Cold air, fog trapped over North India as western disturbances stay away 

January has remained frigid for North India with cold to severe cold wave conditions made worse by extended fog as weather-maker western disturbances continue to stay away. This is unusual during an El Nino phase when active western disturbances become the norm than exception, bringing along with them snow in the high reaches and rain/thunderstorms over the plains. 

This is also contrary to forecasts made by India Meteorological Department (IMD) as well as international weather models. For instance, the IMD predicted monthly rainfall to be above normal (>122 per cent of the Long Period Average, LPA). For the country as a whole, it had forecast above normal (>118 per cent LPA) for most parts of the country except parts of North-West India pockets of North-East India it would be below normal to normal. 

All-India rain deficit

All-India deficit until January 23 came in at 56 per cent, with four Met subdivisions in deficient category; 15 in large-deficient category; while five did not receive any rain at all. With only a week to go for January to end, the West Coast and adjoining South Peninsula have received excess or large excess rain while North-West, East and East-Central India ran into significant deficits. 

The rain deficit is attributed to the lack of western disturbances after howling winds as part of the jet stream in the upper levels stay put over North India, forcing seasonal cold Arctic winds to descend to the ground. They trapped not just the cold air but also seasonal fog within an expansive hold, and have refused to yield for close to a week now. The IMD has said on Wednesday these conditions are expected to rule the roost for another 4-5 days, including the Republic Day. 

Moisture transport 

Winter precipitation and snow are overseen by the moisture transport in the upper level of the atmosphere (jet stream) combined with induced circulations at the lower levels over the Arabian Sea. The Arabian Sea moisture is the principal factor driving heavy precipitation in the Himalayan vicinity due to rapid vertical ascent and intense cloud formation due to orographic (mountain) effects. The moisture quickly condenses to form clouds, rain or snowfall vital for the region during winter.

Induced circulations develop under the watch of active/intense western disturbances, which have been rare so far during the winter. But they also require differential heating generated from cold air to the North of the jet stream and warm air to the South, which may have been impacted this year for some reason, though El Nino years normally produce adequate number of western disturbances. Active disturbances get embedded in the sub-tropical jet stream over North India, but not so far this year.

Excess rain in South 

While North India has so far drawn a blank, the West Coast and the South Peninsula saw excess/large excess precipitation from west-bound disturbances over the Bay of Bengal/Arabian Sea. Coastal Karnataka was the largest beneficiary with an excess of +2,346 per cent, followed at distance by Konkan & Goa (+1,126 per cent); and Lakshadweep (+992 per cent). Kerala & Mahe managed has +907 per cent; South Interior Karnataka, +383 per vent; and Tamil Nadu, Puducherry & Karaikal, +363 per cent.

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