. ENSO Modoki is also an important factor that affects LSWV variations by influencing the vertical velocity at the tropopause. The quasi-biennial oscillation (QBO) affects the CPT, and is the third process modulating the LSWV changes. The simulations also support the results.

Inconsistencies between chemistry–climate models and observed lower stratospheric ozone trends since 1998

Abstract. The stratospheric ozone layer shields surface life from harmful ultraviolet radiation. Following the Montreal Protocol ban on long-lived ozone-depleting substances (ODSs), rapid depletion of total column ozone (TCO) ceased in the late 1990s, and ozone above 32 km is now clearly recovering. However, there is still no confirmation of TCO recovery, and evidence has emerged that ongoing quasi-global (60∘ S–60∘ N) lower stratospheric ozone decreases may be responsible, dominated by low latitudes (30∘ S–30∘ N). Chemistry–climate models (CCMs) used to project future changes predict that lower stratospheric ozone will decrease in the tropics by 2100 but not at mid-latitudes (30–60∘). Here, we show that CCMs display an ozone decline similar to that observed in the tropics over 1998–2016, likely driven by an increase in tropical upwelling. On the other hand, mid-latitude lower stratospheric ozone is observed to decrease, while CCMs that specify real-world historical meteorological fields instead show an increase up to present day. However, these cannot be used to simulate future changes; we demonstrate here that free-running CCMs used for projections also show increases. Despite opposing lower stratospheric ozone changes, which should induce opposite temperature trends, CCMs and observed temperature trends agree; we demonstrate that opposing model–observation stratospheric water vapour (SWV) trends, and their associated radiative effects, explain why temperature changes agree in spite of opposing ozone trends. We provide new evidence that the observed mid-latitude trends can be explained by enhanced mixing between the tropics and extratropics. We further show that the temperature trends are consistent with the observed mid-latitude ozone decrease. Together, our results suggest that large-scale circulation changes expected in the future from increased greenhouse gases (GHGs) may now already be underway but that most CCMs do not simulate mid-latitude ozone layer changes well. However, it is important to emphasise that the periods considered here are short, and internal variability that is both intrinsic to each CCM and different to observed historical variability is not well-characterised and can influence trend estimates. Nevertheless, the reason CCMs do not exhibit the observed changes needs to be identified to allow models to be improved in order to build confidence in future projections of the ozone layer.

Stratospheric radiative feedback limited by the tropospheric influence in global warming

Whether the stratospheric radiative feedback amplifies the global warming remains under debate. The stratospheric water vapor (SWV), one of the primary feedbacks in the stratosphere, is argued to be an important contributor to the global warming. On the other hand, the overall stratospheric feedback, which consists of both the SWV feedback and the stratospheric temperature (ST) feedback, does not amount to a significant value. The key to reconciling these seemingly contradictory arguments is to understand the ST change. Here, we develop a method to decompose the ST change and to quantify the decomposed feedbacks. We find that the SWV feedback, which consists of a 0.04 W m−2 K−1 direct impact on the top-of-the-atmosphere radiation and 0.11 W m−2 K−1 indirect impact via ST cooling, is offset by a negative ST feedback of − 0.13 W m−2 K−1 that is radiatively driven by the tropospheric warming. This compensation results in an insignificant overall stratospheric feedback.

Stratospheric water vapor feedback and its climate impacts in the coupled atmosphere–ocean Goddard Earth Observing System Chemistry-Climate Model

The stratospheric water vapor feedbacks on climate for abrupt CO2 quadrupling are investigated with the coupled atmosphere–ocean Goddard Earth Observing System Chemistry-Climate model. A feedback suppression method is used to quantify the stratospheric water vapor climate feedback parameter and the impacts of stratospheric water vapor increases on temperature and circulation. It is found that increases in stratospheric water vapor change the model’s net climate feedback parameter by 0.11 W m−2 K−1, contributing to 0.5 K, or 10%, of the global-mean surface warming under abrupt CO2 quadrupling. Stratospheric water vapor increases lead to significant impacts on stratospheric temperature and dynamics. The increases induce stratospheric dynamical changes that strongly modify stratospheric cooling patterns. About 30% of the acceleration of the stratospheric Brewer-Dobson circulation under 4 × CO2 is attributed to the stratospheric water vapor increases. In the troposphere, the stratospheric water vapor feedback plays a role in Arctic amplification and is responsible for 14% of the Arctic warming. It also affects tropospheric circulation, causing greater poleward shift of the northern hemisphere tropospheric midlatitude jet.

Erythemal Radiation, Column Ozone, and the North American Monsoon

AbstractRecently, Anderson et al. (2012, https://doi.org/10.1126/science.1222978, 2017, https://doi.org/10.1073/pnas.1619318114) and Anderson and Clapp (2018, https://doi.org/10.1039/C7CP08331A) proposed that summertime convectively injected water vapor over North America could lead to stratospheric ozone depletion through halogenic catalytic reactions. Such ozone loss would reduce the ozone column and increase erythemal daily dose (EDD). Using 10 years of observations over the North American monsoon region from the Aura Ozone Monitoring Instrument, we find that the column ozone and EDD has a ~0.8–0.9 spatial correlation with lower stratospheric water vapor measured by the Aura Microwave Limb Sounder. We show that this correlation appears to be due to the elevation of the monsoonal tropopause and associated monsoonal convection. The increase in tropopause altitude reduces the ozone column and increases EDD. We see no apparent evidence of substantial heterogeneous chemical ozone loss in lower stratospheric ozone coincident with the stratospheric monsoonal water vapor enhancement.

Stratospheric Water Vapor Feedback Disclosed by a Locking Experiment

AbstractMost global climate models project a considerable stratospheric moistening during global warming. It is important to quantify how significantly the increase of stratospheric humidity affects the radiation budget and contributes to the surface warming. Here, we conduct a mechanism denial experiment to investigate the warming effect of the stratospheric water vapor (SWV) using a global climate model. By locking the SWV in a quadrupling CO2 experiment, we find that the surface warming effect of SWV is not as significant as previously thought, increasing the global mean surface warming by about 2%. This is due to compensating changes in other feedback, especially those of tropospheric temperature and cloud, affected by SWV.

Tropical water vapour in the lower stratosphere and its relationship to tropical/extratropical dynamical processes in ERA5

AbstractStratospheric water vapour (SWV), in spite of its low concentration in the stratosphere as compared to the troposphere, contributes significantly to the surface energy budget and can have an influence on the surface climate. This study investigates the dynamical processes that determine SWV on interannual to decadal time‐scales. First, we evaluate two SWV reanalysis products and show that SWV is better represented in a new‐generation reanalysis product, ERA5, than in its predecessor, ERA‐Interim. In particular, it is shown that SWV in ERA5 is highly consistent with observational data obtained from the SPARC Data Initiative Multi‐Instrument Mean (SDI MIM). Second, we investigate the variability of tropical SWV and its relationship to dynamical stratospheric variables. The analyses show that the interannual variability in the tropical lower‐stratospheric water vapour is closely linked to the tropical Quasi‐Biennial Oscillation (QBO). When westerlies occupy the middle stratosphere and easterlies the lower stratosphere, a decrease is observed in lower‐stratospheric water vapour due to a colder tropical tropopause and a QBO‐induced enhanced residual circulation. On decadal time‐scales, the composite analysis of the boreal winter in two typical periods shows that less SWV is related to a warm anomaly in the North Atlantic sea‐surface temperature, which leads to stronger upward propagation of planetary wave activity at high latitudes, a weaker polar vortex and an enhanced residual circulation. The opposite occurs during periods with higher concentrations of SWV.