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Vinson Kurian

Recently in Tokyo The year 2009 Indian monsoon could forfeit the likely gains from La Nina conditions in the equatorial east Pacific in the context of a developing negative Indian Ocean Dipole (IOD) event of seesawing sea surface temperatures.

The monsoon is known to thrive when rear-ended by a positive IOD, which arises from an anomalous warming of the West Indian Ocean in contrast with a cooling anomaly to the east.The pattern is reversed during a negative IOD.

La Nina, on the other hand, cools down the east equatorial Pacific while warming up the west to aid convection and precipitation, a condition generally considered beneficial for the June to September monsoon. There is no evidence to suggest a cause-effect relationship between these two, but a negative IOD could literally pour cold water on prospects of expected good tidings by weaning away moisture into the southeast. This provides for a significant shift from the recent past, coming as it does after a rare third-in-a-row positive IOD event helped bring about a ‘normal’ monsoon as late as even the last year.

Researchers assembling at an international symposium hosted by the Japan Agency for Marine-Earth Science and Technology (Jamstec), where this correspondent was invited to make a presentation, said ocean conditions were being continuously monitored for the IOD signals. The correlation between the IOD and a concurrent Indian monsoon has been showing an increasing trend, according to researchers at Jamstec and the sibling Frontier Research Centre for Global Change (FRCGC), which discovered the IOD phenomenon. The high correlation observed in the early sixties is attributed to intense IOD events during 1961, 1963, and 1967, which strongly influenced the monsoons. But there were not many strong IOD events till the 1980s and the correlation values were very weak during this period. Return of intense IOD events in the 1980s and 1990s have helped re-establish the correlation. When the IOD event occurs in the absence of an intense El Nino/La Nina, it can strongly influence the season’s rainfall. The La Nina event evolving this year has not yet shown signs of rapid intensification.

Meanwhile, minimum temperatures are likely to fall by another 1-2 deg C over the Indo-Gangetic plains during the next two days and increase thereafter as an incoming western disturbance weighs in over the region. As for the south peninsula, the European Centre for Medium-Range Weather Forecasts has been indicating some activity over the southwest Bay of Bengal late this week and early into the next (February 4-11) with occasional rains for extreme south peninsula. According to the Climate Prediction Centre of the US National Weather Services, there is an increased chance for above-average rainfall for parts of equatorial Indian Ocean (adjoining south Sri Lanka), Indonesia, Malaysia, the Philippines and northern Australia during the week ending Monday next.

This is being attributed to the combined effect of a passing upper-level Madden-Julian Oscillation (MJO) wave, which amplifies weather over regions below its footprint, and the neutral-to-La Nina conditions in the equatorial Pacific. The MJO wave tracks west to the east periodically, and has alternating dry and wet phases. It is known to have crucial influence on the performance of the southwest monsoon, responsible for precipitating strong intra-seasonal variations.

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