‘North Atlantic Oscillation may have played a role in this time's cold wave’

Cold day and cold wave conditions in the north and north western parts of India made global headlines in December 2019. The capital Delhi recorded one of its lowest maximum and minimum temperatures in over a 100 years.

But why was the month so cold when the India Meteorological Department (IMD) had stated in November 2019 that the current winter season will be warmer than normal? Down To Earth spoke to Rajendra Kumar Jenamani, scientist at the IMD, about the reasons for the persistent cold conditions in the region.

What were the meteorological causes of intense and extended cold day and cold wave conditions in the north Indian region this winter?

An active western disturbance affected north and north west India from December 10-14. This first caused rainfall and snowfall in the Western Himalayan Region between December 10-12 and then in the northern plains region from December 11-13.

This includes Punjab, Haryana, Delhi, Uttar Pradesh, western Bihar, northern Rajasthan and northern Madhya Pradesh. At some stations in Delhi, the rainfall broke records held for more than 50 years.

The accumulated rainfall recorded on the morning of December 13 was 33.5 mm at Safdarjung Airport (SFD) and 40.2 mm at Palam airport. This rainfall broke records for the 24-hour rainfall in the month of December since 1952 (out which the observatory has been functional for 52 years) for SFD and 68 years for Palam (for which data was analysed).

This rainfall brought moisture into the region, with relative humidity levels crossing 80 per cent and led to the formation of a low-lying cloud in the atmosphere. This cloud did not allow sunlight to fall on the surface, bringing down day temperatures, resulting in cold day conditions.

The conditions were further aggravated by the low speed of prevailing winds at 5-9 km/hr which could have disrupted the clouds. The presence of pollutants in the atmosphere at the same time also did not help the cause.

All these factors resulted in an atmospheric system known as an ‘anti-cyclone’ which is a region of high pressure in the atmosphere in the central part of north west India. Another anti cyclone ran from North India towards West Bengal.

Such anticyclones cause weather conditions to become stagnant unless externally disturbed by another weather system, which in this case should have been a new western disturbance. But between December 15 and December 30, there was no new western disturbance which caused the severe cold day conditions to persist.

Further as the days became colder and there was no sunlight, the radiative heat stored by the Earth when sunlight falls on it and given out at night also went down, causing night temperatures to dip, resulting in cold wave conditions from December 25 onwards in the north and north western regions of the country.

This caused the unique event of parallel cold day and cold wave conditions that made the situation worse and broke maximum and minimum temperature records in places like Delhi, Jaipur, Gwalior and Gurugram between December 27 and 31.

What is the relationship between cold day and cold wave conditions?

Cold day conditions are determined by measuring the maximum temperature of a place during a day while cold wave conditions are measured via minimum temperatures during the night.

Normally, places record their minimum temperatures during early mornings. But in these recent events, the minimum temperatures were recorded during the early night, which is not usual and indicates that nights were really cold, giving way to cold waves from cold days. 

What global climatic factors impact the occurrence of cold day and cold wave events in India?

Some role might have been played by the North Atlantic Oscillation (NAO) and the Siberian High. We can say this because of the absence of a major western disturbance for 15 days, fixed wind pattern at lower levels and the resultant severe cold spell. All of these are linked. 

Factors like pollution across the Indo-Gangetic Plain also played a role as it favours such large-scale fog / low clouds over a period of two weeks. NAO is an irregular fluctuation of atmospheric pressure over the North Atlantic Ocean that has a strong effect on winter weather in Europe, Greenland, northeastern North America, North Africa, and northern Asia.

The NAO can occur on a yearly basis or the fluctuations can take place decades apart. It is an ‘oscillation’ because the changes in atmospheric pressure are essentially a back and forth switching between two prevailing patterns, or modes.

A ‘positive mode’ is one in which a strong subtropical high is located over the Azores archipelago in the central North Atlantic while a strong low-pressure system is centred over Iceland and a ‘negative mode’ is one in which weaker high and low pressure systems are found over the same locations.

We are looking at its role as large scale features. The absence of western disturbances are also linked to circulation over mid-latitude right from the Mediterranean Sea. 

The conditions have arisen even after the IMD had put out a press release in November 2018 that the current winter season will be warmer than usual. Why?

IMD had just said that central India was likely to be warmer not north India.

How are cold wave conditions forecasted by the IMD?

Cold day and cold wave conditions depend on the occurrence and non-occurrence of western disturbances and their impact on local weather conditions that we presently monitor and successfully forecast 3-5 days in advance, using state-of-the-art global and regional models. Then we follow the severe weather with continuous current weather observations, weather radars and satellites at second to minute scale and hourly scale. 

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