In a different approach for rainfall forecast, scientists of Indian Institute of Tropical Meteorology (IITM) and Indian Institute of Science Education and Research (IISER) have used variations in sea temperatures to predict monsoon in the subcontinent.
A team from the Seasonal Prediction Group of IITM and IISER, Pune used historical meteorological data and modern modelling techniques to predict monsoon pattern.
They have investigated how variations (anomalies) of sea surface temperatures (SSTs) from the extra-tropical, i.e. temperate, latitudes in the Northern Hemisphere, affected the Asian monsoon rainfall.
Traditionally, seasonal rainfall in the tropics have been predicted based on extended periods of warm-cold cycles of sea surface temperatures of the tropical central and eastern Pacific ocean.
This is referred to as the slow coupled oceanu2013atmosphere mode called the El Nino and Southern Oscillation or ENSO.
The cycles develop off the western coast of South America and cause a broad range of climatic changes across the tropics and subtropics, through a process dubbed as ‘teleconnection’.
The extra-tropical SSTs affect the north-south tropospheric temperature gradients in the Asian monsoon region via their effects on local jet streams — fast flowing air currents at around 10 km above sea-level.
The research group carefully analysed annual rainfall and associated SSTs in each monsoon season from 1960 to 2013. They found that the 2013 monsoon season shared similar data parameters with that of the year 1961.
Both these years witnessed a higher than normal rainfall and also had a weak El Nino and Southern Oscillation (ENSO) influence. The team showed that when equatorial SSTs associated with the ENSO mode were weak, the SST anomalies over the north Pacific and north Atlantic provided an additional reference point for predicting the south Asian monsoon.
“We find that the extra-tropical SSTs are ineffective when the tropical SST anomalies associated with ENSO are strong as during a mature El Nino or a La Nina. However, we also identify two patterns of extra-tropical SSTs that could drive monsoon rainfall above normal or below normal; and occur when ENSO is going through a transition from El Nino to La Nina or vice versa,” said scientist Rajib Chattopadhyay, the lead author of the paper.
“Although much research has gone into trying to exploit this ENSOu2013monsoon teleconnection for seasonal rainfall prediction, its relatively weak influence on the south Asian monsoon region has rendered it an ineffective tool,” said the paper which was published by the scientists in the Quarterly Journal of the Royal Meteorological Society.
“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 per cent of inter-annual variability of the Indian monsoon rainfall,” added B N Goswami, an author of the paper.
Future work by the group will now focus on including more parameters such as the Atlantic Multidecadal Oscillation and the Pacific Decadal Oscillation — both of which are robust, recurring patterns of ocean-atmosphere climate variability, and are expected to affect their model’s accuracy in predicting the Indian summer monsoons.