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Forecast Forum - May 2001

The canonical correlation analysis (CCA) forecast of SST in the central Pacific (Barnett et al. 1988, Science, 241, 192-196; Barnston and Ropelewski 1992, J. Climate, 5, 1316-1345), is shown in Figs. F1 and F2. This forecast is produced routinely by the Prediction Branch of the Climate Prediction Center. The predictions from the National Centers for Environmental Prediction (NCEP) coupled ocean/atmosphere model (Ji et al. 1998, Mon. Wea. Rev, 126, 1022-1034) are presented in Figs. F3 and F4. Predictions from the latest version of the LDEO model (Chen, D., M. A. Cane, S. E. Zebiak, Rafael Canizares and A. Kaplan, 2000, Geophys. Res. Let., accepted) are shown in Figs. F5 and F6. Predictions using linear inverse modeling (Penland and Magorian 1993, J. Climate, 6, 1067-1076) are shown in Figs. F7 and F8. Predictions from the Scripps / Max Planck Institute (MPI) hybrid coupled model (Barnett et al. 1993, J. Climate, 6, 1545-1566) are shown in Fig. F9.

The CPC and the contributors to the Forecast Forum caution potential users of this predictive information that they can expect only modest skill.

Outlook

Near-normal conditions are likely in the tropical Pacific during Northern Hemisphere summer, followed by near-normal or slightly warmer-than-normal conditions into early 2002.

Discussion

Near-normal atmospheric and oceanic conditions prevailed in the tropical Pacific during May. Weak negative SST anomalies across the central and east-central tropical Pacific continued a trend towards 0C during the month (Fig. T9). The oceanic thermocline remained deeper-than-normal in the equatorial west-central and western Pacific (Fig. T15), with temperatures averaging up to 4C above normal at thermocline depth (Fig. T17). The negative temperature anomalies that have characterized the subsurface thermal structure in the eastern Pacific since late 1998 have continued to weaken and decrease in areal extent (Figs. T15, T17). The pattern of tropical convection [as inferred from anomalous outgoing longwave radiation (OLR)] remained generally consistent with weak cold episode conditions, with above-normal precipitation over Indonesia (Fig. T25). Consistent with the decrease in magnitude of the negative SST anomalies across the central equatorial Pacific in recent months, the low-level easterly wind anomalies across the central and western tropical Pacific were weak (Fig. T20).

Over the past two years there has been a gradual eastward expansion of the area of positive subsurface temperature anomalies into the central Pacific (Fig. T15) and a gradual decrease in the strength of the negative SST anomalies (Fig. T9). This evolution is consistent with a slow decay of the subsurface thermal structure, which characterizes the mature phase of cold episodes, towards neutral conditions in the tropical Pacific. Thus, it is likely that near-normal conditions will continue during the summer of 2001. This assessment is generally supported by the most recent NCEP statistical (Figs. F1 and F2) and coupled model forecasts (Figs. F3 and F4), as well as by other available coupled model and statistical model predictions (Figs. F5, F6, F7, F8, F9), which indicate near-normal conditions during Northern Hemisphere summer. Thereafter, the models indicate near-normal or slightly warmer-than-normal conditions during late 2001 and early 2002.

Weekly updates of 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).


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