Wednesday, January 25, 2012

Snowfall in Punjab, Winter rain in Rajasthan: An Arctic Link to an Extended Winter (Live Mint)

From Mr. Lalloobhoy Battliwala

The story below mentions first snow in Punjab in over half a century. I heard about a rainstorm in Rajasthan from a laborer.

Climate changes. So?

An Arctic link to an extended winter
On 10 January, when the index dipped to its negative phase, snow dramatically fell over much of Europe, India and Pakistan - Jacob P. Koshy (Live Mint, India)

New Delhi: Blame a combination of tropical storms and a whimsical drop in air pressure in the distant Arctic for the surprise snowfall in Punjab, the first in over half a century of the state’s meteorological history.

While the weather over northern India may have been mild for most of the winter months, international forecast agencies and analysts suggest that the January chill over north India is likely to be in line with global trends of a long-drawn winter that is likely to extend well into February.

While it’s well known that snow in Europe closely ties in with the intensity of winters in India, international interest in the relationship between global warming and its impact on ice cover in the Arctic has dramatically increased scientific scrutiny of how weather patterns here - such as fluctuations in atmospheric pressure, wind speeds, sea surface temperatures - may have a global impact on weather in far-flung continents.
This scrutiny has led climate scientists to develop and monitor the so-called Arctic Oscillation (AO) index, a barometer of the variation in pressure over the Arctic region.

When the AO index is positive, surface pressure is low in the polar region. This helps the middle-latitude jet stream (gusts of warm air above the Mediterranean) to blow strongly and consistently from west to east and trap the cold Arctic air within the polar region.

When the AO index is negative -as it is now-there tends to be high pressure in the polar region, weaker zonal winds and greater movement of frigid polar air into middle latitudes.

While the pressures were rising and positive until January, there’s been a steady, unexpected dip in the index since 4 January.

In fact on 10 January, when the index dipped to its negative phase, snow dramatically fell over much of Europe, India and Pakistan.

Moreover, current predictions from the National Oceanic and Atmospheric Administration’s National Climatic Data Center, as well as the European Centre for Medium-Range Weather Forecasts suggest that this negative phase is likely to last well into January.

“They could be wrong too, because AO is quite erratic that way. But these models’ ability to predict the AO patterns over the next week or fortnight has considerably improved over the last few years,” said Madhavan Rajeevan, meteorologist at Indian Space Research Organisation.

Last year, too, the index was negative throughout January and triggered a longer, but relatively mild winter over India.

For most of the 20th century, during which systematic global climate records were maintained, the AO index was usually positive. Since the 1970s, almost coinciding with global concerns over greenhouse gases and warming, the index has seen fluctuating patterns.

“There are some correlations between the Indian monsoon and AO index, but it’s still to be well fleshed out, since the fluctuations of the ’70s in the ice cover (over the Arctic) are being closely monitored and they seem to be more erratic,” according to Rajeevan. “It’s not known why.”

Scientists at the India Meteorological Department (IMD) said that they didn’t monitor the AO index daily.

“Arctic seasons are known to have global repercussions, but given that hailstorms and blizzards aren’t too frequent over India, we only monitor Arctic patterns for monsoon,” said D.S. Pai, director (forecasting) at IMD.

Consistently low temperatures over January and February could affect India’s rabi or winter crop. The agriculture ministry has already issued an advisory that stronger hailstorms and colder weather could affect some of the oilseed and cotton crop, though there was unlikely to be significant damage to the key wheat crop, which is the staple of India’s rabi season.

No comments: