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, that is, 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 ocean-atmosphere 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.