El Niño - Southern Oscillation (ENSO)

As we have seen, the major circulation patterns across the globe, run meridionally (that is from north to south or vice versa). However, across the Pacific Ocean, a lateral circulation pattern was observed by Sir Gilbert Walker, the Director-General of British Observatories in India in the early 20th Century. Known as the Walker Circulation, these east-west winds are driven by the pressure differences between the Australian East coast and the west coast of South America.

"Normal" ocean circulation

The Humboldt current brings cold water northward along the west coast of South America. Due to upwelling along the Peruvian coast, this cold water is forced upwards as well as northwards towards the Equator. This cold water then flows westward along the Equator, forced along by the Trade Winds, where is it warmed before reaching the Australian and Indonesian coastlines. Here the water is approximately 3 to 8°C warmer than where it originated on the Equator at South America.

Due to the temperature difference of the water of the western Pacific and the eastern Pacific, air is differentially warmed. Near Australia and Indonesia, the water is warmer and therefore heats up the air immediately above it. Low pressure cells develop as a result of air rising, and precipitation is therefore high over these regions. To the east near the coast of South America, the water is relatively cool, and the air descends to create a high pressure cell. This results in dry weather conditions over western South America.

Ocean circulation during an El Niño

Every six to seven years for reasons as yet unknown, the easterly Trade Winds diminish. This results in the reduced movement of cold water westerly across the Pacific Ocean. In addition, much of the cold water brought up by upwelling falls to the ocean floor again. This results in water temperatures staying relatively much the same in both the west and east Pacific therefore slowing down the Walker Circulation. In fact the warmer water moves away from Northern Australia and Indonesia, eastwards towards the centre of the Pacific. Thus low pressure cells develop over the central Pacific as well as eastern Pacific (near South America) resulting in an increased amount of precipitation in the central and eastern Pacific. Conversely, high pressure cells develop over Northern Australia and Indonesia, resulting in drier the usual conditions. The circulation patterns can be seen in the following diagram.

This phenomenon occurs around Christmas time, and has been named El Niño (or boy child) by the anchovy fishermen in Peru, in recognition of this. Because the cold water brought to the shores of South America by the Humboldt Current, stay deep, this results in a poor anchovy catch for these fishermen.