Characteristics of Upper Surface of the Bay of Bengal to Indian Summer Monsoon Deficit in June

The Bay of Bengal (BoB) plays a vital role in pumping moisture to the monsoonal circulation during Indian summer monsoon period. In last two decades there are three years: 1995, 2009 and 2012 in which severe Indian Summer Monsoon (ISM) deficit occurred in the month of June (more than 20% deficit from the normal precipitation). In all these years we found significantly less cyclonic eddy activity in the northern BoB compare to that of the normal ISM years. It is found that during these years the strength of the spring Wyrtky Jet was reduced compare to that of the normal years. Hence, there was reduction in the Kelvin wave activities in the equatorial wave guide as well as in the Eastern Equatorial Indian Ocean. Thus, the propagation of coastal Kelvin wave to the BoB during premonsoon and monsoon time is reduced. Furthermore, it is found that intraseasonal winds in the BoB did not favour northward propagation of coastal Kelvin wave during premonsoon time of these years. The reduction in coastal Kelvin wave activities reduces the Rossby waves to the northern BoB, which results in decrease in cyclonic eddy activities. Hence, upwelling in the northern BoB is reduced during pre-monsoon season. Very less chlorophyll-a concentration at the northern BoB was observed in premonsoon time of these three years compare to the normal ISM years. Also, increase in oceanic heat content and deeper 23^o C isotherm depth (D23) in the northern Bay during premonsoon period is observed during above years. Reduction of upwelling deepen the D23 resulting in thick mixed layer and more heat content.