New Delhi: Scientists at the Indian Institute of Tropical Meteorology (IITM) have made a breakthrough in predicting monsoon rainfall over India after studying more than half-a-century of data for the June-September rainy season.
The predictor, the scientists said after studying data from 1960 to 2013, could forecast monsoon rainfall with a high degree of accuracy in years when the El Nino weather phenomenon, associated with droughts in South Asia, is weak or absent.
The accuracy of the forecast for the monsoon season by the India Meteorological Department is crucial for policymakers, farmers and for the wider economy in a country where more than half the farmlands are rainfed. In 2012, then earth sciences minister Ashwani Kumar told lawmakers in Parliament that the accuracy of Long Range Forecasting for seasonal rainfall was about 50% during 2007-11.
Normally monsoon prediction uses mathematical models based on physical factors, one of which is El Nino. El Nino Southern Oscillation (ENSO) refers to warm-cold cycles of sea surface temperatures of the tropical central and eastern Pacific ocean.
The cycles which develop off the western coast of South America cause several climatic changes across the tropics and subtropics. Despite research to explore the ENSO–monsoon connection for seasonal rainfall prediction, it’s considered a weak influence on the South Asian monsoon region, making it an ineffective tool.
“The predictability of Indian summer monsoon, so far, has been considered primarily to arise from tropical sea surface temperature (SST) associated with the ENSO. However, ENSO only explains about 40% of inter-annual variability of the Indian monsoon rainfall," explained B.N. Goswami, one of the authors of a research paper published by an IITM team of scientists in the Quarterly Journal of the Royal Meteorological Society in May.
One hundred years ago, several years that did not experience the El Nino weather phenomenon had weak monsoon rainfall, explained the paper’s lead author and IITM scientist Rajib Chattopadhyay. Hence his team of scientists tried to search for the source of variability. “Our breakthrough was when we found out that predictability can also come from extra-tropical regions which mean northern Pacific and northern Atlantic to be specific," said Chattopadhyay.
The IITM team observed in the paper that strong extra-tropical sea surface temperature anomalies in the North Pacific and North Atlantic along with weak tropical sea surface temperature anomalies were responsible for the above-normal rainfall over India during 2013.
The scientist explained that while this predictor was already used in India’s mathematical seasonal prediction, their study confirms the importance of this factor in predicting rainfall.
The team from the seasonal prediction group of IITM used historical meteorological data and modern modelling techniques to investigate how variations of sea surface temperatures from the extra-tropical regions affected the Asian monsoon rainfall.
“These sea surface temperatures will prove to be important predictors on those years when El Nino is not there. You might recall 2013 when monsoon was above normal. That time, the sea surface temperatures in North Pacific and Atlantic proved to be important predictors," said Chattopadhyay.