Onset and withdrawal of Indian summer monsoon

The onset of the Indian summer monsoon (ISM) is defined in terms of zonal asymmetric temperature anomaly and withdrawal is defined in terms of vertical wind shear between 850 hPa and 200 hPa. The onset and withdrawal dates determined by these two indexes are also consistent with the onset/withdrawal timing of the real monsoon circulation. The onset and withdrawal day values are also reasonably correlated with the strength of the monsoon and to some of its boundary forcings such as SST anomalies over the Nino-3 and Bay of Bengal regions. Thus the proposed criteria are both physically and practically viable to study the interannual variability of the Indian summer monsoon.