Cold episode (La Niña) conditions weakened during April 2001, as sea surface temperature (SST) anomalies trended toward 0°C throughout the tropical Pacific. However, the persistent pattern of stronger-than-normal low-level easterlies over the central equatorial Pacific, which has been characteristic of the La Niña conditions since mid-1998, continued during March-April 2001 (Fig. 1). Beginning in early February 2001, SSTs became anomalously warm in many sections of the eastern tropical Pacific, while remaining below normal in the central equatorial Pacific. Similar conditions were observed in the eastern equatorial Pacific during March-April 1999 and 2000. In both of those years the anomalous warming of the eastern equatorial Pacific SSTs lasted until late April or early May and then rapidly disappeared as cross-equatorial flow from the Southern Hemisphere into the Northern Hemisphere became established and seasonal rainfall began to increase over Central America, southern Mexico and the southeastern tropical North Pacific. As in the last two years, the positive SST anomalies rapidly dissipated during late April-early May 2001 (Fig. 2), as the low-level easterlies became anomalously strong over the eastern tropical Pacific (Fig. 1).

Since the demise of the 1997-98 El Niño, many ENSO indices have shown distinct annual cycles, with the northern winter seasons featuring 1) minima in the SST (Fig. 3), 2) maxima in the OLR anomalies, and 3) maxima in the low-level easterly winds over the central equatorial Pacific. The slope of the oceanic thermocline has been greater than normal throughout this period, with positive (negative) subsurface temperature anomalies in the west-central (eastern) equatorial Pacific. The strength of this anomalous subsurface pattern has also displayed an annual cycle since mid-1998. The evolution of the atmospheric and oceanic anomaly patterns since mid-1998 is similar to, but stronger than, that observed during 1984-1986, which followed the strong 1982-83 El Niño. During both of these post-strong El Niño periods the anomalous annual cycles were accompanied by an enhanced Australasian monsoon system.

Over the past two years there has been a gradual expansion of the area of positive equatorial subsurface temperature anomalies into the central Pacific and a gradual decrease in the strength of the negative SST anomalies (Fig. 3). This evolution is consistent with a slow decay of the subsurface thermal structure that characterizes the mature phase of cold episodes. Thus, it is likely that cold episode conditions will continue to weaken over the next few months, with near-normal conditions likely during the summer of 2001. This assessment is generally supported by the most recent NCEP statistical and coupled model forecasts, as well as by other available coupled model and statistical model predictions, which indicate a gradual weakening of cold episode conditions during the next few months. Thereafter, the models indicate near-normal or slightly warmer-than-normal conditions during late 2001 and early 2002.

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. This ENSO Diagnostic Discussion, which replaces the ENSO Advisories, will appear regularly around the 10^th of each month on the CPC web site.