El Niño and La Niña
Why in news? India is experiencing a colder than normal winter as north-south winter flow set up by the climate phenomenon known as La Niña. Forecasts for the 2023 winter are predicting that there is a 50% possibility for its companion phenomenon, the El Niño to occur.
What are El Niño and La Niña?
- El Niño refers to a band of warm water spreading from west to east in the equatorial Pacific Ocean.
- The years in which an El Niño occur are called ‘El Niño years’, and global weather patterns in that year tend to be anomalous in certain ways.
- Similarly, a La Niña occurs when the band of water spreads east-west and is cooler and both phenomena can have drastic effects on economies that depend on rainfall.
How the prediction is done ?
- El Niño forecasts before spring tend to be unreliable because the Sun transitions across the equator, from one hemisphere to the other, creating dilemma in predictions.
- In a La Niña year, the tropical Pacific Ocean soaks up heat and accumulates warm water.
- During the El Niño, this warm water spills from the western part of the Pacific Ocean to the eastern part.
- Earth has had three straight La Niña years, which means the Pacific’s warm-water volume is fully loaded and likely to birth an El Niño soon.
- An El Niño year creates a miniature global-warming crisis, since the warm water spreading across the tropical Pacific releases a large amount of heat into the atmosphere
What will happen to cyclone formation and monsoons?
- A transition from a La Niña winter to an El Niño summer tends to produce a large monsoon deficit and means pre-monsoon and monsoon circulations tend to be weaker in an El Niño year.
- The vertical shear (change in the intensity of winds from the surface to the upper atmosphere) tends to be weaker as well and can favour enhanced cyclone formation.
- Intraseasonal or subseasonal timescale variability in sea-surface temperature and winds is also very important for cyclogenesis over the northern Indian Ocean.
- These timescales denote the durations for which certain temperature and wind characteristics persist in the pre- and post-monsoon periods and the net effect is for cyclogenesis to be subdued in an El Niño year.
- For monsoons if an El Niño state does emerge by summer its likely have a deficit in 2023.
How is pre-monsoon cyclones effects ?
- If pre-monsoon cyclones are susceptible to warming in the Arctic region, and could in turn affect the onset of the summer monsoon.
- The Bay of Bengal has of late been receiving freshwater from heavy rains and anomalously high river-runoffs and these waters tend to sneak into the Arabian Sea, produce surface warming, and build up subsurface heat.
- These changes together may create favourable conditions for the formation of bigger and badder cyclones, especially if the circulation and the vertical shear are weak.
How Indian ocean dipole helps in this process?
- Some research has indicated that the Indian Ocean dipole ,a seesawing of sea-surface temperature over the tropical Indian Ocean could compensate for the negative effects of an El Niño.