Transport of ice into the stratosphere and the humidification of the stratosphere over the 21st century (original) (raw)
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Atmospheric Chemistry and Physics
Variations in tropical lower-stratospheric humidity influence both the chemistry and climate of the atmosphere. We analyze tropical lower-stratospheric water vapor in 21st century simulations from 12 state-of-the-art chemistry–climate models (CCMs), using a linear regression model to determine the factors driving the trends and variability. Within CCMs, warming of the troposphere primarily drives the long-term trend in stratospheric humidity. This is partially offset in most CCMs by an increase in the strength of the Brewer–Dobson circulation, which tends to cool the tropical tropopause layer (TTL). We also apply the regression model to individual decades from the 21st century CCM runs and compare them to a regression of a decade of observations. Many of the CCMs, but not all, compare well with these observations, lending credibility to their predictions. One notable deficiency is that most CCMs underestimate the impact of the quasi-biennial oscillation on lower-stratospheric water ...
Quarterly Journal of the Royal Meteorological Society, 1999
Stratospheric humidity analyses produced operationally by the European Centre for Medium-Range Weather Forecasts (ECMWF) are discussed for the period since late January 1996 when the practice of resetting the upperlevel specific humidity to a fixed value at each analysis time was abandoned. Near-tropopause analyses are in reasonable overall agreement with independent observations. Very low humidities occur in conjunction with deep convection and a particularly cold tropopause over the equatorial western Pacific during the northern winter. Drying occurs also in the cold core of the Antarctic polar-night vortex. The lower stratosphere is moistened in the outer tropics and subtropics in summer and autumn, predominantly in the northern hemisphere. Changes associated with the latest occurrence of El Niiio are illustrated.
Simulation of stratospheric water vapor and trends using three reanalyses
The domain-filling, forward trajectory calculation model developed by Schoeberl and Dessler (2011) is extended to the 1979–2010 period. We compare results from NASA's MERRA, NCEP's CFSR, and ECMWF's ERAi reanalyses with HALOE, MLS, and balloon observations. The CFSR based simulation produces a wetter stratosphere than MERRA, and ERAi produces a drier stratosphere than MERRA. We find that ERAi 100 hPa temperatures are cold biased compared to Singapore sondes and MERRA, which explains the ERAi result, and the CFSR grid does not resolve the cold point tropopause, which explains its relatively higher water vapor concentration. The pattern of dehydration locations is also different among the three reanalyses. ERAi dehydration pattern stretches across the Pacific while CFSR and MERRA concentrate dehydration activity in the West Pacific. CSFR and ERAi also show less dehydration activity in the West Pacific Southern Hemisphere than MERRA. The trajectory models' lower northern high latitude stratosphere tends to be dry because too little methane-derived water descends from the middle stratosphere. Using the MLS tropical tape recorder signal, we find that MERRA vertical ascent is 15 % too weak while ERAi is 30 % too strong. The trajectory model reproduces the observed reduction in the amplitude of the 100-hPa annual cycle in zonal mean water vapor as it propagates to middle latitudes. Finally, consistent with the observations, the models show less than 0.2 ppm decade −1 trend in water vapor both at mid-latitudes and in the tropics.
Variations of stratospheric water vapor over the past three decades
We examine variations in water vapor in air entering the stratosphere through the 15 tropical tropopause layer (TTL) over the past three decades in satellite data and in a trajectory 16 model. Most of the variance can be explained by three processes that affect the TTL: the quasi-17 biennial oscillation, the strength of the Brewer-Dobson circulation, and the temperature of the 18 tropical troposphere. When these factors act in phase, significant variations in water entering the 19 stratosphere are possible. We also find that volcanic eruptions, which inject aerosol into the 20 TTL, affect the amount of water entering the stratosphere. While there is clear decadal 21 variability in the data and models, we find little evidence for a long-term trend in water entering 22 the stratosphere through the TTL over the past 3 decades. 23 24
Diurnal variations of humidity and ice water content in the tropical upper troposphere
Atmospheric Chemistry and Physics, 2010
Observational results of diurnal variations of humidity from Odin-SMR and AURA-MLS, and cloud ice mass from Odin-SMR and CloudSat are presented for the first time. Comparisons show that the retrievals of humidity and cloud ice from these three satellite instruments are in good agreement. The retrieved data are combined from four almost 5 evenly distributed times of the day allowing mean values, amplitudes and phases of the diurnal variations around 200 hPa to be estimated. This analysis is applied to six climatologically distinct regions, five located in the tropics and one over the subtropical northern Pacific Ocean. The strongest diurnal cycles are found over tropical land regions, where the amplitude is in the order of 6 %RHi for humidity and 50% for ice mass.
Climate Dynamics, 2000
This work uses an energy balance climate model (EBCM) with explicit infrared radiative transfer, parametrized tropospheric temperature and humidity pro®les, and separate stratosphere, troposphere, and surface energy balances, to investigate claims that a downward redistribution of tropospheric water vapor in response to surface warming could serve as a strong negative feedback on climatic change. A series of sensitivity tests is carried out using: (1) a variety of relationships between total precipitable water in the troposphere and temperature; (2) feedbacks between surface temperature and the vertical distribution of tropospheric water vapor at low latitudes; and (3) feedback between surface temperature or meridional temperature gradient and lapse rate. Fixed relative humidity (RH) enhances the global mean surface temperature response to a CO 2 doubling by only 50% compared to ®xed absolute humidity, giving a response of 1.8 K. When water vapor is assumed to be redistributed downward between 30°S± 30°N such that a 1 K surface warming reduces total precipitable water above 600 hPa by 10%, the global mean surface air temperature response is reduced to 1.2 K. Assuming a stronger downward redistribution in relation to surface temperature change has a rapidly diminishing marginal eect on global mean and tropical surface temperature response, while slightly increasing the warming at high latitudes due to the parametrized dependence of middle-to-high latitude lapse rate on the meridional temperature gradient. A modest downward water vapor redistribution, such that absolute humidity in the upper troposphere at subtropical latitudes is constant as total precipitable water increases, can reduce the tropical temperature sensitivity to less than 1 K, while increasing the equator-to-pole ampli®cation of the surface air temperature response from a factor of about three to a factor of four. However, it is concluded that whatever changes in future GCM response might occur as a result of new parametrizations of subgrid-scale processes, they are exceedingly unlikely to produce a climate sensitivity to a CO 2 doubling of less than 1 K even if there is a strong downward shift in the water vapor distribution as climate warms.
Quarterly Journal of the Royal Meteorological Society, 2014
This study investigates the potential contribution of observed changes in lower stratospheric water vapour to stratospheric temperature variations over the past three decades using a comprehensive global climate model (GCM). Three case studies are considered. In the first, the net increase in stratospheric water vapour (SWV) from 1980-2010 (derived from the Boulder frost-point hygrometer record using the gross assumption that this is globally representative) is estimated to have cooled the lower stratosphere by up to ∼0.2 K decade −1 in the global and annual mean; this is ∼40% of the observed cooling trend over this period. In the Arctic winter stratosphere there is a dynamical response to the increase in SWV, with enhanced polar cooling of 0.6 K decade −1 at 50 hPa and warming of 0.5 K decade −1 at 1 hPa. In the second case study, the observed decrease in tropical lower stratospheric water vapour after the year 2000 (imposed in the GCM as a simplified representation of the observed changes derived from satellite data) is estimated to have caused a relative increase in tropical lower stratospheric temperatures by ∼0.3 K at 50 hPa. In the third case study, the wintertime dehydration in the Antarctic stratospheric polar vortex (again using a simplified representation of the changes seen in a satellite dataset) is estimated to cause a relative warming of the Southern Hemisphere polar stratosphere by up to 1 K at 100 hPa from July-October. This is accompanied by a weakening of the westerly winds on the poleward flank of the stratospheric jet by up to 1.5 m s −1 in the GCM. The results show that, if the measurements are representative of global variations, SWV should be considered as important a driver of transient and long-term variations in lower stratospheric temperature over the past 30 years as increases in long-lived greenhouse gases and stratospheric ozone depletion.
On the influence of stratospheric water vapor changes on the tropospheric circulation
Geophysical Research Letters, 2006
1] Observations suggest that the mixing ratio of water vapour in the stratosphere has increased by 20 -50% between the 1960s and mid-1990s. Here we show that inclusion of such a stratospheric water vapour (SWV) increase in a state-of-the-art climate model modifies the circulation of the extratropical troposphere: the modeled increase in the North Atlantic Oscillation (NAO) index is 40% of the observed increase in NAO index between 1965 and 1995, suggesting that if the SWV trend is real, it explains a significant fraction of the observed NAO trend. Our results imply that SWV changes provide a novel mechanism for communicating the effects of large tropical volcanic eruptions and ENSO events to the extratropical troposphere over timescales of a few years, which provides a mechanism for interannual climate predictability. Finally, we discuss our results in the context of regional climate change associated with changes in methane emissions. Citation: Joshi, M. M., A. J. Charlton, and A. A. Scaife (2006), On the influence of stratospheric water vapor changes on the tropospheric circulation, Geophys. Res. Lett., 33, L09806,
The impact of temperature vertical structure on trajectory modeling of stratospheric water vapor
Lagrangian trajectories driven by reanalysis meteorological fields are frequently used to study water vapor (H 2 O) in the stratosphere, in which the tropical cold-point temperatures regulate the amount of H 2 O entering the stratosphere. Therefore, the accuracy of temperatures in the tropical tropopause layer (TTL) is of great importance for understanding stratospheric H 2 O abundances. Currently, most reanalyses, such as the NASA MERRA (Modern Era Retrospective -analysis for Research and Applications), only provide temperatures with ∼ 1.2 km vertical resolution in the TTL, which has been argued to miss finer vertical structure in the tropopause and therefore introduce uncertainties in our understanding of stratospheric H 2 O. In this paper, we quantify this uncertainty by comparing the Lagrangian trajectory prediction of H 2 O using MERRA temperatures on standard model levels (traj.MER-T) to those using GPS temperatures at finer vertical resolution (traj.GPS-T), and those using adjusted MERRA temperatures with finer vertical structures induced by waves (traj.MER-Twave). It turns out that by using temperatures with finer vertical structure in the tropopause, the trajectory model more realistically simulates the dehydration of air entering the stratosphere. But the effect on H 2 O abundances is relatively minor: compared with traj.MER-T, traj.GPS-T tends to dry air by ∼ 0.1 ppmv, while traj.MER-Twave tends to dry air by 0.2-0.3 ppmv. Despite these differences in absolute values of predicted H 2 O and vertical dehydration patterns, there is virtually no difference in the interannual variability in different runs. Overall, we find that a tropopause temperature with finer vertical structure has limited impact on predicted stratospheric H 2 O.
The impact of temperature resolution on trajectory modeling of stratospheric water vapour
Lagrangian trajectories driven by reanalysis meteorological fields are frequently used to study water vapour (H 2 O) in the stratosphere, in which the tropical cold-point temperatures regulate H 2 O amount entering the stratosphere. Therefore, the accuracy of temperatures in the tropical tropopause layer (TTL) is of great importance for tra-5 jectory studies. Currently, most reanalyses, such as the NASA MERRA (Modern Era Retrospective-Analysis for Research and Applications), only provide temperatures with ∼ 1.2 km vertical resolution in the TTL, which has been argued to introduce uncertainties in the simulations. In this paper, we quantify this uncertainty by comparing the trajectory results using MERRA temperatures on model levels (traj.MER-T) to those 10 using temperatures in finite resolutions, including GPS temperatures (traj.GPS-T) and MERRA temperatures adjusted to recover wave-induced variability underrepresented by the current ∼ 1.2 km vertical resolution (traj.MER-Twave). Comparing with traj.MER-T, traj.GPS-T has little impact on simulated stratospheric H 2 O (changes ∼ 0.1 ppmv), whereas traj.MER-Twave tends to dry air by 0.2-0.3 ppmv. The bimodal dehydration 15 peaks in traj.MER-T due to limited vertical resolution disappear in traj.GPS-T and traj.MER-Twave by allowing the cold-point tropopause to be found at finer vertical levels. Despite these differences in absolute values of predicted H 2 O and vertical dehydration patterns, there is virtually no difference in the interannual variability in different runs. Overall, we find that the finite resolution of temperature has limited impact on 20 predicted H 2 O in the trajectory model. Brewer's seminal work on stratospheric circulation that tropical tropopause tempera-29210 ACPD