Mature cold episode (La Niņa) conditions continued
during February 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 (Fig. 1). Since early February 2001, SSTs have become anomalously warm
in many sections of the eastern tropical Pacific (Fig. 2). Similar
features were observed at about the same time of the year in both 1999 and 2000 (Fig. 3). In both of those years the anomalous warming of the eastern
equatorial Pacific SSTs lasted until 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 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. 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.