Sea surface temperatures in the equatorial Pacific Ocean have not behaved in the expected manner in the last couple of months, once again affecting rainfall prospects in India this monsoon season.
The Indian Meteorological Department had forecast an above normal rainfall this season — 104 per cent of the long-period average — but with just 10 days remaining in the season, the overall rainfall, in the country as a whole, is five per cent below normal.
“We expect some good rainfall in the remaining days in some parts of the country, especially in the southern peninsula but it is unlikely that the five per cent deficiency will be compensated. So, most likely, the season will end with about 2-4 per cent deficiency. That would keep the seasonal rainfall in the ‘normal’ category,” D S Pai of the IMD said.
Rainfall between 96 and 104 per cent of the long-period average is categorised as ‘normal’.
Pai said a major reason why the country had received less than expected rainfall this season is the lack of strength of the La Nina phenomenon in the equatorial Pacific Ocean. La Nina, or the abnormal cooling of sea surface temperatures in the Pacific Ocean, is opposite of the El Nino phenomenon, and is known to help the Indian monsoon.
“From April onwards, not just our own models, but all global models indicated the early development of La Nina. That did not happen. Even now, we observe a situation that is close to weak La Nina conditions. The assistance that the monsoon would have received from that kind of condition prevailing in the Pacific Ocean has been lost. The result is a less than expected rainfall,” Pai said.
The latest bulletin from the Climate Prediction Centre of the National Oceanic and Atmospheric Administration (NOAA), released on Monday, says neutral conditions were prevailing in the Pacific Ocean as of now. “Negative (sea-surface) anomalies (a reference to cooler temperatures and La Nina) strengthened during March-May 2016, followed by weakening in June and July 2016. Since mid-July, negative anomalies have gradually weakened,” the bulletin said.
The Indian Ocean Dipole (IOD), a phenomenon similar to El Nino and La Nina but closer home, in the Indian Ocean, has also been strongly negative in this monsoon season, hampering the rainfall.
“If these two conditions (La Nina and IOD) had remained favourable, we would have seen much better rainfall in the second half of the season,” Pai said.
The country had received more than 100 per cent rain in the first two of the four-month June-September season, but August returned an eight per cent deficiency.
However, the withdrawal of monsoon has been delayed and some more rainfall is still expected in many parts of the country, even beyond this month. The withdrawal of the monsoon begins in the first week of September and is complete by the end of the month. But three weeks into the September, the withdrawal has happened only from the extreme northwestern parts of the country.
Pai said the withdrawal of the monsoon looked like it would be stalled for another week at least.