Focus Areas E2- Oceanography

How does El Niño work?

The Pacific Ocean is a vast body of water.  At the equator it is far wider than the Indian or Atlantic Oceans. In typical years (i.e. non-El Niño, non-La Niña years, when the SOI hovers around zero), cold waters rising along the west coast of South America flow westward across the Pacific Ocean along the equator. During these ‘normal’ years, these westward flowing currents are heated by the tropical sun resulting in the western Pacific Ocean being 3°C to 8°C warmer than the eastern Pacific.

In non-typical years, the circulation in the Pacific Ocean changes. In ‘El Niño’ years, for example, the westward flow of cold water across the Pacific reduces in strength and so the warm water pool shifts east; the central and eastern Pacific Ocean becoming warmer than usual relative to the temperatures in the western Pacific. On the other hand, in ‘La Niña’ years the warm water pool shifts west towards the Australian coast under the influence of stronger cold water currents from the east.

Associated with these patterns of circulation in the waters of the Pacific Ocean are predicable patterns in the atmosphere above the ocean. During typical (i.e. non-El Niño, non-La Niña) years, hot moist air rises over the wet, tropical Indonesian region and travels eastward at a height of about 10-15 kilometres.  As the air travels eastward it cools and dries out, then sinks to become cool, dry air near the Pacific Coast of Peru.  As a result, this part of South America is usually dry.  On the surface of the Earth the trade winds move in the opposite direction across the ocean (from east to west this time) completing the circulation of air over the Pacific Ocean.  These easterly trade winds typically bring warm moist air towards the Indonesian region. The rising air over Indonesia is associated with a region of low air pressure, towering Cumulonimbus thunder clouds and heavy rain.

In non-typical years, the circulation in the atmosphere changes. During El Niño years, as the pool of warm oceanic waters shifts eastward the associated rising flow of moist warm air shifts away from Indonesia and towards the east. Rainfall decreases over Indonesia and the western Pacific region and increases over the east-central and eastern Pacific Ocean. In contrast, during a La Niña, the rising moist air currents move westwards. As a result, in a La Niña, Indonesia and eastern Australia experience wetter than usual conditions.

Figure 1:  The circulation of air above the Pacific Ocean during a typical year compared with during an
El  Niño year (Source: Australian Bureau of Meteorology)

Students learn about El Niño and La Niña
Students learn to identify features of El Niño and La Niña