Mature cold episode (La Niņa) conditions continued
during March 2001, as sea surface temperatures (SSTs) remained more than 1.0°C below
average across portions of the central equatorial Pacific between 160°E and 160°W. The
persistent pattern of stronger-than-normal low-level easterlies over the central
equatorial Pacific, which has been characteristic of the La Niņa conditions since
mid-1998, continued during February-March 2001 (Fig. 1). Since
early February 2001, SSTs have become anomalously warm in many sections of the eastern
tropical Pacific, while remaining below normal in the central equatorial Pacific (Fig. 2). Positive SST anomalies were also observed in the eastern
equatorial Pacific during March-April 1999 and 2000 (Fig. 3). 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.
Since the demise of the 1997-98 El Niņo, many ENSO indices have shown distinct annual
cycles, with the northern winter seasons featuring 1) minima in the SST, 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 Niņo. During both of these post-strong El
Niņo periods the anomalous annual cycles were accompanied by an enhanced Australasian
Over the past two years there has been a gradual expansion of the area of positive
equatorial subsurface temperature anomalies into the central Pacific. 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
gradually weaken over the next several 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 the second half of 2001.
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 Niņo/La Niņa 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 10th of each month on the CPC web site.