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This Winter's Temperature: Is It a Boy or a Girl?

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Bill Chameides, recently appointed to America's Climate Choices, blogs regularly at theGreenGrok.com.

Predicting climate variations from year to year is dicey. So many variables can play a role. Of particular importance is the state of the southern Pacific Ocean. It now looks like the ocean may be headed for another La Niña - if so, colder winter temperatures, among other things, are likely.

Think of the southern Pacific Ocean as a huge bathtub of seawater that from time to time sloshes back and forth between its eastern boundary (South America) and its western boundary (Asia). Because these sloshing effects often first hit South America around Christmas time, this state of the oceans was named El Niño, or "Little Boy" for the Christ child. By comparison, the opposite slosh state is called La Niña, or the Little Girl. Scientists, who love complicated sounding anagrams, call the entire sloshing back-and-forth phenomenon the El Niño-Southern Oscillation, or ENSO.

What makes ENSO so interesting is that all this sloshing can profoundly affect the climate around the globe - leading, for example, to heavy precipitation in some areas and drought conditions in others. For instance, El Niño events bring unusually wet conditions to the southern United States, while La Niña events bring dry, drought-like conditions to this same region (more info on La Niña here).

El Niños and La Niñas Are Global Temperature Wild Cards

Perhaps even more importantly, the El Niño and La Niña states can affect the year-to-year variations in global temperatures. High surface ocean temperatures typical of El Niños favor a release of heat from the ocean to the atmosphere and thus relatively warm temperatures. La Niñas do the opposite. Look, for example, at 1998, the warmest (or second warmest depending upon which analysis you use) year on record - it was also a year with a very strong El Niño. This was followed by a moderate La Niña in 1999 and 2000, years with relatively cool temperatures.

2009-01-07-enso_index2.jpg
The El Niño/Southern Oscillation (ENSO) phenomenon affects year-to-year climate variability. During El Niños (represented by red) Pacific Ocean waters tend to be warmer than normal; during La Niñas (in blue) they tend to be cooler. Predictions show another cold winter might be on the way, thanks to an oncoming La Niña. (Source: NOAA - http://www.cdc.noaa.gov/ENSO/enso.mei_index.html)

It's important to bear in mind that while ENSO can strongly affect year-to-year variations on global temperatures, all this sloshing back and forth does not necessarily lead to long-term climate change. Doing so would require a net transfer of heat to or from the ocean over many ENSO cycles; a canceling out of heat transfer over El Niños and La Niñas will not lead to a temperature trend. In fact, since ocean temperature observations show that the heat content of the oceans has been increasing over the past 40 or so years, if anything, the ocean has been cooling the atmosphere (working against global warming) as opposed to warming the atmosphere.

Nevertheless, because the ENSO can affect global conditions from year to year, governments, farmers, fishermen, and others could really benefit from accurate predictions of what's coming down the pike. We are getting better and better at doing just this, thanks to our models. Knowing what kinds of conditions lead to La Niñas and El Niños, scientists continuously look at the state of the atmosphere and oceans and based on what's happening, they use a model to predict future state of the ocean. The further out the prediction, the less accurate the model becomes.

In general, this approach does a pretty good job of forecasting ENSO trends about one to three seasons ahead of time. These forecasts are posted and discussed here.

What's in Store for this Year

So where are we headed? Well, the emerging picture indicates chill. Remember how last winter was particularly cold? It turns out that ENSO probably had something to do with that, as a strong La Niña paid a visit in late 2007 and 2008. The La Niña ended around May of 2007 and we have been in so-called ENSO-neutral conditions since. I thought that perhaps we could now expect an El Niño. But maybe not.

Forecasts from December suggest that there is about a 50 percent chance of a La Niña coming our way. So do we have another cold winter to look forward to? Updated forecasts will be posted on Thursday -- stay tuned.

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