THE La Niña, the main cause behind the wet summer, has ended.

La Niña prevails when there is a cooling of surface water of the eastern and central Pacific Ocean. In the past few weeks, ocean and weather conditions in the Pacific Ocean have returned to normal.

“As was expected during autumn the 2011/2012 La Niña has now come to an end,” The Weather Channel senior meteorologist, Tom Saunders said.

“Near average conditions have developed in the Pacific Ocean and will remain until at least winter.

“A few key indicators are used to make a call on the end of a La Niña event. Firstly, since the beginning of February sea surface temperatures (SSTs) over the central equatorial Pacific have rapidly warmed, moving us from La Niña into neutral conditions,” he continues.

“When temperatures are at least 0.5C below average La Niña conditions are present. During the peak of the current La Niña SSTs dropped to around 1C below normal, causing increased rainfall and flooding over most of northern and eastern Australia. Temperatures have now warmed to 0.3C, signalling La Niña conditions are no longer present.

“A second indicator is the Southern Oscillation Index (SOI), which measures the pressure difference between Tahiti and Darwin. Late last year the SOI reached +24, well above La Niña thresholds of about +8, but during February the SOI averaged only +2.5.

“Other indicators have been slower to respond, with date line cloud and trade winds remaining well within La Niña thresholds during February, but both have approached normal levels during the past few weeks.”

Saunders says it’s too early to forecast what phase the Pacific Ocean will move to in winter.

“With Australia’s northern wet season also coming to an end we can expect drier weather through most northern and central parts of the country during April and May,” he says.

“The exception is eastern New South Wales which often receives its heaviest rain during autumn, meaning La Niña’s demise will not necessarily bring the sunshine most Sydneysiders are hoping for.”