Atlantic Nino to play tricks with Indian summer monsoon

Move over El Nino; the Atlantic Nino is here to play tricks with Indian summer monsoon.

The little-known weather phenomenon has slowly risen its head in the last four decades to now come to a stage when weather scientists feel that its influence should not be ignored in order to have more accurate monsoon forecast.

Much like the dreaded El Nino which involves unusual warming (or cooling) of the Pacific Ocean, the Atlantic Zonal Mode – more popular as Atlantic Nino – is unusual warming (or cooling) of the Atlantic Ocean. For years, it was known to influence the weather in Africa.

But a new analysis of the 100 years of weather data suggests that the AZM cast its shadows on the Indian summer monsoon too with a visibly increasing trend since 1975 coinciding with the arrival of global warming. The impact is more on those years when El Nino is absent.

“The tele-connection between Indian summer monsoon and Atlantic Zonal Mode particularly strengthened since 1975. This strengthened relationship is concurrent with the increase in variability of sea surface temperature over the tropical Atlantic Ocean connected with the global warming scenario. For seasonal monsoon forecast, this weather phenomenon should be factored into,” said R S Ajay Mohan, a weather scientist at the New York University, Abu Dhabi and the leader of a team that researched on the Atlantic Nino.

In a recent study published in Geophysical Research Letters, Ajay Mohan and his colleagues at the University of Wisconsin-Madison examined India Meteorological Department's high-resolution weather data between 1903 and 2010.

The data was grouped into three periods: 1903-1938; 1939-1974 and 1975-2010 to check whether the Atlantic Nino told upon the monsoon.

“While some weak links were seen between 1939 and 1974, the relationship gets particularly strengthened 1975 onward. The models show vast tracts in western India may be influenced by the AZM in the future. The tele-connections of Indian summer monsoon are changing and one single phenomenon like the El Nino (ENSO to be more accurate) and Indian Ocean Dipole are not sufficient for an accurate monsoon prediction,” Ajaymohan told DH.

IMD currently uses a statistical model for its long-range monsoon forecast. But factors like El Nino and IOD (temperature sea saw of the Indian Ocean) are taken into consideration before the operational forecasts are announced.

In the four summer months between June and September, India receives 80% of its annual rainfall, which drives the trillion dollar economy.

Quoting an earlier study, he said if Atlantic Nino was not factored into monsoon forecast, the predictions would not be very accurate.