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The onset of South West monsoon over Andaman and Nicobar Islands may have missed the normal May 15-20 timeline this year, but is only in keeping with similar aberrations witnessed in the past 100 years.
Based on data available from 1901 to 2000, deviation of onset dates from the normal are marginal except for Andaman and Nicobar Islands where it has been greater (late onsets, that is) by about five days.
SHIFT IN MONSOON
This was stated here by Mr K. Santhosh, Director and Head, Meteorological Centre, Thiruvananthapuram, while addressing a brainstorming session on SouthWest monsoon. The mean withdrawal dates for the monsoon are found to be later than the existing normal with respect to the 30-year slots of 1941-1970 and 1971-2000 by about seven to 10 days, he added, quoting from various studies.
A general late onset coupled with a late withdrawal suggests a shift in monsoon activity.
Decadal and epochal variability indicate near-30-year periodicity in onset, withdrawal and duration of monsoon.
However, annual an all-India June to September rainfall data for the country as a whole for the period 1901 to 2008 does not show any significant trend.
Rainfall for the country as a whole for the same period for individual monsoon months also does not show any significant trend. When it comes to individual meteorological subdivisions, at least three – Jharkhand, Chhattisgarh and Kerala – show a significant decreasing trend.
Eight Met subdivisions show significant increasing trend, and these are Gangetic west Bengal, west Uttar Pradesh, Jammu and Kashmir, Konkan and Goa, Madhya Maharashtra, Rayalaseema, Coastal Andhra Pradesh and north interior Karnataka.
Sub-divisional rainfall data for individual monsoon months also provide for a profile marked by worryingly fluctuating trends.
A significant increasing trend is being observed in June rainfall for the Western Ghats and southwestern parts of the country but significant decreasing trend for the central and eastern parts.
JULY RAINFALL
The crucial July rainfall quantum has witnessed significant slippage over most parts of the central and peninsular India but equally significant surplus in the Northeastern region.
August realisation have significantly increased over the sub-divisions of Konkan and Goa, Marathwada, Madhya Maharashtra, Vidarbha, west Madhya Pradesh, Telengana and west Uttar Pradesh.
September rains have significantly contracted for the sub-divisions of Vidarbha, Marathwada and Telangana while they have increased over sub-Himalayan West Bengal, Mr Santhosh added.
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