Tropical Pacific Ocean Edges Further Toward El Niño

Australian Bureau of Meteorology

Issued on Tuesday 20 May 2014

·         The tropical Pacific Ocean continues a general trend toward El Niño, with just over half of the climate models surveyed by the Bureau suggesting El Niño thresholds will be exceeded by August. An El Niño ALERT remains in place, indicating at least a 70% chance of an El Niño developing in 2014.

·         The tropical Pacific Ocean surface has warmed steadily since February, with sea surface temperature anomalies increasing by 0.5 to 1.0 °C. For El Niño to be established and maintained, the sea surface needs to warm further, and be accompanied by a persistent weakening of the trade winds and a consistent increase in cloudiness near the Date Line. In the past fortnight, trade winds have generally been near normal, though have weakened once again in recent days.

·         El Niño has impacts on many parts of the world, for example, below-average rainfall in the western Pacific and Indonesian regions and increased rainfall in the central and eastern Pacific. For Australia, El Niño is usually associated with below-average rainfall over southern and eastern inland Australia, with about two thirds of El Niño events since 1900 causing major drought over large parts of the continent.

·         The Indian Ocean Dipole (IOD) is currently neutral. Model outlooks suggest the IOD is most likely to remain neutral through winter, with two of the five models surveyed suggesting a positive IOD may develop during spring. Positive IOD events often coincide with El Niño and are typically associated with large parts of southern and central Australia experiencing lower rainfall than usual.

INTERNATIONAL EL-NINO FORCASTS

 May 8 2014Commentary from Michelle L’Heureux, NOAA Climate Prediction Center

Much media attention has been directed at the possibility of an El Niño brewing this year. Many outlets have drawn comparison with the 1997-98 super El Niño. So, what are the odds that El Niño will occur? And if it does, how strong will it be? To track El Niño, meteorologists at the NOAA/NWS Climate Prediction Center (CPC) release weekly and monthly updates on the status of the El Niño-Southern Oscillation (ENSO). The International Research Institute (IRI) for Climate and Society partner is part of www.climate.gov (co-sponsored by the NOAA Climate Programs Office).

As far back as November 2013, the CPC and the IRI have predicted an elevated chance of El Niño (relative to historical chance or climatology) based on a combination of model predictions and general trends over the tropical Pacific Ocean. Once the chance of El Niño reached 50% in March 2014, an El Niño Watch was issued to alert the public that conditions are more favorable for the development of El Niño.

Current forecasts for the Nino-3.4 SST index (as of 5 May 2014) from the NCEP Climate Forecast System version 2 model.

More recently, on May 8th, the CPC/IRI ENSO team increased the chance that El Niño will develop, with a peak probability of ~80% during the late fall/early winter of this year. El Nino onset is currently favored sometime in the early summer (May-June-July). At this point, the team remains non-committal on the possible strength of El Niño preferring to watch the system for at least another month or more before trying to infer the intensity. But, could we get a super strong event? The range of possibilities implied by some models allude to such an outcome, but at this point the uncertainty is just too high. While subsurface heat content levels are well above average (March was the highest for that month since 1979 and April was the second highest), ENSO prediction relies on many other variables and factors. We also remain in the spring prediction barrier, which is a more uncertain time to be making ENSO predictions.

Could El Niño predictions fizzle? Yes, there is roughly a 2 in 10 chance at this point that this could happen. It happened in 2012 when an El Nino Watch was issued, chances became as high as 75% and El Niño never formed. Such is the nature of seasonal climate forecasting when there is enough forecast uncertainty that “busts” can and do occur. In fact, more strictly, if the forecast probabilities are “reliable,” an event with an 80% chance of occurring should only occur 80% of the time over a long historical record. Therefore, 20% of the time the event must NOT occur.

While folks might prefer total certainty in our forecasts, we live in an uncertain world. El Niño is most likely to occur this year, so please stay attentive to the various updates.

Reference:                                                                                             

·         CPC: Climate Protection Centre.                             NOAA: National Oceanic and Atmosphere Administration. 

·         NSES: National Science Education Standards.       ENSO: ‘El Niño–Southern Oscillation‘.  

·         NWS: National Weather Service.                           IRI: International Research Institute.                                    

EL-NINO DESCRIPTION (Wikipedia)

El Niño is defined by prolonged warming in the Pacific Ocean sea surface temperatures when compared with the average value. The accepted definition is a warming of at least 0.5°C (0.9°F) averaged over the east-central tropical Pacific Ocean. Typically, this anomaly happens at irregular intervals of two to seven years, and lasts nine months to two years. The average period length is five years. When this warming occurs for only seven to nine months, it is classified as El Niño “conditions”; when it occurs for more than that period, it is classified as El Niño “episodes”. Similarly, La Niña conditions and episodes are defined for cooling.

The first signs of an El Niño are:

1. Rise in surface pressure over the Indian Ocean, Indonesia, and Australia
2. Fall in air pressure over Tahiti and the rest of the central and eastern Pacific Ocean
3. Trade winds in the south Pacific weaken or head east
4. Warm air rises near Peru, causing rain in the northern Peruvian deserts
5. Warm water spreads from the west Pacific and the Indian Ocean to the east Pacific. It takes the rain with it, causing extensive drought in the western Pacific and rainfall in the normally dry eastern Pacific.

El Niño’s warm rush of nutrient-poor water heated by its eastward passage in the Equatorial Current, replaces the cold, nutrient-rich surface water of the Humboldt Current. When El Niño conditions last for many months, extensive ocean warming and the reduction in easterly trade winds limits upwelling of cold nutrient-rich deep water, and its economic impact to local fishing for an international market can be serious.