Global models see return of La Nina during Sept-Nov

Vinson Kurian Thiruvananthapuram | Updated on July 21, 2021

Low-pressure area likely to develop over Bay of Bengal by July 27, says IMD

Global climate models suspect that the ‘neutral’ conditions (neither El Nino nor La Nina) in the Equatorial Pacific might give way to a return of La Nina conditions into the autumn and winter even as the annual monsoon in India, a known La Nina beneficiary, readies to cross the half-way stage.

La Nina conditions last year had helped the Indian monsoon to a bumper season. But the first several months of this year had seen sea-surface temperatures (SSTs) in the tropical East Pacific Ocean trending towards normal, ringing in an ENSO (El Nino-Southern Oscillation)-neutral conditions.

Indian Ocean Dipole phase

Closer home, a similar seesawing of SSTs s currently on over the Indian Ocean with the East Indian Ocean warming relative to the West (negative Indian Ocean Dipole, IOD), not the ideal setting for a concurrent Indian monsoon. It remains to be seen how the Indian Ocean responds to the changes in the Pacific.

The monsoon has thrived during a positive IOD as best evidenced in 2019 when it drove itself up to 110 per cent of normal and extended the season into mid-October (against the September-30 normal). The prolonged stay over India delayed its arrival over Australia, triggering setting off wild bush fires there.

La Nina watch declared

The US Climate Prediction Centre and International Research Institute for Climate and Society have declared a ‘La Nina watch’ already and sees a 51 per-cent chance of ENSO-neutral state being maintained during August-October with La Nina potentially emerging during September-November.

Model predictions for the causative ENSO pattern collated by the above agencies suggest that resurgence of La Nina conditions may already be under way. They track SST anomalies in what is called the ‘Nino 3.4’ region of the Equatorial Pacific for nine overlapping 3-month periods for confirming.

The ENSO is a recurring climate pattern involving changes in SSTs in the Central and Eastern Tropical Pacific Ocean.

El Nino means warmer waters in East Equatorial Pacific (cooler in West), while a La Nina represents cooler waters in the East and warmer in the West (closer to India and South-East Asia).

Warm waters pack a great amount of latent heat and generate massive convection leading to cloud formation and storminess. The Pacific Ocean is the largest on the planet and accounts for more than 30 per cent of its surface. SST patterns here have a tremendous influence on global weather and climate.

Back-to-back lows

On Wednesday, outlook for full-blown monsoon conditions for the country for the rest of July received a further boost with India Meteorological Department (IMD) signaling that the Bay of Bengal may host another low-pressure area by July 27 on the back of one expected to form on Friday.

The second one in the back-to-back formations may show up over the North Bay around the area ceded by the first one and will trigger widespread rainfall with isolated heavy to very heavy falls over a likely saturated East, adjoining Central and North-West India including the hills and plains of the region.

Published on July 21, 2021

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