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Forecast Forum - July 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

The overall patterns of oceanic temperature and atmospheric circulation in the tropical Pacific were consistent with continued weak cold-episode conditions during July. The pattern of sea surface temperature (SST) anomalies remained similar to that observed in recent months, with weak negative anomalies throughout the equatorial Pacific east of 160 E (Figs. T9, T18). Consistent with this, tropical convection was weaker-than-normal over the western and central equatorial Pacific (Fig. T25) and low-level easterlies were stronger-than-normal in the vicinity of the date line (Fig. T20).

The pattern of subsurface temperature anomalies has also been highly persistent, with positive anomalies in the western equatorial Pacific and negative anomalies in the eastern equatorial Pacific (Fig. T17). This pattern is consistent with a deeper (shallower) than normal thermocline in the western (eastern) equatorial Pacific (Figs. T15, T16). The positive subsurface temperature anomalies in the west-central equatorial Pacific have shown little evidence of an eastward shift indicating that the present cold episode is likely to continue for at least the next several months. This expectation is supported by the latest NCEP coupled model forecast (Figs. F3 and F4) and other available coupled model and statistical model predictions that indicate cold episode conditions persisting through the remainder of 1999 and into early 2000.

The continuation of cold episode conditions has contributed to an overall enhancement of the monsoon systems over southeastern Asia, Africa and Central America/ Mexico. Rainfall has been above normal in these regions during the last two months. Especially noteworthy has been 1) well-above-normal monsoonal rainfall over southern China, Southeast Asia and the Philippines, 2) abundant rainfall in sub-Saharan Africa and 3) very wet conditions in the southwestern United States. This rainfall pattern has been accompanied by anomalous upper-tropospheric anticyclonic circulation (ridging) throughout the middle latitudes of the Northern Hemisphere and the subtropics and lower mid-latitudes of the Southern Hemisphere. Historically these patterns of large-scale rainfall and atmospheric circulation have been accompanied by above-normal tropical storm activity in the tropical Atlantic. This association has prompted NOAA scientists to issue an outlook for this Atlantic hurricane season stating that conditions are favorable for an active hurricane season.

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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