Monday, September 13, 2010

El Nino and La Nina

For many centuries Peruvian fishermen were aware of a warm ocean current that periodically appeared around Christmas time along the northwest coast of South America. It was dreaded by the locals because it decimated their main fish catch – the cold water anchovy.

They called this phenomenon El Niño – Spanish for “boy child” – and it was realised in more modern times that this was not just a local event but part of a broad warming of the equatorial Pacific Ocean waters that stretched thousands of kilometres westward from the South American coastline.

Further research into the phenomena showed that El Niño was 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 reversed and the El Niño develops.

The El Niño pattern of sea surface temperatures. The thermocline referred to is the line that separates the mixed upper layers of the ocean from the calm, deep waters below.
(Image from Wikipedia Commons)

And, as with most things in nature, there is an opposite phase, and this is when sea surface temperatures cool over the central and eastern equatorial Pacific Ocean, leaving the temperatures over the western Pacific far warmer. This configuration was given the name La Niña – Spanish for “girl child”.

Like the Peruvians with El Niño , the Fijians were aware of La Niña and they related it to the behaviour of the mango tree. They believed when the mango tree flowered early, a bad tropical cyclone season would follow. And there is science behind this observation as the tree will flower early when waters in the surrounding ocean are warmer than normal, and this in turn, promotes tropical cyclone development.

In modern terms we would say the same thing in a different way. That is, during times of La Niña, there is usually increased tropical cyclone frequency over the western Pacific Ocean – a neat convergence of ancient and modern science.

The La Nina pattern of Sea surface temperatures. (Image from Wikipedia Commons)

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.

Both these situations have a profound effect on Australian rainfall, including much of eastern Australia. The El Niño is normally associated with drier than average conditions for east and southern parts of the country, whilst La Niña conditions tend to promote above average rainfall over most parts of eastern Australia.

At present La Niña conditions predominate and we have seen a persistence of well above average rainfall over much of inland Australia during the last six months, with record totals falling over northern parts of South Australia, southwest Queensland, much of Victoria and southern parts of the Northern Territory.

At the moment we also have warmer than average sea surface temperatures to the northwest of Australia, and together with the prevailing La Nina conditions, this increases the possibility of above average rain over much of the continent during the remainder of spring.

Footnote 1: Whilst we have Spanish names for both the opposite phases - El Niño and La Niña – there is no such term for neutral conditions. I’ll bite the bullet and suggest “La Bamba”.

Footnote 2: Since this was originally posted in September 2010, much of eastern Australia has experienced its wettest ever spring and summer in what turned out to be one of the most powerful La Niñas in recent times.