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CUSAT develops method to predict monsoon onset over Kerala

Vinson Kurian

Thiruvananthapuram , May 29

RESEARCH scientists at the Centre for Monsoon Studies of the Cochin University of Science and Technology (CUSAT) have developed an objective method to predicting three to four weeks in advance, the onset of monsoon over Kerala.

The model predicts an onset five to six days later than the normal this year, said Dr C.K. Rajan, Professor and Honorary Director of the Centre, and a co-author of the research paper. Other authors are renowned meteorologist and monsoon expert Dr P.V. Joseph, and Dr K. P. Sooraj. The research is part of a project taken up under the Respond Project funded by the Indian Space Research Organisation (ISRO).

The objective model for operationally determining the onset date assumes an observational timetable on evolving atmospheric parameters May onwards. Initially, the depth and strength of the monsoon current's westerly component in a `box' just south of Kerala will be daily monitored beginning May 5, till it reaches specific speeds and the desired levels.

If a possible onset date is found during the period from May 5 to 25, the model checks that it is the onset proper, and not the pre-monsoon rainfall peak or what meteorologists call a `bogus onset.' Finally, the model assesses the slow and steady movement of organised convection (vertical development of clouds) from the equatorial area to the latitudes of Kerala to confirm that the date chosen represents the real date for onset.

The India Meteorological Department (IMD) has determined the onset dates operationally every year, for more than 100 years. These are subjective estimates based primarily on the rainfall reports from rain gauge stations of the synoptic network. The research paper sees the process of the onset spread over a period of two intra-seasonal cycles lasting about 70 days in most of the years. Moisture gets pumped up into the atmosphere in a large area around India sufficient to trigger the onset of monsoon, a phenomenon of planetary scales. There is active interaction between the atmosphere and the North Indian Ocean as well.

Signals to the onset can be derived from the low-level wind field, formation of the convective clouds and the ocean surface temperature, which are all monitored by satellites.

Around 35 days to the onset, a spatially large area of deep convection forms near the equator south of the Bay of Bengal. This convective area moves to South-East Asia, marking the onset of the South China Sea monsoon 20 to 25 days prior to the onset over Kerala.

Thirty days prior to the onset, a large area of convection forms near the equator south of the Arabian Sea, which generates a Low Level Jetstream (LLJ) that crosses the equator close to the East African Coast. This convective heat source and the associated LLJ grow steadily in strength. At the time of the onset, the convective heat source passes through Kerala latitudes and the core of the LLJ is located just south of Kerala.

Sea Surface Temperature (SST) over the tropical Indian Ocean and monsoon onset are closely related. Seventy days before the onset, there is a warm pool over the Bay of Bengal and the area of active convection is to its south near the equator. Thirty days before the onset, as the Bay of Bengal SST cools, the warm pool is over the Arabian Sea and an active convection area forms south of it. This area of convection moves north and brings about the onset over Kerala.

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