The Hindu Business Line : Weather trends throw up confusing signals

Thiruvananthapuram, April 15 A prevailing western disturbance in northwest India, a typhoon-in-making in the South China Sea basin and a decaying La Nina have combined to throw up confusing signals for evolving weather over the sub-continent.

The prevailing western disturbance and offspring thunder squalls/hailstorms have doused parts of north and northwest India and wrecked the build-up of the seasonal ‘heat low’ for the time being. But mercury is forecast to reverse the falling trend and touch the 40s over parts of mainland after April 18.

India Met Department has warned of isolated thunder squall/hailstorm events over the Jammu division of Jammu and Kashmir, Himachal Pradesh, Uttarakhand, Haryana, Chandigarh, Delhi, West Uttar Pradesh and Rajasthan during the next 48 hours.

The westerly flows have twisted out of shape the ‘line of wind discontinuity’, which now swings away to drop into the Arabian Sea and into Lakshadweep through Vidarbha, Marathwada and north and coastal Karnataka. This ‘line’ is an evolving trough where winds from opposing directions converge to create unstable weather.

TROPICAL STORM NEOGURI

Meanwhile, Tropical Storm Neoguri roaring across the South China Sea may just about churn to typhoon status, but models differ as to the direction it might take for lateral movement. A west-northwest movement would cause it to make a landfall over Vietnam, leaving behind a likely steaming trail in the Bay of Bengal.

This could in turn go to set up the eagerly awaited pre-monsoon ‘low’ in the Bay, but there are other models suggesting that Neoguri could run into invading westerlies to be blown away north-northeast across the Chinese mainland.

DECAYING LA NINA

The third but significant meteorological event with implications for weather in these parts is the decaying La Nina, the colder counterpart of El Nino. The cooling trend over the eastern equatorial Pacific is breaking apart and might get ‘neutralised’ ahead of model projections.

La Nina also means concurrent warming of the western Pacific and enhanced convection that aids storm build-up and rain events. The Indian monsoon in the immediate neighbourhood has been observed to benefit from this scenario, though no direct cause-effect has been established.

Moderate La Niña conditions are present across the tropical Pacific Ocean, says an update by the International Research Institute (IRI) for Climate Prediction and Society at the University of Columbia.

Recent sea-surface temperature trends and model forecasts indicate La Nina will continue for the next three months.

MODEL SPREAD

Thereafter, there is considerable spread in the models, with nearly one-half indicating La Nina could continue well into the second half of the year, the IRI said.

According to the Australian Bureau of Meteorology (BoM), the event has continued to weaken during the past two weeks. While western Pacific surface temperatures remain typical for La Nina, eastern Pacific temperatures have now been out of range for the last five weeks.

These observations from the eastern Pacific show a faster decline in La Nina conditions than was forecast by most computer models. But the models do not suggest warmer El Nino conditions will return during 2008.