6:15 PM - 7:30 PM
[ACG37-P03] Seasonal Variations of the Mascarene High and Related Changes in Jetstreams and a Stormtrack
Keywords:subtropical high, Indian Ocean, Agulhas Return Current, SST front, jetstream, stormtrack
The subtropical high in the Southern Indian Ocean, called the Mascarene high, is an integral part of the climate system there, influencing not only weather conditions in the surrounding regions but also the oceanic state. The present study examines the mechanisms for the seasonal variations of the Mascarene high. The high resides over the eastern portion of the basin in summer, while it shifts westward in winter toward the Agulhas storm-track core in strengthening. This large seasonal displacement is a distinct feature of the Mascarene High from other subtropical highs. Our analysis reveals that, while low-level thermal contrasts between the Australian continent and southeastern Indian Ocean is important for the formation of the high in summer, its wintertime formation is owing primarily to eddy-feedback forcing due to the seasonally-enhanced storm-track activity that is maintained in the presence of pronounced SST gradient along the Agulhas Return Current. In winter, the mid-tropospheric subsidence over the surface high is associated with upper-tropospheric convergence of the cross-equatorial divergent flow, indicative of a connection between the high and the Asian summer monsoon. From the viewpoint of vorticity budget, the cyclonic tendency by the upper-level convergence is balanced with the westerly advection of the anti-cyclonic vorticity. While the converging upper-tropospheric flux of Rossby wave activity from lower and higher latitudes acts to reinforce the high in winter, the high itself acts as a source of the climatological-mean planetary waves with the net local divergence of the flux, which is suggestive of the importance of the high even on the hemispheric scale.