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Description

Triple Drip La Nina

Context: During the 3 consecutive years of La Niña (2020-23) - a rare “triple-dip” phenomenon - air quality improved in north India while peninsular India recorded an increase in pollution levels in the 2022-23 winter season.

Introduction

In the Pacific Ocean, near the equator, the Sun makes the water especially warm on the surface.

Normally, a surface low pressure system forms in northern Australia and Indonesia and a high-pressure system develops off the coast of Peru.

As a result, the trade winds blow strongly from east to west over the Pacific Ocean, transporting warm surface waters westward.

This leads to convective storms (thunderstorms) to Indonesia and coastal Australia.

La Nina

During La Niña events, trade winds are even stronger than usual, pushing more warm water toward Asia. Off the west coast of the Americas, upwelling increases, bringing cold, nutrient-rich water to the surface.

These cold waters in the Pacific push the jet stream northward. This tends to lead to drought in the southern U.S. and heavy rains and flooding in the Pacific Northwest and Canada. During a La Niña year, winter temperatures are warmer than normal in the South and cooler than normal in the North. 

A "triple-dip La Niña" refers to a rare phenomenon where three consecutive La Niña events occur in close succession in the equatorial Pacific Ocean.

Typically, La Niña events last for about 9-12 months, but occasionally, there can be a sequence of multiple La Niña events back-to-back. When three such events occur in succession, it's referred to as a "triple-dip" La Niña.

This can result in prolonged periods of cooler sea surface temperatures in the equatorial Pacific, which can influence global weather patterns, including increased chances of drought in some regions and increased rainfall in others, depending on the location and strength of the La Niña events.

How La Nina Impacted Air Quality in India?

·         By changing wind direction:

During this time, wind usually blows in the north-westerly direction: for example, from Punjab towards Delhi and further into the Gangetic plains.

This is one of the main reasons why agricultural waste pollutants in Punjab and Haryana flow into Delhi.

However, the wind circulation was in the north-south direction in the winter of 2022. The pollutants from Punjab and Haryana bypassed Delhi and flew over Rajasthan and Gujarat to southern regions.

·         By changing the local circulation of wind near Mumbai:

Wind currents alternate between blowing from the land to the sea every few days.

When blowing from the land towards the sea, the winds carry pollutants out of the city.

However, instead of changing direction every four to five days, the winds persisted in one direction for more than a week or 10 days, leading to greater accumulation of pollutants in Mumbai in 2022.

Impact of La Nina on Air Quality in India:

Normally, northern Indian cities (particularly Delhi) experience very high concentrations of PM2.5 during October to January.

However, the winter of 2022 showed a significant deviation from this normal.

Northern Indian cities (including Delhi) were cleaner than usual, while cities in the west and the south, like Mumbai, Bengaluru and Chennai, experienced worse-than-usual air quality.

The study said PM2.5 concentrations in Delhi saw a reduction of about 10%. Simultaneously, the concentrations in Mumbai rose by 30%, while Bengaluru registered a 20% rise.