A comprehensive overview of the climatological composition of the Asian summer monsoon anticyclone based on 10 years of Aura Microwave Limb Sounder measurements

Abstract

Intense deep convection associated with the Asian summer monsoon (ASM) lofts surface pollutants to the upper troposphere/lower stratosphere (UTLS), where strong winds and long chemical lifetimes allow intercontinental transport, affecting atmospheric composition around the globe. The Aura Microwave Limb Sounder (MLS), launched in 2004, makes simultaneous colocated measurements of trace gases and cloud ice water content (a proxy for deep convection) in the UTLS on a daily basis. Here we exploit the dense spatial and temporal coverage, long‐term data record, extensive measurement suite, and insensitivity to aerosol and most clouds of Aura MLS to characterize the climatological (2005–2014) composition of the ASM anticyclone throughout its annual life cycle. We use version 4 MLS data to quantify spatial and temporal variations in both tropospheric (H2O, CO, CH3Cl, CH3CN, CH3OH) and stratospheric (O3, HNO3, HCl) tracers on four potential temperature surfaces (350–410 K). Inside the mature anticyclone, all species exhibit substantial changes, not only from their premonsoon distributions in the ASM region but also from their summertime distributions in the rest of the hemisphere. Different tracers exhibit dissimilar seasonal evolution, and the exact location and timing of their extreme values vary. Although individual aspects of the anticyclone have been described previously, we present a uniquely comprehensive overview of the climatological seasonal evolution of the ASM and its impact on UTLS composition. This work provides valuable context for planned in situ measurements as well as a benchmark for model evaluation and future investigations of interannual variability and long‐term changes in monsoon processes.

Dynamics and composition of the Asian summer monsoon anticyclone

“…EMAC-simulated O 3 at different isentropic surfaces. Multi-annual monthly averages are calculated for the same period as in Santee et al (2017) and agree well with their figures (370 and 390 K). However, at the level corresponding to the HALO flight altitude (360 K), O 3 is enhanced in the ASMA in September (i) and the observed sudden increase at the southern ASMA edge over Oman is also reproduced (arrow).…”

“…Ten-year averages of simulated O 3 reproduce the low-O 3 ASMA interior of satellite climatologies (Santee et al, 2017) as well as increased O 3 found by HALO ESMVal at slightly lower potential temperatures (Fig. 1).…”

“…Recent papers discussed climatological trace gas distributions in the monsoon region (Santee et al, 2017), CO distributions in the context of daily ASMA dynamics and monthly budgets of CO and O 3 . Building on the ASMA observations during the HALO ESMVal campaign, the second objective of our study is to complement these papers by considering additional tracers on a 10-hourly scale to characterise key processes relevant for the O 3 distribution in the monsoon region.…”

Transport of trace gases via eddy shedding from the Asian summer monsoon anticyclone and associated impacts on ozone heating rates

“…Confinements of high concentrations of trace gases, including ozone precursors (e.g. hydrogen cyanide (HCN), CO, hydrochloric acid (HCl), NO x and PAN), and low ozone in the anticyclone are evident in satellite and aircraft observations (Randel et al, 2010;Vogel et al, 2014;Fadnavis et al, 2015;Ungermann et al, 2016;Santee et al, 2017). The observed ozone minimum is still an open question in spite of high amounts of its precursors in the anticyclone.…”

“…Randel et al, 2010;Fadnavis et al, 2015;Santee et al, 2017). The uplifted air masses become confined into the anticyclone enclosed by jets (westerly and easterly jets to the north and south, respectively), which act as a strong transport barrier and restrict isentropic mixing into the extratropical lower stratosphere or the equatorial tropics (Ploeger et al, 2015.…”