Neutral El Nino Leads to Slightly Cooler Winter Forecasts

Researchers and scientists at the National Oceanic and Atmospheric Administration (NOAA) are monitoring the formation of a possible weak El Nino and predict that the United States could experience very weak-to-marginal impacts late winter to early spring 2002, which typically means a colder, wetter winter. However, recent indications show a more neutral El Nino pattern developing, NOAA said. As a result, forecasters, such as EarthSat, now are predicting normal to slightly below normal temperatures this winter.

"Although slightly warmer-than-normal ocean waters are being observed in the equatorial Pacific, current conditions in the tropical Pacific are closer to neutral than either El Nino or La Nina," said NOAA's Climate Prediction Center meteorologist Dr. Vernon Kousky. "Over the last two months we have been monitoring warmer than average temperatures in the tropical central Pacific, but slightly warm sea surface temperatures alone do not make an El Nino."

August climate data indicate that the waters in the central equatorial Pacific, near the International Date Line, have averaged at least 84 degrees Fahrenheit, which is approximately one degree Fahrenheit above normal. Researchers say they have not seen any changes in the global temperature and precipitation pattern that would be consistent with an El Nino.

El Nino is an abnormal warming of the ocean temperatures across the eastern tropical Pacific that affects weather around the globe. El Nino episodes usually occur approximately every four-to-five years. It has been a little over four years since the last El Nino event. For NOAA to officially announce the formation of an El Nino, there would have to be persistent weakening of trade winds, precipitation over the warmer than normal waters and sustained sea surface temperatures (SST) of at least a degree Fahrenheit above normal for a number of consecutive months.

Although most oceanic and atmospheric indices reflect ENSO-neutral conditions, there are indications that a warm episode will develop during the remainder of 2001, NOAA said. Over the past two years there has been a gradual eastward shift of the area of positive equatorial subsurface temperature anomalies from the western equatorial Pacific into the central equatorial Pacific. This evolution is consistent with the decay of the subsurface thermal structure that characterizes the mature phase of cold episodes and the development of conditions usually found just prior to warm (El Nino) episodes. Accompanying this evolution has been a gradual transition from negative to positive SST anomalies and a gradual return to near normal low-level winds in the central equatorial Pacific.

"If the forecast and actual values begin to slip toward that weak El Nino side, then the forecast may need to be shifted colder once again," EarthSat said in its latest Seasonal Outlook Discussion. "At this point, the most likely outcome is for a seasonal to below normal winter. A weak El Nino status would more likely trigger a below to possibly much below normal winter period. The least likely outcome is a moderate to strong event (either El Nino or La Nina) that would favor a warmer winter episode (above to much above normal)," the EarthSat outlook stated.

EarthSat said the latest data indicate December and January have the best opportunity for the coldest weather this winter, in contrast to last winter when the November-to-December period was the coldest. The months of February and November look closest to normal if not slightly warmer, while March looks like it could be colder than normal.

"Another interesting change is the colder outlook in the western states," EarthSat said. "It appears that cold air events that slide southward through the Plains and Rockies may occasionally spill over into the Western cities as well, offering cooler anomalies in many western area."

EarthSat said the initial look at next summer shows above normal temperatures in the central third of the nation. "The coasts look more seasonal if not occasionally cooler, but the main producing area of Texas and the Main Midwestern areas appear hotter in the current forecasts for next summer..."

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