Forecast of Tropical Pacific SST Using a Simple Coupled Ocean-Atmosphere Dynamical Model

contributed by Stephen E. Zebiak, Mark A. Cane and Dake Chen

Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York

Since the middle to late 1980s, forecasts of the NINO3 SSTA have been regularly made at Lamont-Doherty Earth Observatory (LDEO) of Columbia University using a simple coupled ocean-atmosphere dynamical model (Cane et al., 1986; Cane and Zebiak, 1987; Zebiak and Cane, 1987). This represented the beginning of a strong movement toward physical approaches to the diagnosis and prediction of climate and its short-term fluctuations. Here we present a few details of this model's current forecasts of SSTA for the tropical Pacific basin. Forecasts using a new version of the model with improved initialization (LDEO3) will be highlighted following a discussion of the standard version of the model (LDEO1).

Figure 1 shows 6, 9 and 12 month lead SST anomaly forecasts by LDEO1 for the tropical Pacific basin, verifying in January, April, and July 1999, respectively. The forecasts are actually ensemble means of forecasts from six consecutive months ranging from February to July 1998. The forecasts are adjusted to have the same mean and standard deviation as observed data on an overall basis (as opposed to individually for each calendar month and lead time, as in Lamont's forecast tables that are not presented here). The forecasts are adjusted for systematic biases. One such bias is an underestimation of the amplitude of anomalies in the central (but not eastern) Pacific, which would cause anomaly maxima to be placed too far east or prevent the central Pacific from fully participating. A statistical correction using singular value decomposition (SVD) is used for this adjustment. The 6, 9 and 12 month lead forecasts shown in Figure 1 indicate that SST will drop to 1-1.5 degrees cooler than normal in the eastern and central equatorial Pacific by January 1999, and then the cooling will gradually become less intense.

A closer look at the forecast integrations for the NINO3 region in particular is provided in Figure 2, where 6 individual SST forecasts beginning from 1-month-apart initial conditions from February to July 1998 are shown along with the ensemble mean used for Fig. 1. All runs indicate a cooling trend toward the end of the year and moderately cool SST throughout 1999.

The new version of the model, LDEO3, follows the same coupled initialization procedure of its predecessor LDEO2 (Chen et al., 1995) except that it assimilates sea level data in addition to winds (Chen et al., 1998). This version retains the good overall skill of LDEO2 for the 1970s and 1980s while performing much better for the 1990s, especially for the recent El Niño event. The improvement is attributed to the extreme effectiveness of the sea level data in the vicinity of the equator in correcting the model ocean state and preconditioning it for prediction. LDEO3 represents the first step of our effort at LDEO to develop a systematic procedure that assimilates multiple data sets into the Lamont forecast system. The current forecasts made by LDEO3 are shown in Figure 3. As compared to LDEO1 (Figure 2), it follows observations more closely at the initial stage and predicts a relatively stronger cooling for 1999.

References

Cane, M. A., S. E. Zebiak and S. C. Dolan, 1986: Experimental forecasts of El Nino, Nature, 321,827-832.

Cane, M. A., and S. E. Zebiak, 1987: Prediction of El Nino events using a physical model, In Atmospheric and Oceanic Variability, H. Cattle, Ed., Royal Meteorological Society Press, 153-182.

Chen, D., S. E. Zebiak, A. J. Busalacchi and M. A. Cane, 1995: An improved procedure for El Nino forecasting: implications for predictability. Science, 269, 1699-1702.

Chen, D., M. A. Cane, and S. E. Zebiak, 1998: The impact of sea level data assimilation on the Lamont model prediction of the 1997/98 El Nino, Geophys. Res. Lett., 25, 2837-2840.

Zebiak, S. E. and M. A. Cane, 1987: A model El Nino-Southern Oscillation. Mon.Wea. Rev., 115, 2262-2278.

Figure 1: LDEO1 predicted SSTA at 6, 9 and 12 month lead times. Predictions are based on ensemble mean individual forecasts initialized from six consecutive months ending in May 1998. Forecasts for each lead time (independent of start month) have been corrected for systematic biases using a singular value decomposition analysis based on the years 1972-92.

Figure 2: LDEO1 time evolution of NINO3 forecasts initialized one month apart from December 1997 to May 1998. The thin solid line is the ensemble mean of the 6 individual forecasts. The thick bold line is the observed NINO3 temperature anomaly.

Figure 3: Same as Fig. 2 except for LDEO3.