Unsteady Vortex Behavior in the Asian Monsoon Anticyclone

Abstract

The Asian monsoon anticyclone (AMA), which is primarily driven by the latent heat released by monsoon precipitation, is one of the dominant features of the Northern Hemisphere summer circulation in the upper troposphere and lower stratosphere. Due to variations in the diabatic heating, interactions with Rossby waves propagating along the subtropical jet, and internal dynamics within the anticyclone, the circulation of the AMA is unsteady. Here we use the ERA-Interim reanalysis and trajectories computed with ERA-Interim winds to show that the AMA contains two or three distinct synoptic-scale sub-vortices 69% of the time, while a single circulation center is present only 23% of the time. More than three simultaneous sub-vortices are uncommon. Observed behaviors of the sub-vortices include (1) splitting of a single vortex into two vortices; (2) merger of two vortices into a single vortex; (3) vortex shedding in the eastward direction; (4) vortex shedding in the westward direction; and (5) formation, movement, and dissipation of a vortex. The evolution of the sub-vortices is closely tied to stirring and transport.