Satellite-observed extratropical forcing of southern hemisphere and enhancement of monsoon systems over the Indian region

In this paper, satellite-observed wind forcing in the form of strong surge of cold air through Mozambique channel and along east African coast, under the influence of eastward propagating baroclinic wave disturbances over southwest Indian ocean has been studied during various monsoon seasons. Further, its impact on the intensification of the monsoon system over the Arabian sea through the excitation of low level jet and associated cross equatorial flow has been investigated. For this purpose, satellite-observed cloud imagery obtained from TIROS-N, NOAA, DMSP, GOES and INSAT satellites during 1979 and 1988-99 have been utilised for finding the latitudinal positions of the cold front across south African Malgassy region (i.e.30S, 40E). GOES(IO) and INSAT derived low level cloud drift winds in the domain 2.5 S to 2.5 N and 40-50E have been used to observe the triggering of low level jet and associated cross equatorial flow. Daily rainfall of ten stations viz. Tiruvanathpuram, Alleppy, Cochin, Kozikhode, Mangalore, Karwar, Goa, Ratnagiri, Mumbai and Dahanu have been used for finding the modulation of rainfall activity along the west coast of India.

From the results of the study, it is inferred that there is an abrupt increase in the strength of low level wind flow along the east African coast 2 to 3 days after the first appearence of northward propagating cold front across south African Malagassy region. This is followed by the intensificaion of the rainbearing monsoon systems (e.g. ITCZ, lows, depressions, off shore troughs etc.)over the Arabian sea giving increase in rainfall activity along the west coast of India. Therefore, it is suggested that above association between northward propagating cold front across south African Malgassy region and excitation of low level cross equatorial flow can be linked to forecast an increase in rainfall activity along the west coast of India 5-7 days in advance.