Why Winter Has Arrived Late
A southward shift of two wind systems has allowed the Westerlies to penetrate into the Indian landmass.
Winter is six weeks later than normal. Scientists say when winters set in late, they usually linger for longer. They also say that February this year is likely to be colder than it usually is. For all of December and the first two weeks of January, temperatures in most parts of India were substantially higher than normal. In some places, the departures were as much as 5-8 degrees more than normal. Besides, the small amount of rainfall that takes place in late December and early January, which brings the chill in the air, was almost absent. Winter rainfall in the country as a whole at the start of the year was about 85 per cent below normal.
REASONS
Scientists attributed the warmer than usual temperatures in December and January to two main factors:
1. The prevailing El Niño in the equatorial Pacific Ocean that is now being counted amongst the strongest in the last 60 years.
2. The abnormal location of two wind systems – a sub-tropical anti-cyclonic formation and the Jetstreams. These wind systems were both located north of their normal positions for this time of the year, and were in effect blocking the entry of the cold Westerlies from Europe into the Indian region.
The Westerlies move in the mid-latitudes, between 30 and 60 degrees, in the northern hemisphere in the west to east direction. These winds move slightly southward during winter, and flow across most of northern and central India, bringing in not just chill, but also rainfall.
WHAT HAS CHANGED
There has been no change in the El Niño situation in the last two weeks. El Niño, which refers to an unusual warming of sea waters in the equatorial Pacific Ocean, off the coast of Ecuador and Peru in South America, influences weather patterns across the world.
In India, it is generally associated with suppressed rainfall in the monsoon season. But past data also show that the winter in India following an El Niño is slightly warmer than usual.
Since the current El Niño has been exceptionally long and strong, scientists were seeing this as one of the reasons for the warmer winter in India.
But what has led to the change in weather in India over the last one week has been the movement of the two wind systems, sub-tropical anti-cyclone and Jetstreams, from where they had been located. Both of them have moved southward, allowing the Westerlies to penetrate the Indian region. As in every other season, the arrival of the Westerlies in northern India has been accompanied by a drop in temperature and some rain. The overall deficiency in winter rainfall over the country has a whole dropped to 74 per cent last week.
The drop in temperature has also been significant. Minimum temperatures across the country started getting aligned to their normal values around January 9, and have remained so since then. The maximum temperature started becoming normal a couple of days later. In fact, the maximum temperatures are now 5-8 degree below normal in some places of Punjab, Uttar Pradesh, Madhya Pradesh, Jharkhand, Assam and Meghalaya. At some other places they are 2-4 degree below normal. But at many other places, in Jammu and Kashmir, Andaman and Nicobar Islands, Tamil Nadu and Kerala, they happen to be higher than normal as well.
While this winter has been unusually warm, scientists say this is not the first time that it has set in so late. The blocking of the Westerlies by anti-cyclonic winds and Jetstreams has happened on earlier occasions as well. Both these wind systems, otherwise independent phenomena, are influenced by the thermal gradient that exists between the equator and the northern latitudes. Scientists say the temperature difference over two latitudes is more pronounced in winter than in summer. The temperature difference between Delhi and Moscow, for example, is higher in winter than in summer. This thermal gradient influences wind patterns across the globe.
A weaker thermal gradient, as was the case this year, leads to northward movement of the sub-tropical anti-cyclonic wind system that is generally located south of the Indian peninsula around December-January. It also gives rise to a shift in the location of the Jetstreams. The movement of these two systems away from their abnormal positions has coincided with the strengthening of the thermal gradient.