The Hindu Business Line : `Unstable' tropics a weatherman's nightmare

Tropical weather, especially the Indian monsoon, continues to be an enigma that has engaged the minds of the best meteorologists over the last few decades.

Thiruvananthapuram , April 25

THE constantly evolving weather in the tropics has tested the wits of meteorologists at the best of times, but rarely more revealing than now when the long-term forecast of the Indian monsoon is being attempted by various agencies.

Though the Indian Meteorological Department (IMD) has made bold to forecast a `normal' monsoon for the impending season, it cannot afford to sit back in the smug relief that the rain would fall to the desired pattern, spatially and temporally. The unstable nature of the weather in the tropics just does not allow them to relax.

Dr M. Rajeevan, Director-Forecasting, IMD, Pune, emphasised that the long-term forecast for the 2005 monsoon has been made on the basis of "some very early indicators''. He said this in response to a query whether there was any element in the forecast that precluded the chances of a repeat of last July's `washout'.

According to him, some crucial developments taking place during the next two to three months could well change the course - for instance, the warming over the Central Pacific as well as the Indian Ocean. "Therefore, we have to keep a careful watch of how these things evolve."

Of the eight parameters of the power regression and parametric forecast model, two were considered `unfavourable' for the progress of monsoon. These two - December Eurasian snow cover and January-February hPa wind pattern - suggest the role of the European climate anomalies on the system.

The threat of El Nino looming has been discounted for the time being, as conditions are reported to returning to `neutral' over the equatorial Pacific.

Suffice to say that tropical weather, especially the Indian monsoon, continues to be an enigma that has engaged the minds of the best meteorologists over the last few decades.

According to Dr Akhilesh Gupta of the National Centre for Medium Range Weather Forecasting, many people who travel to advanced countries (mostly extra-tropical ones) have argued that weather forecasting in these countries is quite accurate presumably due to the country's advancement in the field. But the basic fact is that, unlike the tropics, weather systems are quite well behaved in these latitudes.

Extra-tropics have a single scale of atmospheric systems that travel in defined path and have preferred areas of development and intensification. Their forecasting becomes much easier in this context.

But the tropics, on the other hand, do not have this blessing. The fundamental laws of fluid dynamics do not apply in tropics. These regions are dominated by all scales of motion ranging from less than a km to tens of thousands of km.

Moreover, the convective development in the tropics could be quite sudden and erratic compared to those in extra-tropics. As if not enough, the tropical areas are covered with ocean where little data is available. These factors are responsible for the poor accuracy of weather predictions in tropics.

It has been shown mathematically that while weather systems in extra-tropics can be predicted with adequate accuracy up to seven to 10 days in advance, the limit of predictability for tropics is only three days, Dr Gupta said.

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