How global climatic phenomena affect India’s weather

New study says there is a connection between fog conditions in Indo-Gangetic Plains and anomalies in movement of air in the Arctic and Eurasian regions

North India has been going through some flip-flop weather this winter. As predicted by the India Meteorological Department (IMD), the season started off with warmth which gave way to cold waves and ground frost across the country.

These conditions abated to warm days once the Western Disturbances (WDs) resumed their activity in January, 2019. But there has been a visible dip in temperatures again since January 27, 2019 after the WD activity relented.

Moreover, changes in climatic conditions in regions far away from India affect its weather, and further influences the lives of people by impacting their livelihoods, health, transportation as well as the environment.

For example, a study published in the journal Climate Dynamics in September 2018, says that there is a connection between fog conditions in the Indo-Gangetic Plains (IGP) and anomalies in the movement of air in the Arctic and Eurasian regions.

Scientists at the Indian Institute of Tropical Meteorology (IITM), Pune, studied 105 different fog days between 1979 and 2013, and concluded that there is a ‘footprint’ of the Arctic Oscillation (AO) and Eurasian (EU) circulations on the anti cyclonic circulations — regions of high pressure areas associated with sinking air — over the north Indian plains.

“During the winter season, when temperatures are cold, these circulations push air towards the surface and trap water vapour in its place, producing foggy weather. The water vapour for this phenomenon could be present in local atmosphere at the time or sourced from nearby oceanic regions by the circulations,” says R Krishnan, one of the authors of the research paper, and a senior climate scientist at IITM.

The research claims that high pressure systems in the Arctic Circle during the negative phase of AO phenomenon increases the occurrence of fog events in the IGP.

During the positive phase of the AO, colder temperatures are confined around the Arctic region and it’s well separated from the tropical regions keeping them warm. But during its negative phase, the Arctic circulation gets disrupted and the cold air can move out of the Arctic region and affect weather in the mid and lower latitudes of North America, Europe and Asia. This can cause frigid winters in these regions.

Another semi permanent climatic phenomenon that the scientists say affects the fog conditions in the IGP is a large anti cyclonic system over the Siberian region which forms during the winter season.

The positive phase of this system, also known as the Siberian High (SH), is also associated with foggy weather, but to a lesser extent. Positive SH sends cold winds to the IGP even during the positive AO phase.

“When both the negative AO and positive SH are aligned with each other they bring cold air towards the south and create the most favourable conditions for the formation of fog in the IGP,” says Ramesh Vellore, another author of the study and senior scientist at IITM.

The current winter season in North India, especially Delhi, has witnessed less than normal fog events.

On December 31, 2018, the IMD had said, “The fog coverage data (hours) at Indira Gandhi International Airport (IGIA), Palam, shows that it was the clearest month in last 22 years in terms of visibility (visibility>= 1000m). Fog hours (shallow to very dense) during the month were only 145 against normal of 300 hours. Dense fog conditions were also subdued with just two nights of nine hours, against the normal of nine nights with 45 hours.”

This dearth of fog events has been attributed to the lack of WDs in December, as well as the fact that the AO, is in its positive phase, could have had an impact.

Anti cyclones and fog conditions when combined with the increasing air pollution levels in cities like Delhi can create deadly smog, which is detrimental to the health of the people. The three years have witnessed smog events in the capital.

In November last year, the situation was so critical that Delhi came close to banning all private vehicles in the city, but was later saved by timely rainfall which brought down the pollution levels.

In the first week of November 2017, a north westerly wind had brought in dust along with smoke from stubble burning from north-west India and Pakistan inducing intense smog.

In 2016, Delhi had seen high levels of pollution around Diwali and the worst smog in 17 years. An anti cyclonic system which had drastically reduced wind speeds in Delhi had worsened air quality during this period.

But anti cyclones and fog conditions are not the only Indian weather phenomena that are affected by global climatic cycles. The AO and the El Nino Southern Oscillation (ENSO) are two major global phenomena that influence WDs, also known as the Indian winter monsoon.

WDs are extra tropical storm systems which originate in the Mediterranean region, suck air from the higher latitudes and then travel towards Afghanistan, Pakistan and India.

The positive phase of the AO increases the precipitation that WDs cause and the negative phase decreases the precipitation. On the other hand, the warm phase of the ENSO, also known as El Nino, decreases WD precipitation.

El Nino refers to the unusual warming of the central and east-central equatorial Pacific Ocean which affects global weather. The warmer waters of Pacific Ocean cause the winds in various regions to reverse, like the trade winds that come towards India. This change of wind direction leads to warmer winters.

A lack of strong WDs combined with the El Nino causes warm and dry winters in India. Such a situation was last witnessed in the winter of 2015-16, during a particularly long and strong El Nino event.

Currently El Nino is in a neutral phase, according to IMD. When there is a lack of strong WDs and El Nino is neutral, then cold and dry winters are observed as was the case in December and early January. The cold wave conditions abated when the WDs became active again.

WDs are essential for the Rabi cropping season in north India as they maintain required moisture in the soil and air. They also bring in one-third of the rain and snowfall to the Himalayan states of Uttarakhand, Himachal Pradesh and Jammu and Kashmir, and becomes one of the major sources of water in these areas. The snow and rain also feed its rivers which are sources of water for the IGP — one of the most densely populated regions in the world.

There is also a relationship between El Nino events and hotter than usual summers and a decrease in rainfall during the monsoon. Most of the time, these events have also led to drought conditions.

In the 135 years between 1880 and 2014, around 90 per cent of all evolving El Nino years have seen below normal rainfall, and 65 per cent of them experienced droughts, according to weather portal Skymet weather.

In fact, six of the worst droughts in the country since 1871 have been triggered by El Nino — the most recent being in 2009. Scientists are also working towards knowing how the Arctic and Antarctic regions will affect the winds bringing the South West Monsoon (SWM) to India.

For example, a research paper published in the Journal of Climatology and Weather Forecasting in July 2017 says that the positive phase of AO generally encourages more rainfall during the SWM because of decreased snowfall over the Eurasian region.

SWM contributes 70 per cent of the total rainfall in India, which is essential for the growth of summer crops that account for 50 per cent of the country’s total food production.