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Forecast Forum - July 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

A gradual transition to warm episode conditions is expected to continue in the tropical Pacific through early 2002.

Discussion

Near-normal atmospheric and slightly warmer-than-normal oceanic conditions prevailed in the tropical Pacific during July. SST anomalies averaged less than 1°C at all locations in the tropical Pacific during the month (Fig. T18). However, SSTs and SST anomalies have steadily increased in the central equatorial Pacific since early 2001 (Fig. T9) rising to their highest levels since the 1997-1998 warm episode by late July. The oceanic thermocline remained deeper-than-normal in the equatorial central and western Pacific (Fig. T15), with temperatures averaging around 3°C above normal at thermocline depth (Fig. T17). 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 and areal extent of the negative subsurface temperature anomalies in the eastern Pacific. This evolution is consistent with a decay of the subsurface thermal structure that characterizes the mature phase of cold episodes and with the development of conditions usually found just prior to warm episodes. The pattern of anomalous tropical convection [as inferred from anomalous outgoing longwave radiation (OLR)] was rather disorganized during the month, with areas of below-normal precipitation extending from Indonesia to the date line (Figs. T25, E4). Consistent with this, the low-level wind anomalies across the central and western tropical Pacific were weak (Fig. T20).

Positive SST anomalies are likely to continue in the equatorial Pacific during the remainder of 2001 and the first half of 2002. This assessment is generally consistent with the most recent NCEP statistical (Figs. F1 and F2) and coupled model forecasts (Figs. F3 and F4), as well as most other coupled model and statistical model predictions (Figs. F5, F6, F7, F8, F9) that indicate warmer-than-normal oceanic conditions through early 2002. The impacts that this warming will have on global temperature and precipitation patterns depend to a large degree on its intensity, areal extent, and location. There is considerable spread in the predicted SST anomalies, with most predictions indicating a weak or moderate warm episode (El Nio) by the end of 2001 and the beginning of 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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