A Potential Vorticity Perspective of the South Asian Monsoon Intraseasonal Oscillation

Concepts of atmospheric instabilities and potential vorticity conservation are fundamental in understanding basic atmospheric processes at high latitudes. However, in the tropics, where the amplitude of diabatic and frictional processes is large, these concepts are rarely used. Analyzing data from ERA-interim reanalysis and NOAA interpolated outgoing long wave radiation (OLR), we reassess the relations between potential vorticity and intraseasonal oscillations during the south Asian monsoon season and seek signatures suggestive of dynamical instabilities.

Despite the large height variations of isentropic surfaces in the presence of diabatic heating, potential vorticity anomalies on isentropic levels during boreal summer are shown to be significantly correlated with OLR. Positive anomalies in the northern hemisphere are associated with enhanced convection and vice versa. The vertical structure of these anomalies is found to be, perhaps surprisingly, barotropic. Composites relative to maximum convection over the Bay of Bengal reveal meridional elongated potential vorticity structures (significant to the 95% level) and anomalous westerlies on the 330K isentropic level which seems to be associated with the northward propagation of the convective envelope from the equator. Finally, to assess further the existence instabilities we decompose potential vorticity as an annual mean plus an intraseasonal perturbation and the statistical properties of these terms are analyzed.

Our study intents to contribute to a better understanding of the Intraseasonal oscillations which, we believe, will eventually lead to better predictions or to better understand inherent prediction limits over the South Asian region.