Mercury seen trending up over North-West India from weekend

As the North-East monsoon wilts in the south peninsula, winter climes seem to have settled down over the North, with a low temperature regime prevailing currently over most parts of the region.

The mercury is trending down due to the absence of weather-altering western disturbances, with the next one in the series expected to report in by Sunday only.

These disturbances originate in the Mediterranean and traverse Iraq, Iran, Afghanistan and Pakistan before crossing the Rajasthan/Gujarat border into India. On an average, six-seven disturbances move across North India during winter.

These low-pressure wind bands (atmospheric rivers) circumnavigate the globe. They have special significance given their capacity to influence weather over North India, pronouncedly so during winter.

The low pressure associated with these waves mean that they also carry moisture, which will warm up the air and raise the ambient mercury. When moisture-laden air rises against the heights of the Himalayas, it cools and condenses to form rain. This is crucial for the region from the viewpoint of the rabi crop.

Thus, the weather over the region is mostly dictated by western disturbances — ‘western’ denotes the point of their origin and contrasts with the mostly eastern origin of rain systems (low-pressure areas, depressions) during the two monsoons.

These disturbance have acquired special significance this year, with pollution levels across North and North-West India have touched alarming levels.

North-Westerly and South-Westerly winds associated with the disturbances could have helped blow away the harmful smog and, along with it, the serious health hazard. But western disturbance activity was reduced to practically nil during the worst phase of the crisis around Diwali. Sustained winds and scattered rain were conspicuous by their absence when they were most needed.

The El Nino/La Nina phase in the Equatorial Pacific is known to influence winter precipitation over North-West India. El Nino brings about increased precipitation and vice-versa.

A largely neutral phase in the Pacific with a bias towards the ‘cold’ La Nina is thought to be working against the prospects of the movement of ‘productive’ western disturbances this year.

The year was also noted for a stridently negative phase of the Indian Ocean Dipole (IOD), which mimics El Nino-La Nina closer home. But scientists rule out any direct correlation between the IOD and the winter rainfall in the North.

Meanwhile, early indications are that the incoming western disturbance by Sunday would be capable of generating significant weather in terms of snowfall over the mountains of North-West India and scattered to moderate rain over the plains.

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