Water vapor increase in the lower stratosphere of the Northern Hemisphere due to the Asian monsoon anticyclone observed during the TACTS/ESMVal campaigns

Christian Rolf,Rolf Müller,Armin Afchine,Martin Riese,Nicole Spelten,Peter Hoor,Stefan Müller,Gebhard Günther,Martina Krämer,Bärbel Vogel

doi:10.5194/acp-18-2973-2018

Abstract

Abstract. The impact of air masses originating in Asia and influenced by the Asian monsoon anticyclone on the Northern Hemisphere stratosphere is investigated based on in situ measurements. A statistically significant increase in water vapor (H2O) of about 0.5 ppmv (11 %) and methane (CH4) of up to 20 ppbv (1.2 %) in the extratropical stratosphere above a potential temperature of 380 K was detected between August and September 2012 during the HALO aircraft missions Transport and Composition in the UT/LMS (TACTS) and Earth System Model Validation (ESMVal). We investigate the origin of the increased water vapor and methane using the three-dimensional Chemical Lagrangian Model of the Stratosphere (CLaMS). We assign the source of the moist air masses in the Asian region (northern and southern India, eastern China, southeast Asia, and the tropical Pacific) based on tracers of air mass origin used in CLaMS. The water vapor increase is correlated with an increase of the simulated Asian monsoon air mass contribution from about 10 % in August to about 20 % in September, which corresponds to a doubling of the influence from the Asian monsoon region. Additionally, back trajectories starting at the aircraft flight paths are used to differentiate transport from the Asian monsoon anticyclone and other source regions by calculating the Lagrangian cold point (LCP). The geographic location of the LCPs, which indicates the region where the set point of water vapor mixing ratio along these trajectories occurs, can be predominantly attributed to the Asian monsoon region.

Highlights

Active trace gases, such as water vapor and methane, play a key role in determining the radiative balance in the upper troposphere and lower stratosphere (UTLS) and have an impact on the surface climate of the Earth (Forster and Shine, 2002; Riese et al, 2012)
We present in situ water vapor measurements of the two phases (August and September) and support the hypothesis based on water vapor–methane correlations
Air masses with potential vorticity (PV) < 8 PVU reveal large variability between phase 2 and phase 1, most likely due to the local variability of the thermal tropopause not captured by the European Centre for Medium-range Weather Forecasts (ECMWF) data and planetary wave activity which distort the location of the sub-tropical jet core

Summary

Introduction

Active trace gases, such as water vapor and methane, play a key role in determining the radiative balance in the upper troposphere and lower stratosphere (UTLS) and have an impact on the surface climate of the Earth (Forster and Shine, 2002; Riese et al, 2012). Only small increases/decreases of stratospheric water vapor on the order of 10 % have a significant influence on the radiative forcing and constitute warming/cooling potential for global surface temperature on the order of 25–30 % (Solomon et al, 2010). Frozen, dry air masses with a low amount of water vapor enter the stratosphere in the tropical tropopause layer (TTL) and are transported vertically via the Brewer– Dobson circulation (BDC) deep into the stratosphere and quasi-horizontally into the extratropical lower stratosphere (Ex-LS; e.g., Gettelman et al, 2011). As a rule, increasing inflow of tropospheric air masses from the tropics in combination with higher temperatures in the tropopause region causes a moistening of the Ex-LS in summer compared to winter months

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