Abstract
Abstract. The Asian monsoon anticyclone (AMA) represents one of the wettest regions in the lower stratosphere (LS) and is a key contributor to the global annual maximum in LS water vapour. While the AMA wet pool is linked with persistent convection in the region and horizontal confinement of the anticyclone, there remain ambiguities regarding the role of tropopause-overshooting convection in maintaining the regional LS water vapour maximum. This study tackles this issue using a unique set of observations from aboard the high-altitude M55-Geophysica aircraft deployed in Nepal in summer 2017 within the EU StratoClim project. We use a combination of airborne measurements (water vapour, ice water, water isotopes, cloud backscatter) together with ensemble trajectory modelling coupled with satellite observations to characterize the processes controlling water vapour and clouds in the confined lower stratosphere (CLS) of the AMA. Our analysis puts in evidence the dual role of overshooting convection, which may lead to hydration or dehydration depending on the synoptic-scale tropopause temperatures in the AMA. We show that all of the observed CLS water vapour enhancements are traceable to convective events within the AMA and furthermore bear an isotopic signature of the overshooting process. A surprising result is that the plumes of moist air with mixing ratios nearly twice the background level can persist for weeks whilst recirculating within the anticyclone, without being subject to irreversible dehydration through ice settling. Our findings highlight the importance of convection and recirculation within the AMA for the transport of water into the stratosphere.
Highlights
Water vapour in the lower stratosphere has a direct impact on surface climate and stratospheric ozone chemistry (e.g. Dessler et al, 2013; Dvortsov and Solomon, 2001; Anderson et al, 2017)
This is consistent with a comprehensive analysis of airborne data from various campaigns by Krämer et al (2020), who pointed out significantly larger amounts of ice water content (IWC) in subsaturated ice crystals above the cold point tropopause (CPT) in the Asian summer monsoon anticyclone (AMA) compared to that in the surrounding tropical regions, which underlines the importance of the AMA as the source of lower stratosphere (LS) water
The occurrence of water vapour enhancements in the lower stratosphere associated with overshooting convection has been reported in several studies based on in situ measurements in the deep tropics over western Africa (Khaykin et al, 2009; Schiller et al, 2009), northern Australia (Kley et al, 1982; Corti et al, 2008), South America (Khaykin et al, 2013), Central America (Sargent et al, 2014), and the western Pacific (Jensen et al, 2020), as well as at midlatitudes over the North American monsoon (Hanisco et al, 2007; Weinstock et al, 2007; Smith et al, 2017) and Asian monsoon (Vernier et al, 2018; Brunamonti et al, 2018; Krämer et al, 2020)
Summary
Water vapour in the lower stratosphere has a direct impact on surface climate and stratospheric ozone chemistry (e.g. Dessler et al, 2013; Dvortsov and Solomon, 2001; Anderson et al, 2017). The variability of global lower stratospheric water vapour is, to first order, regulated by the minimum temperature in the tropical tropopause layer (TTL) – the main gateway for stratospheric entry of tropospheric moisture (e.g. Fueglistaler et al, 2009, and references therein). The AMA is characterized by a persistent maximum of water vapour extending up to 68 hPa (Park et al, 2007; Santee et al, 2017), which makes it the wettest region in the boreal summer lower stratosphere. This large-scale maximum is conditioned by convective uplift of moist air in the Asian monsoon region and its horizontal confinement within the anticyclone (Dethof et al, 1999; Ploeger et al, 2015).
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