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Forecast Forum - September 1999

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 Cane and Zebiak model (Cane et al. 1986, Nature, 321, 827-832; Zebiak and Cane 1987, Mon. Wea. Rev., 115, 2262-2278) are shown in Figs. F5 and F6. Predictions from the modified Cane and Zebiak model (Chen et al. 1998, Geophys. Res. Let., 103, 2387-2840), referred to in the figures as LDEO3, are shown in Figs. F7 and F8. Predictions using linear inverse modeling (Penland and Magorian 1993, J. Climate, 6, 1067-1076) are shown in Figs. F9 and F10. Predictions from the Scripps / Max Planck Institute (MPI) hybrid coupled model (Barnett et al. 1993, J. Climate, 6, 1545-1566) are shown in Fig. F11.

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

Discussion and Outlook

Cold episode (La Nia) conditions continued in the tropical Pacific during September. Sea surface temperatures (SSTs) averaged more than 1C below normal across the tropical Pacific between 150W and the South American coast, with weak negative anomalies extending westward to 160E (Figs. T9, T18). Consistent with this, tropical convection [as inferred from anomalous outgoing longwave radiation (OLR)] remained suppressed across the western and central equatorial Pacific during the month (Fig. T25) and the low-level equatorial winds remained stronger than normal across the same region (Fig. T20).

The pattern of subsurface temperature anomalies has been highly persistent, with positive anomalies in the western equatorial Pacific and negative anomalies farther east (Fig. T17). Consistent with this pattern, the oceanic thermocline remained deeper (shallower) than normal in the western (eastern) equatorial Pacific (Fig. T16). Over the last couple of months temperatures at thermocline depth have decreased to more than 4C below normal across the east-central and eastern tropical Pacific. Also, the positive sub-surface temperature anomalies in the west-central equatorial Pacific continue to show little evidence of an eastward shift, indicating that cold episode conditions are likely to persist in the tropical Pacific through the Northern Hemisphere winter 1999/2000. This assessment is supported by the most recent NCEP coupled model forecast (Figs. F3 and F4) and by other available coupled model and statistical model predictions that indicate cold episode conditions persisting through the Northern Hemisphere spring of 2000.

Weekly updates of SST, 850-hPa wind and OLR are available on the Climate Prediction Center homepage at: http://www.cpc.ncep.noaa.gov (Weekly Update).


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