Transport above the Asian summer monsoon anticyclone inferred from Aura Microwave Limb Sounder tracers

Abstract

Tracer variability above the Asian summer monsoon anticyclone is investigated using Aura Microwave Limb Sounder (MLS) measurements of carbon monoxide, ozone, water vapor, and temperature during Northern Hemisphere summer (June to August) of 2005. Observations show persistent maxima in carbon monoxide and minima in ozone within the anticyclone in the upper troposphere–lower stratosphere (UTLS) throughout summer, and variations in these tracers are closely related to the intensity of underlying deep convection. Temperatures in the UTLS are also closely coupled to deep convection (cold anomalies are linked with enhanced convection), and the three‐dimensional temperature patterns are consistent with a dynamical response to near‐ equatorial convection. Upper tropospheric water vapor in the monsoon region is strongly coherent with deep convection, both spatially and temporally. However, at the altitude of the tropopause, maximum water vapor is centered within the anticyclone, distant from the deepest convection, and is also less temporally correlated with convective intensity. Because the main outflow of deep convection occurs near ∼12 km, well below the tropopause level (∼16 km), we investigate the large‐scale vertical transport within the anticyclone. The mean vertical circulation obtained from the ERA40 reanalysis data set and a free‐running general circulation model is upward across the tropopause on the eastern end of the anticyclone, as part of the balanced three‐dimensional monsoon circulation. In addition to deep transport from the most intense convection, this large‐scale circulation may help explain the transport of constituents to tropopause level.