‘Despite delay, monsoon picked up due to positive EQUINOO’

The Indian summer monsoon picked up steam despite a delayed start due to a positive phase of Equatorial Indian Ocean Oscillation (EQUINOO), says a study by scientists from the Indian Institute of Science (IISc), Bengaluru, and the Hyderabad-based National Centre for Ocean Information Services.

The paper published on Tuesday in the peer-reviewed Current Science journal of the Indian Academy of Sciences described EQUINOO as a “seesaw” or an “oscillation” between enhanced cloud formation and rainfall over the western equatorial Indian Ocean (WEIO) and suppressed clouding over the eastern equatorial Indian Ocean (EEIO) west of Sumatra. A positive EQUINOO phase is when the surface sea temperature in WEIO is above 27.5 degree Celsius leading to enhanced clouding, which is then suppressed in the eastern equatorial Indian Ocean, the paper said.

As on September 9, the country has received 792mm of rainfall compared to the normal of 737 mm – or 3% more than normal – as per the India Meteorological Department (IMD). The southwest monsoon is normal to excess in 28 meteorological subdivisions out of the 36 subdivisions, and deficient in just 8.

The summer monsoon season was first marked by a delayed onset over Kerala followed by long and weak spell until June 20. By the end of the June, India faced a large rainfall deficit of about 33%, which the paper said was because of El Nino, a phenomenon caused when warm water from the western Pacific Ocean flows towards the east increasing the chances of a drought.

Sulochana Gadgil, lead author and emeritus professor, Centre for Atmospheric and Oceanic Sciences, IISc, said unlike June 2014, EQUINOO was favourable in June this year and fortunately El Nino weakened from June to July this year. “A favourable EQUINOO helped rainfall to pick up around June 22, and monsoon subsequently recovered with 4.2% above-normal rainfall in July. Rainfall has continued to be above the average continued in August and so far in September,” said Gadgil.

DS Pai, head, climate research services at IMD, Pune, said several low-pressure areas that over the Bay of Bengal moved from Odisha coast to Gujarat and even Rajasthan kept the monsoon trough almost across central India. “As a result till August, central India and southern peninsula received a lot of rain with slight deficiency in northwest India ,” he said.