Behavior of the Sub-Vortices within the Asian Monsoon Anticyclone

The Asian Monsoon Anticyclone (AMA) is one of the dominant circulation features of the upper troposphere and lower stratosphere (UTLS) during the boreal warm season. Convectively-lofted tropospheric air exported from the AMA affects the composition of the global atmosphere. Because of variations in diabatic heating, interactions with Rossby waves propagating along the subtropical jet, and instabilities and nonlinearities, the circulation of the AMA is unsteady. Conventionally, the AMA is characterized as being bimodal with two preferred centers, i.e., the Tibetan High and the Iranian High. This impression can be misleading, however, because the AMA usually contains multiple distinct regional anticyclonic sub-vortices. Using 39 years of the ERA-Interim reanalysis we show that a single circulation center is present only 14% of the time, while two to four sub-vortices are simultaneously present 81% of the time. More than four simultaneous sub-vortices are uncommon. Observed behaviors of the regional 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 and dissipation of a vortex. We discuss the role of these different processes in entraining and detraining air from the AMA.