Objective Detection of Indian Summer Monsoon Onset Using QuikSCAT Seawinds Scatterometer

Surface winds from the QuikSCAT scatterometer over a region parallel to southern peninsular (SP) India are analyzed to monitor the daily evolution of the monsoon. For this purpose, the daily flow direction (DFD) and the corresponding optimal angle are derived from scatterometer winds for the period 2003-2009. These are correlated with the modern era retrospective analysis for research and application reanalysis land surface temperatures averaged over the SP. In the time series for DFD, positive DFD period is wet monsoon with a well-defined onset, followed by a short wet-to-dry transition where DFD is mostly negative and decreasing, then negative DFD period is dry monsoon, and followed by a dry-to-wet transition during which the sign and amplitude of DFD alternates daily between positive and negative. The correlation of DFD with averaged land surface temperatures shows that maximum temperature is achieved during the dry-to-wet transition season and is more pronounced in the years 2004-2007. Though 2005 and 2006 have the longest period of precursor cooling, in 2008 the land surface temperature continues to increase right up to the onset of monsoon. The hypotheses derived from these are: every year, the land surface cooling begins weeks before the monsoon onset over Kerala (MOK); the potential for convection over land is most favorable where DFD is positive, and is inhibited when negative. This study also reveals that one can extract the onset dates with a standard deviation of 3-4 days from these time series data sets, and the latter are used to objectively define the MOK.