Williams Weather Watch: The Return of El Nino?

There are definite indications that El Nino is returning. Since spring of 2001, we have not been in either an El Nino or La Nina state, but, rather, a state meteorologists call ENSO-Neutral. ENSO stands for El Nino/Southern Oscillation, a cycle that refers to the coherent, large-scale fluctuation of ocean temperatures, rainfall, atmospheric circulation, vertical motion and air pressure across the tropical Pacific. That means that there has been neither abnormally warm sea-surface temperatures across the eastern tropical Pacific – that would make up an El Nino event – nor abnormally cold sea-surface temperatures across the eastern tropical Pacific – that would constitute a La Nina event.

However, Dr. Vernon Kousky, Chief of the ENSO Analysis Branch of the Centers for Environmental Prediction in Camp Springs, MD, explains that “although most oceanic and atmospheric indices reflect ENSO-neutral conditions, there are indications that a warm episode will develop during the remainder of 2001.” In other words, the Climate Prediction Center’s series of long-term global climate models are suggesting the reemergence of El Nino – the first since 1998 – by the end of this year, possibly continuing into the first half of 2002. However, Dr. Kousky cautions that “considering both the sea-surface-temperature predictions and the observed oceanic and atmospheric circulation patterns, it seems most likely that the intensity of the warming will be weak or moderate.” Of course, the extent to which a reemergence of an El Nino may affect our weather this winter and spring is, in fact, largely linked to its strength.

When I talked with Kousky, he cautioned that “the Northeast does not have a consistent relationship with El Nino.” While he did add that the last two very strong El Nino episodes, which occured during 1982-1983 and 1997-1998, provided generally warmer than normal conditions for most of the contiguous United States, he warned that the effects of a weak El Nino are uncertain at best: “Weak events have sometimes been accompanied by coast-to-coast warmth or by frigid conditions in the eastern two-thirds of the nation.”

One might ask what good ENSO does in helping meteorologists predict seasonal climate variability. The answer is that ENSO is only one part of the long-term climate puzzle. There are often many other factors, which meteorologists and scientists take into consideration, such as the Pacific Decadal Oscillation and the North American and Arctic Oscillations. At the end of our conversation, Kousky told me that during weak El Nino events – what he predicts this winter to be – the Arctic Oscillation “is the main player for the Northeast.” “Under weak conditions it is the Arctic Oscillation that becomes the dominant player. We can’t predict the Arctic Oscillation, but the trend in recent years has been downward” meaning frequent, strong cold air outbreaks over the northern half of the country (like last winter) may, in fact, be the biggest weather story this winter.