El Niño and La Niña

 

Both El Niño and La Niña have a strong effect on Australian rainfall, with both having major impacts on Australian agriculture. El Niños are often associated with below average rainfall over much of eastern Australia with severe droughts sometimes the result. La Niñas, on the other hand, are usually associated with wetter than normal periods, and have been linked to several major flooding events during the 20th century.

 

 

El Niño is part of an ocean temperature “see saw” that constantly rocks back and forth across the equatorial Pacific. Normally the sea surface temperatures across the western equatorial Pacific Ocean are much warmer than those in the east because of prevailing winds and ocean currents. However on occasion, this situation is changed and the El Niño develops. When this happens the warmer water tends to accumulate off the coastline of South America.

 

 

La Niña conditions are opposite to El Niño, when sea surface temperatures cool over the central and eastern equatorial Pacific Ocean. This leaves the temperatures over the western Pacific, adjacent to Australia, far warmer.

 

 

For much of the time sea surface temperatures across the equatorial Pacific are somewhere between an El Niño and a La Niña with cycles between the two occurring in an irregular fashion. When the temperature patterns are between the recognised El Niño and La Niña thresholds, the situation is said to be neutral.

 

El Niños tend to occur roughly every 3 to 8 years, and typically last for 12 to 18 months. La Niñas or neutral conditions predominate for the rest of the time. 

 

 

El Niños and La Niñas can be effectively monitored by looking at the monthly-averaged difference in mean sea level barometric pressure between Darwin and Tahiti. If this is markedly positive for a period in excess of 3 months, it can point to a developing La Niña, whilst continuing negative values may indicate the onset of an El Niño.