Context: A “La Nina watch” has been issued with some early signs pointing to the weather event that brings rains and floods to Asia, particularly India, forming in the Pacific Ocean later this year by the Australian Bureau of Meteorology (BoM).
El-Nino Southern Oscillation
- East-West zonal circulation of tropical winds is an important variant from general atmospheric circulation.
- This typical east-west circulation of tropical wind is called Walker Circulation named after a famous scientist G.T. Walker.
- Walker circulation is convective cell of air circulation, which is formed due to development of pressure gradient from east to west in the equatorial Pacific Ocean.
- After two-three years this general condition of east-west pressure gradient is reversed. i.e. pressure gradient becomes west to east.
- Walker called such oscillations as southern oscillations.
- In normal conditions high pressure develops on the sea surface of the equatorial east Pacific Ocean and the western coastal lands of South America due to subsidence of air from above and upwelling of cold oceanic water.
- The low pressure is formed in the equatorial western Pacific Ocean due to rise of air from the warm sea surface.
- This pressure gradient from east to west generates east-west circulation of trade winds on the surface while there is reverse upper air circulation i.e. from west to east which completes a convective cell.
El Niño:
- It is a climate pattern that describes the unusual warming of surface waters in the eastern tropical Pacific Ocean.
- El Niño is the “warm phase” of a larger phenomenon called the El Niño-Southern Oscillation (ENSO). La Niña, the “cool phase” of ENSO, is a pattern that describes the unusual cooling of the region’s surface waters. El Niño and La Niña are considered the ocean part of ENSO, while the Southern Oscillation is its atmospheric changes.
- During El Niño, the trade winds weaken or even reverse:
- Instead of blowing from east (South America) to west (Indonesia), they could turn into westerlies.
- As the winds blow from the west to east, they cause the masses of warm water to move into the central and eastern equatorial Pacific Ocean.
- The rise in SSTs there, thus, produces increased rainfall along western Latin America, the Caribbean, and US Gulf Coast, while depriving Southeast Asia, Australia and India of convective currents.
- El Niño occurs simultaneously with the Southern Oscillation. The Southern Oscillation is a change in air pressure over the tropical Pacific Ocean. When coastal waters become warmer in the eastern tropical Pacific (El Niño), the atmospheric pressure above the ocean decreases.
- An El Niño event can be identified by the variations in sea surface temperature (SST) over the equatorial Central Pacific.
- El Niño events are classified as weak, moderate, strong, and very strong depending on the strength of the positive SST variations.
- An El Niño event is announced when monthly Nino 3.4 SST deviations reach +0.50 °C, along with consistent atmospheric features, and when these anomalies persist for three consecutive months.
La Niña:
- La Niña basically refers to an abnormal cooling of the central and eastern Pacific Ocean waters off the coasts of Ecuador and Peru.
- Such cooling (sea surface temperatures i.e. SSTs falling) is a result of strong trade winds blowing west along the equator, taking warm water from South America towards Asia.
- The warming of the western equatorial Pacific, then, leads to increased evaporation and concentrated cloud-formation activity around that region, whose effects may spread to India as well.
Indian monsoon: Indian Ocean Dipole (IOD)
- IOD measures differences in sea surface temperatures between the western and eastern parts of the Indian Ocean. It is basically like the El Nino weather system that develops in the Pacific Ocean. It is characterized by an irregular oscillation of sea-surface temperatures in the eastern and western Indian Ocean
- Impact on weather patterns: IOD alters the wind, temperature, and rainfall patterns in the Indian Ocean region.
- Positive IOD event is known to bring floods to eastern Africa and droughts and bushfires to eastern Asia and Australia. Ex. 2020 Australian Bushfires.
- Positive IOD is known to increase the intensity of Monsoon in the Subcontinent and leads to above normal rainfall. A simultaneous occurrence of Positive IOD and El Nino balances the negative impact of El Nino on the Indian Monsoon rainfall. Ex. above normal rainfall in India in 2019.
- In contrast, Negative IOD coupled with El – Nino leads to poor Monsoon rainfall. Ex. Deficient rainfall in 1992.
Findings
- La Nina Watch Issued: Australian Bureau of Meteorology (BoM) issues a "La Nina watch" indicating potential formation of La Nina later in the year. La Nina brings rains and floods to Asia, especially India.
- ENSO-Neutral Conditions: El Nino-Southern Oscillation (ENSO) remains neutral until at least July 2024. Positive Indian Ocean Dipole (IOD) development has stalled.
- Current Climate Patterns: ENSO-neutral conditions observed currently. Sea surface temperatures (SSTs) in the central Pacific have been cooling steadily since December 2023.Recent cloud and surface pressure patterns indicate ENSO-neutral conditions.
- US Prediction: Transition from El Nino to ENSO-neutral likely in the upcoming month. La Nina may develop with 49% chance in June-August 2024 or 69% chance in July-September 2024, as per the US Climate Prediction Center (CPC).
- Global Temperature Trends: Emergence of El Nino in June 2023 resulted in warmest global temperatures for the respective 11 months. Global sea surface temperatures (SSTs) have been recording warm for each month between April 2023 and April 2024.
- Indian Ocean Dipole (IOD): IOD currently neutral; recent index value within historically neutral thresholds. Potential positive IOD development stalled; SST observations suggest recent development may have halted. Weaker positive IOD forecast compared to earlier predictions.
- Monsoon Booster: Positive IOD can enhance India's southwest monsoon performance. Example: In 2019, a strong IOD event compensated for a 30% rainfall deficit in June.