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EL NIŅO/SOUTHERN OSCILLATION (ENSO)

DIAGNOSTIC ADVISORY 2000/7

Cold Phase

issued by

CLIMATE PREDICTION CENTER/NCEP

July 11, 2000

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.

 

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