Weak monsoon due to warmer Indian Ocean, say scientists

Despite its early onset over Kerala, the southwest monsoon played truant for most of June this year due to unusual heating up of the Indian Ocean as compared to the Bay of Bengal, senior meteorologists P A Francis and Sulochana Gadgil have found.
Indian monsoon is sustained by formation of clouds over a warm Bay. Normally, the sea surface temperatures in the Bay are slightly higher than sea surface temperatures in the equatorial Indian Ocean region.

Analysis of sea surface temperatures for June showed that the Bay was colder than the equatorial Indian Ocean which was warmer than average.
"The large deficit rainfall over the Indian region in June 2009 was associated with the suppression of convection over the Bay of Bengal," Francis and Gadgil wrote in Current Science.

Francis and Gadgil are senior scientists at the Indian National Centre for Ocean Information Services and the Indian Institute of Science, respectively.
The country received 48 per cent deficit rains in June while the overall monsoon deficiency for the season was 23 per cent.

During June 1-24, the convection over the Bay appeared intermittently over the region between 10 degrees North latitude and 20 degrees north latitude, the scientists said adding that each spell was only for about 3–4 days, and no northward propagations occurred in this period.
The scientists dubbed the phenomenon as unusual as it was for the first time such conditions had extended over a three week period.
Convection is a process where the excess water vapour in rising air parcels condenses to form a cloud.

"No low pressure systems were generated over the head Bay and the westward propagation across the monsoon zone, which is characteristic of the onset phase of the monsoon, did not occur," Gadgil and Francis said.
This suggests that the massive deficit in the all-India rainfall for this period is a consequence of the suppressed convection over the Bay of Bengal, they said.
Similar conditions had existed earlier in 1995 when the Bay was cooler than eastern equatorial Indian Ocean for several days in June. Convection over the Bay was suppressed during that period also and the all-India rainfall for June 1995 was 24 per cent below normal.

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