El Niño – Vicki Billings

El Niño Southern Oscillation (ENSO)

This is a major disruption of the ocean/atmosphere system in the south Pacific Ocean which has devastating consequences for the weather in other parts of the world. Places that are normally arid may have floods and those that normally have high rainfall may experience drought. Freak weather conditions can occur in distant regions.

Records of El Niño events go back to the sixteenth century when the Spanish conquistadors arrived in South America. It was noted that every few years the sea off the coast of Peru was exceptionally warm and the Peruvian anchoveta fishery failed. The name ‘El Niño’ (the boy child in Spanish) arose because the first signs of an impending event generally appeared soon after Christmas. El Niño events occur at intervals of between 2 and 7 years and in between, the pendulum swings back to neutral conditions, then through to the opposite situation termed La Niña (the girl child).

Homing in on the tropical Pacific, just south of the equator the South Equatorial Current flows from E to W, as the northern component of the South Pacific gyre which circulates in an anticlockwise direction. This transports surface water from the coast of S America towards Indonesia. Beneath this is the Equatorial Undercurrent or Cromwell current which flows in the opposite direction, W to E, and which is located within the thermocline at a depth of 50m to 250m. The Cromwell current is in the order of 400km wide, so it is like a sub-surface ribbon of water.

Flowing northwards along the coast of S America is the Peru or Humboldt current bringing cold water from Antarctic regions.

The thermocline is a region of rapid decrease in temperature with depth, the water above is well-mixed and is where primary production takes place in the phytoplankton. Nutrients that sink below the thermocline will be lost to the deep ocean unless brought back up to the surface by upwelling.

As a result of offshore winds (the SE Trade winds) upwelling occurs along the coast of S America and the cold nutrient rich waters are highly productive, hence the normally very successful Peruvian anchoveta fishing industry.

The Southern Oscillation.

  1. Under ‘normal’ or neutral conditions, the SE Trade winds blow strongly across the Pacific gaining moisture and heat as they cross the ocean, bearing in mind that at the equator the Pacific is about 12,000km wide. An area of intense low pressure occurs over Indonesia, the warm moist air rises and huge cumulus clouds form, resulting in heavy precipitation in this region. The air then circulates back at high altitude towards S America where the cool dry air sinks in a region of high pressure. The SE Trades blowing over the surface of the ocean cause warm water to slope up towards the west and a lowering of the thermocline. Upwelling of cold nutrient rich waters occurs off the coast of Peru and also the Galapagos Islands which straddle the equator.
  2. An El Niño event develops when the SE Trade winds weaken, the area of intense low pressure and high precipitation shifts eastwards towards the mid-Pacific, and winds may even reverse, especially over the W Pacific. The sea surface slope is no longer supported by the winds, and these sudden changes in conditions in the W Pacific initiate internal Kelvin waves (see below) in the thermocline. These waves are of long wave length and low amplitude, and travelling at about 2m/s they take 2 to 3 months to cross the ocean, but they transmit changes from one side of the ocean to the other. This has the effect of shifting the thermocline and allowing warmer water to be transported eastwards. The weakening of the SE Trades combined with the lowering of the thermocline results in reduced upwelling of water that is nutrient poor and a dramatic reduction in productivity in coastal waters of Peru and the Galapagos Islands. When the Kelvin waves reach the coast of S America they split, travelling both N and S along the coastline, and they are also reflected as Rossby waves (see below) which act as a negative feedback mechanism, taking anything up to a year to traverse the ocean from E to W.
  3. Eventually the pendulum swings back to neutral again, but the SE Trade winds may become even stronger, then a La Niña event develops. At this stage the Indonesian low becomes more intense and precipitation increases, high pressure over S American coastal areas increases, the sea surface slopes up to the W, the thermocline develops a steeper slope and upwelling intensifies along E Pacific coasts. These extreme conditions do not seem to cause the major climatic disasters around the world that accompany El Niño.

The Southern Oscillation Index (SOI) is used to plot the progress of events. This is simply a comparison of atmospheric pressure between Darwin and Tahiti. If the pressure is high in Tahiti and low in Darwin the SOI is positive and conditions are neutral, or La Niña is in evidence. If pressure is higher at Darwin than Tahiti the SOI is negative and if this persists for a month or more then an El Niño event is predicted.

These events have been scientifically recorded since early in the twentieth century, but it was an intense El Niño in 1972/3 which prompted more detailed research. There was another severe event in 1982/3 during which the wildlife of the Galapagos Islands was seriously affected due to warm water and lack of upwelling causing low productivity. In the 1990s there were 3 El Niño events one after the other. The most recent El Niño was in 2014/15, then a neutral period followed. We are currently in a mild La Niña phase.

Records show that conditions periodically swing from one extreme to the other, but science is no nearer to predicting what will happen next, or to identifying what triggers these dramatic changes.

Kelvin waves

  • initiated by changes in ocean/atmosphere system
  • travel along a topographic boundary eg a coastline, or a ‘wave guide’ such as the equator under the influence of Coriolis effect
  • have a long wave length (10km or more) and small amplitude
  • equatorial Kelvin waves propagate from W to E, travelling in the thermocline
  • transmit changes in ocean conditions from W Pacific to E Pacific
  • travel at about 2m/s and take 2 to 3 months to cross the Pacific

Rossby waves

  • propagate from E to W in the ocean
  • transverse waves travelling in the thermocline
  • low amplitude and very long wave length
  • slow rate of travel, taking anything up to a year to cross Pacific
  • influenced by Coriolis effect and conservation of potential vorticity

Demonstration of El Nino thermocline oscillation