The Hindu Business Line : ‘Change of guard’ aids shift in weather

Thiruvananthapuram, Jan. 10 An abominably dry and cold winter in the north and northwest India has given way to the prevailing wet and humid climes thanks largely due to a fortuitous substitution of the ‘weather guard’ dictating terms from across the border.

A resident semi-permanent ridge (anti-cyclone or high-pressure area with clear skies and sinking, dry air) proving the nemesis of the rain-fed rabi crops in the region has now been replaced by a semi-permanent trough (cyclonic circulation marked by low pressure and moist, rising air).

Inbound western disturbances have recoiled at the sight of the stubborn ridge, and were made to either fall apart or divert from the usual path that would have taken them to India. It took the first deep westerly system covering much southerly latitudes in extent to unseat the ridge during the first week of the New Year and in turn occupy the strategic perch across the border (over Afghanistan-Pakistan) as a semi-permanent trough.

CATALYST SYSTEM

Unlike in the case of the ridge that revels in scaring away westerly systems, this facilitating trough catalyses the formation and passage of pulses bound for India. At times, even the feeblest of the pulses become activated to become much bigger systems that rain their contents over this side of the border.

Responsible for the change in weather in the north and northwest, this semi-permanent trough is expected to stay anchored for the best part of the remainder of January, according to forecasts by international models. This would mean that moisture-laden westerly troughs with varying strengths would continue to amble in for sometime more to come.

Importantly, this would in turn bring about alternating bouts of showers and cold snaps, which are vital for the proper growth and sustenance of the standing rabi crops. ‘Average to cold’ conditions have been forecast to prevail during January-February-March as per long-range forecasts put out by various international weather models.

BIG STORM BREWING

According to Mr Jim Andrews of AccuWeather.com, two brewing winter storms over the Caspian Sea and Black Sea far to India’s north-northwest would merge to become the next big South Asian snowstorm over the next 4-5 days. The tropical warmth in relative proximity to south of the storm-carrying jet stream from southern Arabia to India provides all the ingredients for unsettled to stormy weather, he says.

Meanwhile, India Meteorological Department on Thursday announced the withdrawal of the northeast monsoon over Tamil Nadu, Kerala and adjoining parts of Andhra Pradesh and Karnataka. But easterly waves would continue to drift in across the Bay of Bengal, though lacking in intensity and frequency, to trigger occasional wet session over south coastal Tamil Nadu and adjoining Kerala.

In fact, the European Centre for Medium-Range Weather Forecasts (ECMWF) sees the prospect of a massive westerly trough venturing far to the south as to set up an interaction with an easterly wave to cause some wet weather over Kerala later this week.