Dynamic variability of the Asian monsoon anticyclone observed in potential vorticity and correlations with tracer distributions

Abstract

The Asian summer monsoon is associated with strong upward transport of tropospheric source gases and isolation of air within the upper tropospheric anticyclone, with a high degree of dynamical variability. Here we study the anticyclone in terms of potential vorticity (PV) as derived from reanalysis data. The strength of the anticyclone, as measured by low PV area, varies on subseasonal time scales (periods of 30–40 days), driven by variability in convection. The convective forcing of low PV areas is associated with heating in the middle troposphere and divergent motion in the upper troposphere, and we find that upper level divergence is a good predictor of the anticyclone strength. Low PV air is often observed to propagate from the forcing region to the west, and occasionally to the east. Carbon monoxide (CO) measured by the Aura Microwave Limb Sounder is used to study the covariability of chemical tracers with the anticyclone strength and location. Concentrations of CO maximize within the upper tropospheric anticyclone, and enhanced CO is well correlated with the spatial distribution of low PV. Time variations of CO concentrations in the upper troposphere (around 360 K) are not strongly correlated with anticyclone strength, probably because CO transport also involves coupling with surface CO sources (unlike PV). Temporal correlations with PV are stronger for CO at higher levels (380–400 K), suggesting that advective upward transport is important for tracer evolution at these levels.