Role of El Nino and Southern Oscillation (ENSO) in Indian Monsoon

Role of El Nino and Southern Oscillation (ENSO) in Indian Monsoon
Posted on 18-08-2023

Impact of El Nino and Southern Oscillation (ENSO) on Climate Patterns

The term "El Nino" translates to 'Child Christ,' referencing its occurrence around Christmas in December. El Nino is a intricate climatic phenomenon that emerges approximately every three to seven years, bringing about diverse weather extremes such as droughts, floods, and other severe events across the globe.

This phenomenon encompasses both oceanic and atmospheric factors, materializing as warm ocean currents along the coastline of Peru in the Eastern Pacific. Its influence extends to numerous regions, including India. El Nino can be regarded as an extension of the warm equatorial current, which momentarily displaces the typically cold Peruvian or Humboldt current. This warm current elevates the water temperature along the Peruvian shoreline by approximately 10°C, leading to disruptions in the usual equatorial atmospheric circulation and irregularities in sea water evaporation.

As warm water accumulates near the coast of Peru, it triggers the formation of a low-pressure system in that vicinity. This weakening of easterly winds is significant, as El Nino has been observed to hinder monsoon rainfall in India. The easterly winds are responsible for carrying moisture-laden clouds, which are essential for Indian monsoon precipitation.

ENSO, encompassing both El Nino and its counterpart La Nina, represents an oscillation between these contrasting phases. The transition between El Nino and La Nina occurs sporadically, typically spanning every two to seven years. La Nina stands in opposition to El Nino, involving a cooling of the waters off the coast of Peru. During La Nina, the easterly winds intensify, resulting in elevated monsoon rainfall levels during India's monsoon period.

Indian Monsoon and El Nino Modoki

El Nino Modoki, distinct from the traditional El Nino, features a distinct sea surface temperature (SST) warming pattern, predominantly centered in the central equatorial Pacific region rather than the eastern equatorial Pacific. Referred to as Central Pacific El Nino or warm pool El Nino, it was first identified in 1986. The conventional ENSO is more closely linked to the tropical Indian Ocean, while El Nino Modoki is associated with the Southern Indian Ocean.

Influence of El Nino and Southern Oscillation (ENSO) on Climate

El Nino signifies a transient warm oceanic current that emerges off the coast of Peru typically around December in certain years.

Variations in atmospheric pressure are observed, with high pressure prevailing over the equatorial south Indian Ocean and low pressure over the equatorial south Pacific, and vice versa.

This pressure disparity instigates a circulation pattern known as the Walker Circulation, operating along the equatorial belt.

The fluctuations in pressure are quantified by the Southern Oscillation Index (SOI), calculated as the pressure discrepancy between Tahiti in French Polynesia, representing the southern Pacific Ocean, and Port Darwin in northern Australia, symbolizing the Indian Ocean.

Extended positive SOI values point toward La Nina conditions, whereas negative values suggest the occurrence of El Nino conditions.

Although there is a tendency for monsoon patterns to be subdued during El Nino years, this correlation isn't entirely deterministic.

El Nino constitutes a phenomenon in the equatorial Pacific, marked by an increase in sea-surface temperatures surpassing a threshold of +0.5 degrees Celsius (and conversely cooling by a similar margin during its counterpart, La Nina).

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