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 SST anomaly 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 SST for the tropical Pacific basin. In addition to the standard LDEO model (LDEO1), forecasts using a new version of the model (Chen et al., 1995; 1998) with improved initialization and data assimilation (LDEO3) will also be presented. The 6-month ensemble averaging used in LDEO1 is not needed in LDEO3 because consecutive forecasts are rather consistent with one another in the latter model.
Figure 1 and Figure 2 show 6, 9 and 12 month lead SST anomaly forecasts by LDEO1 and LDEO3 for the tropical Pacific basin, verifying in January, April and July 2000, respectively. Both model forecasts indicate continuing negative SST anomalies in the eastern and central equatorial Pacific in the next spring. A closer look at the forecast integrations for the NINO3 region in particular is provided in Figures 3-6. Figure 3 and Figure 4 show the time series of NINO3 SST anomaly for 6 different lead times, while Figure 5 and Figure 6 show 6 individual forecasts beginning from 1-month-apart initial conditions from February to July 1999 along with the ensemble mean. It is obvious that LDEO3 did a better job in predicting the 1997/98 El Nino as compared to LDEO1. The current forecasts by both models indicate that SST anomaly will be slightly negative to normal in the eastern and central equatorial Pacific over the next few seasons.
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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 July 1999. 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: Same as Figure 1 except for LDEO3 without ensemble averaging.
Figure 3: LDEO1 forecasts of NINO3 SSTA for 6 different lead times. Each forecast is actually the mean of forecasts from 6 consecutive months, adjusted to have the same mean and standard deviation as the observed. Note that 0 month lead can differ from the observed because SST data are not used in initialization. For each lead time, forecast values are indicated by x's and observed values by a solid line. Error bars represent root-mean-square error, based on the years 1972-1987.
Figure 4: Same as Figure 3 except for LDEO3 without ensemble averaging.
Figure 5: LDEO1 forecasts of NINO3 SSTA initialized one month apart from February to July 1999 (dashed curves). The thin solid line is the ensemble mean of the 6 individual forecasts. The thick bold line is the observed NINO3 SSTA.
Figure 6: Same as Figure 5 except for LDEO3.