What is La Nina, expected to set in later this year, and how it could impact global weather

• The India Meteorological Department (IMD) forecasted above-normal rain in the upcoming monsoon season in India, with “favourable” La Nina conditions expected to set in by August-September.

Key highlights

• Another recent bulletin from the IMD noted how the strength of El Niño conditions has weakened since the beginning of this year.

What are El Niño and Nina?

• El Niño (meaning “little boy” in Spanish) and La Nina (meaning “little girl” in Spanish) are climate phenomena that are a result of ocean-atmosphere interactions
  • which impact the temperature of waters in the central and eastern tropical Pacific Ocean.
  • These affect global weather.
• The Earth’s east-west rotation causes all winds blowing between 30 degrees to the north and south of the equator to slant in their trajectory.
• As a result, winds in the region flow towards a southwesterly direction in the northern hemisphere and a northwesterly direction in the southern hemisphere.
• This is known as the Coriolis Effect.
• Due to this, winds in this belt (called trade winds) blow westwards on either side of the equator.
• Under normal ocean conditions, these trade winds travel westwards along the equator from South America towards Asia.
• Wind movement over the ocean results in a phenomenon called upwelling, where cold water beneath the ocean surface rises and displaces the warm surface waters.
• At times, the weak trade winds get pushed back towards South America and there is no upwelling.
• Thus, warmer-than-usual sea surface temperatures are recorded along the equatorial Pacific Ocean, and this is known as the emergence of El Niño conditions.
• Conversely, during La Nina, strong trade winds push warm water towards Asia.
• Greater upwelling gives rise to cold and nutrient-rich water towards South America.
• Thus, climatologically, El Niño and La Nina are opposite phases of what is collectively called the El Niño Southern Oscillation (ENSO) cycle. It also includes a third neutral phase.
• El Niño events are far more frequent than La Nina ones.
• Once every two to seven years, neutral ENSO conditions get interrupted by either El Niño or La Nina.
• Recently, La Nina conditions prevailed between 2020 and 2023.

How could the incoming La Nina impact global weather?

• Due to ENSO and the warming or cooling of ocean waters, the air circulation loop in this region also gets affected.
• That, in turn, affects the amount of precipitation over the neighbouring regions, and impacts the Indian monsoon as well.

La Nina’s impact on India

• Except in east and northeast India, all remaining regions are expected to receive normal or above seasonal rainfall, the IMD said in April.

La Nina’s impact on the world

• Similar to India, Indonesia, the Philippines, Malaysia and their neighbouring countries receive good rainfall during a La Nina year.
• On the other hand, droughts are common in southern regions of North America, where winters become warmer than usual.

Is climate change affecting ENSO?

• Over India, El Niño is known to suppress the southwest monsoon rainfall and drive higher temperatures and intense heat waves, like the present summer season.
• In the past, monsoon seasons during years following an El Niño were 1982-1983 and 1987-1988, with both 1983 and 1988 recording bountiful rainfall.
• At present too, a similar situation could play out.