Mon­soon to be nor­mal, says IMD

The Hindu - - FRONT PAGE - Ja­cob koshy

The coun­try is likely to see “nor­mal” mon­soon rains, the In­dia Me­te­o­ro­log­i­cal Depart­ment (IMD) said in its fore­cast on Mon­day. How­ever ocean tem­per­a­tures in the Pa­cific and In­dian oceans, around Au­gust, pose con­cerns.

In­dia is likely to get 97% rain­fall, de­fined as be­tween 96% and 104% of the Long Pe­riod Av­er­age (LPA) of 89 cm it typ­i­cally gets be­tween June and Septem­ber. The IMD has ruled out chances of a “drought,” as­crib­ing only a 14% chance of ‘de­fi­cient’ rains and given a 42% like­li­hood of ‘nor­mal’ mon­soon rains.

In any given year, there’s a 16% chance of de­fi­cient and 33% odds of nor­mal rains.

Not al­ways ac­cu­rate

The IMD’s fore­casts in April aren’t al­ways ac­cu­rate. In April 2016, for in­stance, it said In­dia would get 106% of the LPA — or ‘above nor­mal’ rains in Met par­lance. In­dia ended up with 97% or ‘nor­mal’ rains.

It also under-es­ti­mated rains in 2015 and 2014, when In­dia ended up with deficits greater than 10%, or drought-like con­di­tions.

Last year’s fore­cast was more ac­cu­rate, with In­dia get­ting 95% as op­posed to the IMD’s 96% es­ti­mate.

De­tails, such as how the mon­soon will pan out over the coun­try and the quan­tum of rain­fall in July and Au­gust — the key mon­soon months — would be made avail­able in the depart­ment’s June up­date, said IMD chief K.J. Ramesh at a press con­fer­ence in New Delhi.

The Met depart­ment used a sta­tis­ti­cal model for ar­riv­ing at its 97% es­ti­mate, which has an in-built er­ror mar­gin of 5%. An­other model, called the dy­nam­i­cal model, fore­casts 99% rain­fall. This how­ever is still a work-in-progress and the IMD hopes that this model will be the main­stay of mon­soon fore­cast­ing in the com­ing years. The IMD bases its op­ti­mism of a nor­mal mon­soon this year on the ab­sence of an El Nino, a rel­a­tive warm­ing of the sur­face-tem­per­a­tures in the Cen­tral Pa­cific, and as­so­ci­ated with a weak mon­soon.

The con­verse phe­nom­e­non, of a La Nina and cool­ing of these wa­ters, gen­er­ally brings good rains. Cur­rently La Nina con­di­tions pre­vail and weather models sug­gest a slight warm­ing or so-called “neu­tral” con­di­tions to pre­vail around Au­gust.

“Our own out­look and weather models from var­i­ous in­ter­na­tional agen­cies sug­gest that an El Nino is un­likely dur­ing the mon­soon and neu­tral con­di­tions are to pre­vail at that time,” said Mr. Ramesh.

How­ever there’s a mass of warm wa­ter that lurks be­neath the West­ern Pa­cific and “…if dur­ing the mon­soon months that mass rises up and warms the sur­round­ing cold wa­ter, it could be a prob­lem,” said a se­nior me­te­o­rol­o­gist in­ti­mately fa­mil­iar with the IMD fore­cast-prepa­ra­tion process. “His­tor­i­cally a tran­si­tion from La Nina to neu­tral con­di­tions has gen­er­ally meant nor­mal mon­soons.”

An­other worry lurks closer home. A pos­i­tive tem­per­a­ture gradient in the West and Eastern In­dian oceans, called a ‘pos­i­tive In­dian Ocean dipole’, is as­so­ci­ated with neu­ter­ing warm-wa­ter hos­til­i­ties from the Pa­cific. Such a pos­i­tive dipole is un­likely this year.

“The IOD con­di­tions are among the rea­sons why we have fore­cast 97% and not 100% mon­soon,” said D.S. Pai, Chief Fore­caster, IMD.


Rainy days: The IMD bases its op­ti­mism of a nor­mal mon­soon on the ab­sence of an El Nino.

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