Principal Investigator Raymond Plumb
Application of the circulation theorem leads one to conclude that steady, inviscid anticyclones forced in a shallow water model by mass relaxation (analogous to Newtonian relaxation of temperature) to an axisymmetric state must have zero absolute vorticity in a finite region, within which all the divergent flow is contained. This is unlike, for example, what is observed in the upper troposphere over monsoon regions.
Numerical calculations in a shallow water model confirm this prediction: with localized forcing on an f-plane, an axisymmetric anticyclone is produced which, for weak viscosity, has absolute vorticity very close to zero and negligible divergence elsewhere. However, when the symmetry is broken, by an imposed mean flow or a beta-effect of sufficient magnitude, the flow changes character. The flow becomes unsteady (thus breaking the steady circulation constraint) as the main anticyclone periodically sheds secondary anticyclones--to the west, on a beta-plane with no mean flow.
Analyses of July and August near-tropopause flow show this behavior occuring in the vicinity of the Tibetan anticyclone: secondary anticyclones, typically two or three per season, are shed westward, reaching about 20oE before re-merging with the westerly jet. These features are vertically shallow, being hard to detect below about 350K.