Mature cold episode (La Niņa) conditions continued
during January 2001, as sea surface temperatures (SSTs) remained more than 1.0°C below
average across the central equatorial Pacific between the date line and 160°W (Fig. 1, top right). The SST anomaly pattern for January is similar to,
but weaker than, the patterns observed during January 1999 and January 2000 (Fig. 1, top left and top middle, respectively). The January patterns
of anomalous 850-hPa zonal wind and precipitation also show remarkable similarity among
the three years, with low-level easterly anomalies and below normal precipitation over the
central and western equatorial Pacific (Fig. 1).
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 (Fig. 2), 2) maxima in the OLR anomalies (Fig. 3),
and 3) maxima in the low-level easterly winds (Fig. 4) 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.
Based on current conditions in the tropical Pacific, on the NCEP SST predictions, and
on results from historical studies on the effects of cold episodes, we expect
wetter-than-normal conditions to prevail over Indonesia, northern Australia, Northeast
Brazil and portions of southern Africa during the remainder of the NH winter. Over the
United States warmer-than-normal conditions are expected along the southern tier of states
from southern California eastward to Florida, while cooler-than-average conditions are
likely over western and central Canada and in the upper Midwest and Great Lakes.
Weekly updates for SST, 850-hPa wind, and OLR 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.