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HOME > Expert Assessments > Climate Diagnostics Bulletin

Forecast Forum - November 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 (La Niņa) episode conditions intensified in the equatorial Pacific during November as sea surface temperature (SST) anomalies became increasingly negative across the central and eastern tropical Pacific (Fig. T18 and Table T2). Consistent with this pattern, the depth of the 20°C isotherm remained deeper (shallower) than normal in the western (eastern) equatorial Pacific during November (Fig. T16), with the thermocline more than 30 m shallower than normal from 135oW to 105oW. Over the past several months the pattern of subsurface temperature anomalies has been very persistent, with positive anomalies in the western equatorial Pacific and negative anomalies in the eastern equatorial Pacific (Fig. T17). Accompanying these conditions, tropical convection remained suppressed across the western and central equatorial Pacific and enhanced over Indonesia and the Philippines (Fig. T25).

The associated low-level wind field featured a continuation of enhanced easterlies over the western and central tropical Pacific and near-normal easterlies over the eastern Pacific (Fig. T20). Low-level easterly anomalies have been confined to the western and central equatorial Pacific since June 1998 (Fig. T7). At upper levels, westerly winds were enhanced over much of the central and eastern tropical Pacific during November (Fig. T21). This overall wind pattern reflected an enhanced equatorial Walker circulation, and was consistent with the La Niņa-related pattern of anomalous tropical convection. All of these overall conditions been very persistent for the past 18 months (Figs. T7, T8, T11), and are consistent with the ongoing cold episode.

In the past the depth of the 20°C isotherm has been a good indicator of transitions to either warm or cold episodes. The onset of warm (cold) episodes is preceded by a deeper-than-normal (shallower-than-normal) thermocline across the central and eastern equatorial Pacific. At the present time the subsurface oceanic conditions show no signs of an evolution toward a pre-warm episode state. Thus, it is likely that cold episode conditions will persist in the tropical Pacific for at least the next several months. 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.

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 occur during the December-March period over Indonesia, northeastern Australia, southern and southeastern Africa, and northeastern South America. In the United States, wetter-than-normal conditions are expected in the Pacific Northwest and Great Lakes regions. Also, drier-than-normal and warmer-than-normal conditions are expected along the southern tier of the United States and cooler-than-normal conditions are expected along the northern tier of the United States.

Weekly updates of SST, 850-hPa wind and OLR are available on the Climate Prediction Center homepage at: (Weekly Update).

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