The South West monsoon has large variability in three important aspects — onset of the monsoon over the Kerala coast (MOK), different phases of monsoon activity within the lifecycle of the monsoon (active, weak and break in monsoon conditions) and the amount of rainfall received during the monsoon season.
In a paper published in the September issue of Meteorological Applications journal, Mr. M.R. Ramesh Kumar and his team have investigated the influence of low pressure systems (convective systems) which form over the North West Pacific (NWP) Ocean on monsoon activity over the Indian subcontinent. Mr. Kumar is Deputy Director, Physical Oceanography Division, National Institute of Oceanography, Goa.
They compared various parameters of the Indian South West monsoon conditions during excess rainfall years and deficit rainfall years in the light of convective activity over the North Pacific Ocean (0-50 degrees N and 100-180 degrees E) during the season (June to September).
The time duration of the NWP systems during 45 days before and 45 days after MOK were investigated to understand the relationship between the two. The study found that during excess monsoon years, formation of low pressure systems (cyclogenesis) did not occur over the NWP about 25 days prior to and after the MOK, barring an isolated case.
In contrast, during deficit monsoon years only the first 15 days were free from cyclogenesis. The number of days of all convective systems that formed during deficit years was also more, compared to that of excess years.
The study period
An examination of the study period (1951-2003) for the deficit and excess years shows that the mean of the number of NWP systems was much greater than that of Bay of Bengal (BB) systems for deficit years and reverse was the case during excess years.
Different characteristics of the two types of systems were examined, such as frequency, geographical location, duration, and direction of movement during excess and deficit monsoon years.
A statistical analysis of the characteristics for the four monsoon months (June to September) for the excess and deficit years showed that only few systems formed over BB during the deficit years compared to NWP systems and vice versa in the case of excess years.
The number of NWP systems was 1.83 times more than BB systems during deficit years and in excess years the number of BB systems was greater.
In another analysis, of the convective systems that formed in the peak monsoon month of July in excess and deficit years, it was found that during excess years a majority of NWP systems formed close to the East Asian coast.
In deficit years the majority of systems formed much far away from the East Asian coast. According to Mr. Kumar this is the reason for monsoon deficit conditions as the NWP systems would have to travel a long way to reach and transport moisture to the Indian subcontinent and are unlikely to do so.
Another important finding was that in an analysis of evaporation rates in the Arabian sea for two contrasting monsoon years — 2002 (deficit) and 2003 (excess) — the rates were lower during active monsoon conditions and higher during weak conditions implying that Arabian sea evaporation is not important for the ensuing monsoon activity over the Indian subcontinent.
Moisture transport, into the subcontinent was less during 2002 compared to 2003.
Further, a study of averaged zonal low level (1.5 km altitude) wind flow for peak monsoon months of July and August of the excess and deficit years showed that there was less wind flow into the subcontinent in deficit years and more in excess years.
“The number of systems that formed in the Bay of Bengal was only two during the season of 2009 compared to the 15 that formed in NW Pacific,” said Mr. Kumar.
“Hence we feel that the role of systems in NW Pacific was very important in the monsoon activity of Indian subcontinent. This could be the cause of deficit or drought-like conditions over India.”