Why in News?

• According to a new study, an unprecedented La-Nina event, extended by climate change, triggered a peculiar trend in the 2022-23 winter season in India.
• 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.

What’s in Today’s Article?

• What are the Normal Climatic Conditions?
• What is El Nino and La Nina?
• Findings of the New Study - Impact of La Nina on Air Quality in India
• How La Nina Impacted Air Quality in India?

What are the Normal Climatic Conditions?

• 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.

What is El Nino and La Nina?

• El Nino and La Nina are two opposing climate trends that deviate from the normal conditions and normally run nine to twelve months, but can often extend.
• These events occur every two to seven years on average (El Nino is more frequent than La Nina), but not on a regular basis and together are referred to as the El Nino-Southern Oscillation (ENSO) cycle by scientists.
• El Nino is typically known as the warm phase (a band of warmer water spreading from west to east in the equatorial Pacific Ocean) and La Nina is identified as the cold phase (a band of cooler water spreads east-west) of ENSO.
• Both El Nino and La Nina can have global effects on weather, wildfires, ecosystems and economics.

Findings of the New Study - Impact of La Nina on Air Quality in India:

• The study is conducted by researchers at the National Institute of Advanced Studies (Bengaluru) and the Indian Institute of Tropical Meteorology (Pune).
• Monsoon rainfall over India is known to be strongly influenced by El Nino and La Nina events, the alternating warming and cooling of the eastern Pacific Ocean that impacts weather across the world.
• According to the study, this is the first time that air quality in Indian cities has been linked to a La Nina event - and indirectly to climate change, which is making El Nino and La Nina more severe.
• 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.
• The researchers had set out to investigate this unusual behaviour when they discovered the potential effects of La Nina.
  • Not all La Nina events might produce noticeable changes in wind circulation over India, but this one was a particularly strong event.
  • The impact on air circulation became evident only in the third year of La Nina. So, there may be an accumulative effect.
• It was not yet entirely clear whether El Nino would produce an opposite effect for air quality over India.

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.