The large-scale oceanic and atmospheric circulation
patterns continued to reflect cold episode (La Niņa) conditions in the tropical Pacific
during June. However, since the beginning of March several atmospheric and oceanic indices
have shown a weakening of La Niņa conditions. Negative SST anomalies have decreased in
magnitude in the central and eastern equatorial Pacific, which has resulted in substantial
increases in the Niņo 3.4 and, Niņo 4 indices (Fig. 1). The
Tahiti-Darwin SOI dropped to near zero in May and to -0.6 in June. The equatorial SOI,
which continues to better reflect the ongoing La Niņa condition, has also decreased,
dropping from its peak of 3.2 in January to 1.2 in June. At the same time the low-level
easterly anomalies in the west-central equatorial Pacific have also diminished in
intensity. This evolution in both atmospheric and oceanic variables is remarkably similar
to that observed during the first six months of 1999 (Fig. 2, Fig. 3).
The most recent NCEP coupled model forecasts and statistical model forecasts, as well
as other available forecasts, exhibit differences in the expected evolution of the SSTs
over the next 3-9 months. The NCEP coupled model forecast and the latest LDEO forecast
indicate that cold episode conditions will continue to weaken during the next 3 months,
followed by near-normal conditions through the end of 2000. The NCEP statistical model
(CCA) forecasts weak cold episode conditions continuing through the end of 2000, with near
normal condition developing in early 2001. The lack of any rapid evolution in the
subsurface thermal structure and the persistence of low-level easterly anomalies over the
central and western equatorial Pacific continues to support a slower decay of the cold
episode conditions than is shown by the NCEP coupled model. Thus, it is likely that cold
episode conditions will gradually weaken over the next 6 months and that near-normal or
slightly cooler than normal conditions will be present in the tropical Pacific at the end
of the year.
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.
Climate Prediction Center
National Centers for Environmental Prediction
NOAA/National Weather Service
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5200 Auth Road
Camp Springs, MD 20746-4304
e-mail: vkousky@ncep.noaa.gov