After the monsoon, it's the winter season which has played truant with India. More than half of the winter season has passed, but the characteristic chill felt in many parts is missing due to persistent above normal temperatures.
Palam in New Delhi witnessed a maximum temperature of 25.7°C on January 12, 2016, which was 6°C above normal. Minimum temperature in this region on the morning of January 13 was 13.9°C, 7°C above normal. Srinagar, which is in the middle of Chilai Kalan (40-day harsh winter period), witnessed minimum temperature of -2.5°C on January 13, which is still 5°C above normal. Most places in Himachal Pradesh, Uttarakhand, Punjab, Haryana, Chandigarh, western Uttar Pradesh, Rajasthan, western Madhya Pradesh and Vidarbha in Maharashtra witnessed maximum temperatures which were 2-5°C above normal on January 12.
This warm weather might be good news for those who don't like the winter, but such weather and temperature fluctuations adversely impact the growth of winter crops. Apples of Himachal Pradesh and strawberries of Maharashtra have been affected by the warm and dry weather.
Generally, during winter, temperatures often fluctuate but never stay above or below normal for extended periods. This means that a few days during winter may not experience cold conditions. People living in northern Indian states observe this rise in temperatures whenever the weather turns cloudy due to arrival of a western disturbance. But once the western disturbance passes, temperatures return to normal or many times fall below normal.
But this winter season (October 2015 onwards), the number of such warm days appears to be high. More importantly, anomalies in the maximum temperatures have been higher than those in the minimum temperatures. This means the days have been quite warm. This weather data will give us an idea of how warm the winter has been.
Temperature anomalies in various cities
Climate Prediction Centre (CPC) of National Centers for Environmental Prediction (NCEP) monitors city temperatures in many parts of the world, including India. Below data represents temperature anomalies between October 10, 2015 and January 10, 2016. For example, Srinagar 1.01°C means temperature in Srinagar during the period was 1.01°C above normal. It must be noted that this data is preliminary and official data from India Meteorological Department (IMD) may differ.
Aurangabad (Maharashtra) 3.05°C
Bhuj (Gujarat) 2.88°C
Solapur (Maharashtra) 2.86°C
Kurnool (Andhra Pradesh) 2.27°C
Veraval (Gujarat) 2.09°C
Safdarjung (Delhi) 1.15°C
Dumdum (Kolkata) 1.08°C
The range -0.5°C to +0.5°C of normal is considered as the normal range. But anything more than +0.5°C is considered as a positive anomaly (warmer than normal). Numerically, the above figures might appear small (such as 0.7°C), but they are significant in the case of temperature anomalies, especially in the case of Aurangabad which was quite warm.
Temperature departures from October-December 2015
Monsoon withdraws from most of India in October and it's after that the temperatures start falling. That's why the period from October is important for analysis of temperatures. IMD calls October to December the post-monsoon period and January to February the winter period. Usually, meteorologists consider December to February as the winter period in the northern hemisphere, but it's better to consider temperature data of October to February for the purpose of analysis.
Above image shows maximum temperature departures between October and December 2015. The scale used is in degree Celsius. Most of India has been shaded in yellow, orange and red colours which suggest above normal maximum temperatures. Grey areas represent normal temperatures whereas blue areas represent below normal temperatures.
This image clearly shows that central India has been the warmest (highest anomaly) as compared to other parts of India. Parts of Maharashtra have seen significant above normal maximum temperatures (1.5-3°C above normal) with that in Marathwada being 2-3°C above normal. Eastern Madhya Pradesh is 1.5-2°C above normal, Gujarat 1-1.5°C, Rajasthan and Uttar Pradesh 0.5C-2°C, Delhi-NCR 1.5-2°C, Andhra Pradesh and Telangana 2-3°C. In northeastern states and Tamil Nadu, the temperature anomaly is 0.5-1°C. In most of Jammu and Kashmir, Punjab and some parts of Rajasthan, maximum temperatures have been in the normal zone.
Talking about minimum temperature anomalies, once again the highest impact is seen in Maharashtra, especially Marathwada where minimum temperatures have been 2-3°C above normal in the crucial winter period. In southern India, anomalies have been lower, in the 0.5°C-1°C range. In parts of Delhi-NCR, they are in the 1°C-1.5°C range. Temperatures were below normal in eastern Sikkim, western Assam and adjoining West Bengal in this period.
Many parts of India had witnessed mini-summer like conditions in October. The Marathwada-Telangana- northeastern Karnataka belt had received very poor monsoon rainfall and now, temperatures in this belt have been above normal. This is likely to cause a significant impact on agriculture in these areas.
Temperature departures in December 2015
An interesting temperature pattern can be observed in the above images. Most of the areas below the Tropic of Cancer were warmer than normal as compared to areas above it. Temperatures were sort of normal in most of Jammu and Kashmir, Himachal Pradesh, Punjab, Uttar Pradesh and Delhi. But maximum temperatures were above normal in many areas of Rajasthan whereas minimum temperatures were below normal (blue). Over eastern India, maximum temperatures were normal but minimum temperatures were slightly above normal in December 2015.
What is causing this pattern?
Strong El Niño conditions in the Pacific Ocean are being held responsible but the mechanism through which El Niño is impacting India's winter isn't clear yet. At first glance, the problem seems to be more local than global.
After the southwest monsoon withdraws, north-westerly winds (winds blowing from the northwest direction) set over India in the lower levels of the troposphere. We know that winds blow from a high pressure region to a low pressure region. During post-monsoon months, the air pressure is higher in northern parts (higher latitudes) than that in southern parts (lower latitudes). This is mainly because of reduction in surface heating in the high latitude areas.
As a result, India gets these north-westerly winds which are responsible for bringing northern cold air to various parts of India.
While the left side of the image above represents the climatology (the usual pattern) of winds at an altitude of 5,000 feet between November and January, the right side shows anomaly (variations) in these winds observed between November 2015 and January 10, 2016. Anomalous winds from the western and southwestern directions were present during this period (shown by arrows). This implies that these anomalous winds didn't allow the penetration of cold air from higher latitudes which kept central and southern India warm (at least during the nights).
Weaker and a lower number of western disturbances are responsible for less cold weather in north India and also in other parts of India. A western disturbance is not only a source of rain or snow but also of cold air. Frequent, stronger and lower latitude western disturbances keep on supplying cold air which is mandatory for proper winter conditions.
The biggest question now is why these western disturbances have been weak and less in number this time. Some research has been done in the past on the relationship between western disturbances and El Niño Southern Oscillation. The research says that western disturbances tend to intensify over northwest India under El Niño conditions. However, we are seeing the opposite occur and, hence, need to understand the reasons behind this trend.