Forecast Forum - February 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

Strong cold episode conditions continued in the tropical Pacific during February, as sea surface temperatures (SSTs) remained well below normal across the central and east-central equatorial Pacific (Fig. T18 and Table T2). Negative subsurface temperature anomalies and a shallower than normal oceanic thermocline continue to dominate the equatorial Pacific east of the date line (Figs. T16, T17). The cooler-than-normal surface waters contributed to a vigorous Walker circulation over the equatorial central Pacific (Fig. T29) characterized by enhanced low-level easterlies (Fig. T20), enhanced upper-level westerlies (Fig. T21 ) and suppressed rainfall (Fig. E4) and deep convection over the equatorial central Pacific (as inferred from Outgoing Longwave Radiation (OLR) measured by NOAA's polar-orbiting satellites) (Fig. T25).

Given the strength and evolution of existing La Niña conditions, we expect the cold episode to continue for the next three to six months. This is supported by the latest NCEP coupled model forecast (Figs. F3 , F4) which indicates that cold episode conditions will continue through the boreal spring, followed by weakening cold episode conditions thereafter. The NCEP statistical model forecast (Figs. F1, F2) is consistent with the NCEP coupled model through the spring, but it suggests continued cold episode conditions through the remainder of the year. This divergence between the NCEP forecasts also appears in other forecasts, and indicates the considerable uncertainty in expected oceanic conditions during the second half of 1999.

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 continue through March over Indonesia, northern Australia, and southern Africa. Wetter-than-normal conditions are likely to develop over Northeast Brazil and continue through May. Over the United States during the next three months, drier- and warmer-than-normal conditions are expected in southern sections from southern California eastward to the Carolinas. Wetter-than-normal conditions are expected in the northern Great Plains and upper Midwest. Cooler-than-normal conditions are likely over western and central Canada and Alaska.

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).