Nonaxisymmetric thermally driven circulations and upper-tropospheric monsoon dynamics

The authors investigate the nonlinear dynamics of almost inviscid, thermall y forced, divergent circulations in situations that are not axisymmetric. I n shallow-water numerical calculations, asymmetry is imposed on a locally f orced anticyclone by imposition of a mean wind, or a planetary vorticity gr adient. Behavior is similar in the two cases. With weak asymmetry, the forc ed anticyclone is distorted but remains intact and is qualitatively unchang ed from the symmetric response. For sufficiently large asymmetry, however, the elongated anticyclone becomes unstable and periodically sheds eddies. T his behavior shows how the circulation constraint can be satisfied, even wh en 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 ev ident 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 belo w about 400 hPa.