Focus Areas E2- Oceanography

How does La Niña 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-La Niña, non-El Niño 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 ‘La Niña’ years for example, the westward flow of cold water across the Pacific increases in strength and so the warm water pool shifts to the west; the central and eastern Pacific Ocean becoming cooler than usual relative to the temperatures in the western Pacific. On the other hand, in ‘El Niño’ years, the warm water pool shifts east away from the Australian coast under the influence of weaker currents from the east.

Associated with these patterns of circulation in the waters of the Pacific Ocean there are changes 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 La Niña years, as the pool of warm oceanic waters shifts westward there is an associated shift of the rising flow of moist warm air towards Indonesia and the west. Rainfall increases over Indonesia and the western Pacific region and decreases over the east-central and eastern Pacific Ocean. In contrast, during an El Niño the rising moist air currents most eastwards. As a result, Indonesia and eastern Australia experience dryer than usual conditions.

Figure 1: Pattern of Pacific Ocean temperatures and atmospheric circulation during a La Niña episode
compared with an El Niño episode
(Source: Bureau of Meteorology, Australia)


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