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HOME > Expert Assessments > ENSO Diagnostic Discussion
issued by
September 7, 2001

Although most oceanic and atmospheric indices reflect ENSO-neutral conditions, there are indications that a warm episode will develop during the remainder of 2001. 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 (Fig. 1). 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 Niño) episodes. Accompanying this evolution has been a gradual transition from negative to positive SST anomalies (Fig. 2) and a gradual return to near normal low-level winds (Fig. 3) in the central equatorial Pacific.

In recent months, many tropical Pacific atmospheric and oceanic variables have shown evidence of modulation by rather strong intraseasonal (30-60 day) fluctuations, associated with the Madden-Julian Oscillation (MJO). In the past, MJO activity has been instrumental in producing low-level wind fluctuations during both the onset and demise phases of warm episodes. During the onset phase, westerly wind bursts, associated with the convectively active phase of the MJO, initiate eastward propagating oceanic Kelvin waves that often result in subsequent oceanic warming in the eastern equatorial Pacific. This process tends to occur more often during the transition seasons (March-May and September-November) when the monsoon systems of both the Northern and Southern Hemispheres are in transition from winter to summer or vice versa and when the westerly wind bursts, associated with the MJO, tend to occur along the equator. Since we are currently entering a transition season, special attention will be focused on the evolution of the MJO, and any effect that it has on surface and subsurface ocean temperatures.

There is no clear consensus amongst the latest statistical and coupled model predictions. However, several of these predictions indicate that positive SST anomalies will continue in the central equatorial Pacific during the remainder of 2001 and into the first half of 2002. The impacts that this warming will have on global temperature and precipitation patterns depend to a large degree on its intensity. At the moment, considering both the SST predictions and the observed oceanic and atmospheric circulation patterns, it seems most likely that the intensity of the warming will be weak or moderate.

Weekly updates for SST, 850-hPa wind, OLR and the equatorial subsurface temperature structure are available on the Climate Prediction Center homepage at: (Weekly Update).  Forecasts for the evolution of El Niņo/La Niņa are updated monthly in CPC's Climate Diagnostics Bulletin Forecast Forum. To receive an e-mail notification when updated ENSO Diagnostic Discussions are released please send your e-mail address to:

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Page Author: Climate Prediction Center Internet Team
Page last modified: September 7, 2001
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