Re: Will El Nino occur this winter?

The El Nino / Southern Oscillation (ENSO) phenomenon is a coupled interaction between the the atmosphere and upper ocean in the equatorial Pacific. The details of how it works are complicated, not fully understood, and not needed to answer your question, but a brief introduction can be found here.

The strength of ENSO is commonly measured using the "NINO3" index. The sea surface temperature (SST) of the equatorial Eastern Pacific is usually relatively cold, but during an El Nino, it becomes several degrees warmer than normal. The NINO3 index is the difference between current SST and the time-averaged SST, averaged over the vertically-hatched box shown in this figure. El Nino conditions occur when the temperature in the box is more than a couple of degrees warmer than average.

There are several models which are currently attempting to forecast El Nino. They are of several different types, including "Coupled General Circulation Models", which describe the physics of the atmosphere and ocean in minute detail, and "statistical models", which attempt to predict what's going to happen based on what was seen before.

The Australian Bureau of Meteorology has a website summarizing the predictions of several models. According to this site,

The consensus among the ocean models is for a slight warming over the next 6 to 9 months, with the NINO3 anomaly remaining NEUTRAL for both January and April 2001. The COLA [model] is the most divergent of the predictions with Cool conditions predicted at both 6 and 9 months lead.

And now, the important caveats. We really don't know yet whether these models are any good at predicting El Nino. Many model designers said they could predict El Nino back in 1996... but then completely failed to predict the 1997 El Nino, which was one of the largest ever recorded. We do have some idea about how the system works and how to model it, but we are prepared for a big surprise... and you should be too.

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