Sunday, October 16, 2011

La Nina saved monsoon, promises good winter rain

Erratic Pacific temperatures stumped the Met's predictions as a resurgent La Nina spurred a late monsoon revival and looks set to deliver a double bonanza by way of plentiful winter rains.
The La Nina, a phenomenon signaling lower ocean temperatures believed to aid the monsoon, also promises normal or below normal temperatures in the next three months raising hopes of cooler weather.

Changes in Pacific waters helped overcome under par monsoon conditions and weak and irregular formations over the Bay of Bengal and led to a surge, often seen in concentrated bursts of rainfall as in Delhi, resulting in 101% of the long-term rainfall average.

The Indian Meteorological Department (IMD) accepted that "most of the operational long range forecasts issued for the 2011 southwest monsoon underestimated actual rainfall and therefore were not very accurate". The Met's June forecast was 95% while its initial 98% forecast in April was closer.

"The June prediction was based on weakening La Nina conditions. By February-March the temperatures over the Pacific Ocean had started rising and neither La Nina nor El Nino were prevailing since May. But around August La Nina conditions became favourable," said Ajit Tyagi, director-general of IMD.

IMD is not certain that the changes La Nina conditions were solely responsible in revving the monsoon but they definitely had a role, feels Tyagi.

With La Nina conditions still strong, Met officials say the country can expect a good northeast monsoon. The coming three months will also witness normal or slightly below normal temperatures.

The recovery in the second half of the season was also due to western disturbances or their interaction with weak monsoon systems over the Bay of Bengal. Delhi recorded 618.5 mm rainfall against a normal 648 mm overcoming a deficiency of between 80-100 mm between June and mid-August. Most of this rain was recorded over 12-15 days between June and September.

IMD director B P Yadav said WDs are usually not so strong or so frequent between June and August and most of the rainfall is usually a result of strong monsoon systems originating over the Bay of Bengal. "This year there were barely any such systems. Delhi, Rajasthan and adjoining Punjab were among the most deficient areas until September when there were three spells of heavy rain," he said.

This year there were more cyclonic systems over land areas that benefited Chhattisgarh and MP while other factors were a series of mid-tropospheric cyclones over Gujarat and the west coast.

"So far we only have the symptoms and a deeper diagnosis is required. The system is yet to withdraw from the entire country and we are in the process of analyzing the phenomenon," said Tyagi.

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