Nonaxisymmetric Thermally Driven Circulations and Upper-Tropospheric Monsoon Dynamics

Abstract

The authors investigate the nonlinear dynamics of almost inviscid, thermally forced, divergent circulations in situations that are not axisymmetric. In shallow-water numerical calculations, asymmetry is imposed on a locally forced anticyclone by imposition of a mean wind, or a planetary vorticity gradient. Behavior is similar in the two cases. With weak asymmetry, the forced anticyclone is distorted but remains intact and is qualitatively unchanged from the symmetric response. For sufficiently large asymmetry, however, the elongated anticyclone becomes unstable and periodically sheds eddies. This behavior shows how the circulation constraint can be satisfied, even when the time-mean absolute vorticity remains finite in the divergent region, and provides a continuous evolution between the nonlinear (symmetric) and linear (highly asymmetric) limits. Westward shedding of anticyclones from the Tibetan anticyclone is indeed evident in NCEP reanalysis data. These eddies are trapped near the tropopause. Cutoff potential vorticity features are confined to within about 20 K of the tropopause; in geopotential, they extend somewhat further, but not below about 400 hPa.