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Close on the heels of the India Meteorological Department (IMD) predicting a normal monsoon this year, an eminent scientist and researcher has said that that its onset over Kerala could happen a week earlier than normal.
PV Joseph, himself a former director of IMD and often consulted by it even now, told BusinessLine that he based the timeline of onset on what has come to be known as the ‘pre-monsoon rain peak’ phenomenon. The ‘pre-monsoon rain peak’ is represented by a band of raining cloud extending from Kerala eastwards into the Bay of Bengal.
Joseph’s pioneering work on it is among the key parameters considered by the IMD for its forecasts.
Rain cloud band
The ‘pre-monsoon rain peak’ can form any time between the first week of April and the second week of May. Its timing gives prior indication of the date of monsoon onset over Kerala.
For a normal monsoon onset that occurs on June 1, the raining band of cloud in the Bay of Bengal should form around April 21, Joseph explained.
In this context, he added that the rains that Kerala (and the South Bay of Bengal) had experienced during the last week (April 8 to 15) was associated with the pre-monsoon rain peak.
Early onset seen
Based on this, the monsoon onset over Kerala is forecast to occur about a week earlier than normal this year, he said. The normal or long period average date of monsoon onset over Kerala is June 1.
The onset forecast is supported by the observation that during the last several weeks, the ocean surface temperature of Indian and Pacific Oceans was colder than normal South of the Equator and warmer than normal to the North.
This would make the Inter tropical Convergence Zone (ITCZ) cross the Equator and move into the Northern Hemisphere earlier than normal in the Indian Ocean as well as the adjacent West Pacific Ocean.
The ITCZ, a zone of ascending air, maximum clouds and heavy rainfall along the Equator, merges with the monsoon circulation and sets up the monsoon trough to drive the seasonal rains into India.
Bay ‘low’ soon?
On occasions in the past, the raining cloud band represented by the pre-monsoon rain peak and associated westerly winds have caused the genesis of a depression or a cyclonic storm in the Bay.
On Monday, a weather tracker of the US Climate Prediction Centre has hinted that the Bay could likely witness some activity (low-pressure area) over the next eight days.
This would be in response to the bigger churn happening just to the South (Equatorial Indian Ocean), but the US model tends to discount the prospects of formation of cyclone just yet.
What it says instead is that the system would move along a North-North-West track from East Bay of Bengal and head towards South Odisha-North Andhra Pradesh coast by May 2.
If this were to happen, it would bring a decent amount of rainfall to Odisha-Andhra Pradesh. But the Arabian Sea and the Bay would need to be watched during the remaining 20 to 25 days for signs of any churn ahead of monsoon onset.
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