Most oceanic and atmospheric indices reflect ENSO-neutral conditions. However, there are indications of a slow evolution towards a warm episode. Since late June 2001 sea surface temperatures (SSTs) have become anomalously warm in the central equatorial Pacific, with anomalies near 1°C just to the west of the date line (Fig. 1). During the same period, subsurface temperature anomalies have remained positive in the central equatorial Pacific between 160°E and 120°W (Fig. 2), indicating a deeper-than-normal thermocline in that region. This pattern has been observed immediately prior to the onset of past warm episodes.

In recent months, many tropical Pacific atmospheric and oceanic variables have been modulated by intraseasonal (30-60 day) fluctuations (Fig. 3), associated with the Madden-Julian Oscillation (MJO). Low-level wind fluctuations over the central and western tropical Pacific have been consistent with this activity. In the past, the convectively active phase of the MJO has been instrumental in producing low-level westerly wind bursts that are linked to subsequent oceanic warming in the eastern equatorial Pacific during the onset phase of warm episodes, especially during the transition seasons (March-May and September-November). A significant westerly wind burst occurred over the western equatorial Pacific during mid-October. This event resulted in additional deepening of the oceanic thermocline and an increase in subsurface temperature anomalies in the central equatorial Pacific by the end of the month. The Climate Prediction Center will closely monitor the future evolution of the MJO, and any effects that it has on surface and subsurface temperatures during the next 1-2 months.

The latest statistical and coupled model predictions indicate either weak warm or near-normal conditions in the equatorial Pacific during the remainder of 2001 and into the first half of 2002. However, all of these prediction techniques have difficulty in making skillful forecasts during ENSO transitions. Considering the SST predictions, the time of year, and the observed oceanic and atmospheric circulation patterns, it seems most likely that a gradual evolution to warm episode conditions will continue in the tropical Pacific over the next several months.

Weekly updates for SST, 850-hPa wind, OLR and the equatorial subsurface temperature structure are available on the Climate Prediction Center homepage at: http://www.cpc.ncep.noaa.gov (Weekly Update).  Forecasts for the evolution of El Niño/La Niña 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: