Contrasting pre-monsoon features associated with early and late onset of southwest monsoon over Kerala

Abstract

Contrasting features pertaining to a year each of early monsoon (2004) and late monsoon onset (2003) are examined during the pre-monsoon season. The weekly evolution of low level winds during the month of May and the monthly features of middle and upper level winds, Sea Surface Temperature (SST), Precipitable water, Outgoing Longwave Radiation (OLR) and mean sea level pressure gradients are analysed during both these years in the present study.

           The result shows that during the early onset year of 2004 the low level wind indicates strong cross equatorial flow with cyclonic flow from the beginning of May associated with negative SST anomaly over the southern Arabian Sea; whereas, during the late onset year of 2003, the low level flow indicates weak cross equatorial flow persisting upto the first week of June associated with positive SST anomaly over the  Arabian Sea. The middle and upper tropospheric flow indicate stronger Tibetan anticyclone during the month of May accompanied by very strong easterly anomalies during the early onset year of 2004 compared to that of late onset year 2003.  The convective activity during May was stronger over the Indian region compared to that over the south China Sea during early onset year of 2004, which is just the opposite during the late onset year 2003.  The precipitable water was higher than normal during May 2004 accompanied by very deep moist layer over the Indian region compared to that of May 2003. The meridional pressure gradient between southern and northern India were stronger than normal during the pre-monsoon months of April and May with earlier reversal of pressure gradients from winter to summer during the early onset year of  2004 compared to that of late onset year of 2003.