El Nino bias retreats

Peter McMeekin, Nidera Australia, May 16, 2017

THE El Niño is a climate cycle in the Pacific Ocean that has an influence on global weather patterns.

Nidera Australia, Peter McMeekin

El Niño refers to the extensive warming of the central and eastern tropical Pacific that leads to a major shift in weather patterns across the Pacific. El Niño events are often accompanied by cooler-than-normal sea-surface temperatures (SSTs) in the western Pacific, and to the north of Australia.

Typically, the equatorial trade winds blow from east to west across the Pacific Ocean. During the El Niño climate phenomenon the prevailing trade winds tend to weaken, or even reverse direction. These changes set up a feedback loop between the atmosphere and the ocean that boosts El Niño conditions.

In Australia, El Niño conditions are generally, but not necessarily, associated with below-average rainfall, particularly over the eastern states. Since 1900 there have been 27 recorded El Niño events. Two thirds of these have resulted in significant moisture stress across some or all of the winter cropping regions of the continent.

In the latest update from the Bureau of Meteorology (BoM), the El Niño-Southern Oscillation (ENSO) for the tropical Pacific region is in neutral territory. However, there has been a warming trend across the tropical Pacific Ocean since the start of 2017 and they are saying that there is still a 50 per cent chance that El Niño will develop in the second half of this year.

Another key indicator is the Indian Ocean Dipole (IOD). The IOD is the difference in sea surface temperature between two areas (or poles, hence a dipole) – a western pole in the Arabian Sea (western Indian Ocean) and an eastern pole in the eastern Indian Ocean, south of Indonesia.

Four out of six climate models currently suggest a positive IOD is likely to develop during the southern hemisphere winter. In years when a positive IOD coincides with El Niño, the pattern of below-average rainfall extends further west than it typically would under El Niño alone.

In recent weeks a number of the long range forecast models have reduced their El Niño bias for the spring. This is primarily due to a colder-than-normal band of subsurface water in the eastern Pacific Ocean remaining a similar size or even growing in recent months, contrary to the normal El Niño trend.

In April, the US model was forecasting a steady increase in the development of El Niño throughout the southern-hemisphere spring. By early May, their confidence in this forecast had reduced, with the mean prediction now below El Niño levels throughout our spring.

The 10-day BoM ACCESS model is forecasting very good, and timely, general rains for the eastern states with at least 25-50mm expected across most of the winter-cropping regions. In Western Australia and South Australia, only showers and thunderstorms are forecast at this stage, with rainfall totals forecast to be much lower.

As we all know, long-range forecasting is not an exact science, but it would be fair to say that the information and models available today are much better and far more accurate than their predecessors. It is also important to note that as we move away from the boreal spring the forecasting accuracy of these models also increases.

As Maddie Stone so aptly wrote in her Gizmodo article last week: “El Niño could be your introverted friend who bails on the party at the last minute, or it could be the dude who arrives liquored up and ready to rage”.


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