El Nino and Southern Oscillation

What is El Nino?

Fig. 1: Humboldt Current; Source: Imagery © 2022 NASA, TerraMetrics, Map Data ©2022 Google, INEGI

El Nino is a recurring warm ocean current on the west coast of Ecuador, Peru and Chile. Normally on the western coast of Latin America, a cold current flows from the south pacific ocean towards the  equator. This cold current is known as Humboldt Cold Current (Fig. 1). The Humboldt current reaches the equator, becomes warm and turns westward due to trade winds. The wind moves westward, rises over the west pacific, turns towards the east pacific at high altitude and sinks at the east pacific coast. This air circulation is called Walker Circulation

Also read, why air rises over warm water or land?

  • This westward flowing warm current is known as South Pacific Equatorial Current. (See White arrows in Fig. 2)
  • The westward flow of warm equatorial waters leads to upwelling of cold ocean water near Peru and Ecuador coast (Equatorial East Pacific) and results in high atmospheric pressure.
  • The accumulation of warm water in the Equatorial West Pacific leads to low atmospheric pressure.
  • This low atmospheric pressure helps to establish south-westerly monsoon winds.
  • The winds in the Walker Circulation reverse.
  • Fig. 2: Ocean Current pattern during El Nino, Normal condition and La-Nina.
    Source: NOAA/PMEL/TAO Project Office, Dr. Michael J. McPhaden, Director

El Nino disrupts the pattern of flow of ocean current in every 2-7 years. The southward flowing warm current replaces the cold Humboldt current along the Latin coast of Peru and Ecuador. Scientists has names this warm current as El-Nino (Child of Christ).

Impact of El-Nino

Normally, the Humboldt cold current brings planktons and food for marine animals along the coast of Peru and Ecuador which promotes growth of fishing.

  • When the El-Nino arrives (Fig. 2), the atmospheric conditions become warm which leads to high intensity rainfall along the West coast of Peru and Ecuador.
  • The warm current prevents the advance of cold Humboldt current, therefore, the planktons and marine food becomes scarce.
  • This leads to the death of a lot of fish.
  • The South Pacific Equatorial current reverses and flows Eastwards.
  • This causes high atmospheric pressure in the West Pacific and weakens south-westerly western winds.
  • The climatologist argue that the advent of El-Nino in Eastern Pacific or west coast of Peru leads to weak monsoon in South and South-East Asia.

What is Southern Oscillation?

The alternation of high and low atmospheric pressure in western and eastern Pacific Ocean is known as Southern Oscillation.

  • In normal condition, when the West Coast of Peru (Equatorial Eastern Pacific) is occupied by cold Humboldt current, the atmospheric pressure remains high due to cold water. Consequently, the warm south equatorial current flows towards Western Pacific (East coast of Australia) which leads to low atmospheric pressure in Western Pacific.
  • Contrarily, the arrival of El-Nino current causes low pressure on the West Coast of Peru (Eastern Pacific) and high atmospheric pressure on the East coast of Australia (Western Pacific).
  • Scientist measure the atmospheric pressure of the Equatorial East Pacific at Tahiti Island and of the Equatorial West Pacific at Darwin (Australia).
  • The high and low atmospheric pressure oscillates between the East and West Pacific. It is so due to alternation of warm and cold currents on western coast of Latin America.
  • This reversal and alternate change in atmospheric pressure is called southern oscillation. The El-Nino and Southern Oscillation (ENSO) is a corresponding phenomena.

What is La-Nina?

  • Sometimes, the Humboldt current becomes very strong and the westward flowing equatorial winds become very strong, too. 
  • This leads to continuous removal of warm water from the Equatorial Eastern Pacific (Fig. 2). The removal of warm water leads to upwelling of cold water in Equatorial East Pacific. The cold water rises to the surface from lower oceanic layers. This results in formation of a strong high pressure zone and dry weather conditions.
  • Contrary to east pacific, the Equatorial winds accumulate the warm ocean water in the Equatorial West Pacific leading to strong low pressure. This low pressure results in above normal monsoon rainfall.

ENSO In Short

The change in the direction of ocean currents changes the air pressure which in turn changes the atmospheric conditions on the east and west pacific coast. The atmospheric conditions oscillate between high and low pressure leading to dry and wet condition, respectively, on the west coast of Ecuador, Peru and Northern Chile.