Performance of August-September Indian monsoon rainfall when June-July rainfall is reported as being in deficit/excess

摘要

The Indian summer monsoon season (June to September) has been divided into two sub-seasons, viz., June + July (JJ) and August + September (AS). The performance of all-India AS rainfall has been examined when JJ rainfall is reported as being in excess/deficit, during the period 1871-2012. The study shows that the performance of AS rainfall is higher/lower than the JJ rainfall when JJ rainfall is in deficit/excess. The mean values of JJ and AS rainfalls are computed when JJ rainfall is in deficit/excess. The difference between the mean values is statistically significant. Sea surface temperature (SST) data over the Arabian Sea and Bay of Bengal for the period 1941-2011 have been used to examine the relationship between JJ SST and JJ, AS and AS - JJ (AS minus JJ) rainfalls. A statistically significant negative correlation is observed between JJ SST and JJ rainfall while significant positive correlation is observed between JJ SST and AS - JJ rainfall. The sub-seasonal relationship has been explained through the changes in the surface meridional temperature gradient over the Indian land and Oceanic regions in the JJ sub-season due to the performance of rainfall during the JJ sub-season. Wind data at 850 and 200 hPa levels for the period 1949-2012 have been analyzed to examine anomalies during deficit and excess JJ years. During deficit JJ rainfall years, easterly wind anomalies are observed at lower tropospheric levels over the Arabian Sea, which influence SST distributions. The SST distribution strengthens the meridional temperature gradient for developing the strong monsoon circulation during the AS sub-season thereby increasing the AS rainfall. A reverse situation is observed during excess JJ sub-seasons.