The Center for Ocean-Land-Atmosphere Studies (COLA) has recently developed an anomaly coupled prediction system, using sophisticated dynamical ocean and atmosphere models, that produces skillful forecasts of the tropical Pacific sea surface temperature anomaly (SSTA) up to 1.5 years in advance. The details of this coupled prediction system are described by Kirtman et al. (1996) and a brief description of the overall skill of the 30 hindcast predictions was given in the March 1995 issue of this Bulletin. The atmospheric component is the COLA atmospheric general circulation model (AGCM; Kinter et al. 1988) that includes a state-of-the-art land surface model (Xue et al. 1991) and physical parameterizations of radiation, convection, and turbulence. The AGCM is a global spectral model that is horizontally truncated at triangular wavenumber 30 and has 18 unevenly spaced sigma levels in the vertical. The oceanic component is a Pacific basin version of the Geophysical Fluid Dynamics Laboratory (GFDL) ocean model (Pacanowski et al. 1993). In the ocean model there are 20 levels in the vertical with 16 levels in the upper 400 m. The zonal resolution is 1.5 longitude and 0.5 latitude between 20N and 20S. Further details of the ocean model are provided in Huang and Schneider (1995).

We have separately tested the ocean and atmosphere component models in order to evaluate their performance when forced by observed boundary conditions and improvements have been made that are also incorporated into the coupled prediction system. The effects of atmospheric model zonal wind stress errors have been ameliorated by using the zonal wind at the top of the boundary layer to redefine the zonal wind stress at the surface (Huang and Shukla 1996). We have also developed an iterative procedure for further adjusting the zonal wind stress, based on the simulated SSTA errors (Kirtman and Schneider 1996), that improves initial conditions for coupled forecasts (Kirtman et al. 1996).

The Niño 3 SSTA root mean squared error (RMSE) and correlation as a function of forecast lead time were shown in the March 1995 issue of this Bulletin. These two verification measures are computed with respect to the observed SSTA. The correlation in the Niño 3 region remained above 0.6 for lead times of up to 12 months and was larger than that of the persistence forecast for all lead times greater than 3 months.

Figure 1 shows the Niño 3 time series of the predicted SSTA for three forecasts initialized on the first day of July, August and September of 1996, respectively. Each forecast is run for 17 months. All three forecasts show a warming trend from late boreal summer 1996 through winter 1996-97. The July forecast has its peak SSTA in January 1997 which is consistent with the forecast shown in the previous issue of this Bulletin. The warm SSTA in the July forecast decays rapidly during the boreal spring 1997. The August forecast indicates warm temperatures in winter 1996-97 with continued warming through summer 1996. While the warming in winter is weaker with the August forecast, the evolution is fairly consistent with earlier forecasts for the first 6-9 months. The September forecast, however, has a warming trend in boreal winter, but the peak warm SSTA does not occur until summer of 1997. The September forecast deviates from the previous 11 forecasts in that the peak warming occurs during summer 1997 as opposed to winter 1996-97 and spring 1997, without even a relative SSTA maximum at that earlier time.

The horizontal structure of the ensemble mean (average of all three forecasts) SSTA for boreal fall 1996, winter 1996-97 and spring 1997 are shown in the three panels of Fig. 2. The spatial structure of the predicted anomalies for winter 1996-97 and spring 1997 are remarkably similar to forecasts initialized three months earlier shown in the June 1996 issue of this Bulletin. The amplitude of the SSTA in the more recent ensemble, however, is reduced by 25-35%, primarily due to the forecast initialized in September 1996. The most recent ensemble forecast for fall 1996 is for near normal conditions, whereas previous forecasts have indicated weak warm conditions. It should be noted that the amplitude of the boreal winter and spring warming has reduced with this most recent ensemble mean and the consistency among the ensemble members is also reduced. Nevertheless, the ensemble forecast still calls for warm conditions from the winter of 1996-97 through spring of 1997.

Acknowledgments: This research is part of a larger group effort at COLA to study the predictability of the coupled system. Many members (D. DeWitt, M. Fennessy, J. Kinter, L. Marx and E. Schneider) of this group have provided invaluable advice. L. Kikas assisted in managing the data. This work was supported under NOAA grant NA26-GP0149 and NA46-GP0217 and NSF grant ATM-93-21354.

Huang, B., and J. Shukla, 1996: An examination of AGCM simulated surface stress and low level winds over the tropical Pacific ocean. Mon. Wea. Rev., 124, in press.

Huang, B., and E. K. Schneider, 1995: The response of an ocean general circulation model to surface wind stress produced by an atmospheric general circulation model. Mon. Wea. Rev., 123, 3059-3085.

Kinter, J. L. III, J. Shukla, L. Marx and E. K. Schneider, 1988: A simulation of winter and summer circulations with the NMC global spectral model. J. Atmos. Sci., 45, 2486-2522.

Kirtman, B. P., J. Shukla, B. Huang, Z. Zhu, E. K. Schneider, 1996: Multiseasonal predictions with a coupled tropical ocean global atmosphere system. Mon. Wea. Rev., 124, in press.

Kirtman, B. P. and E. K. Schneider 1996: Model based estimates of equatorial Pacific wind stress. J. Climate, 124, 1077-1091.

Pacanowski, R. C., K. Dixon, A. Rosati, 1993: The GFDL modular ocean model users guide, version 1.0. GFDL Ocean Group Tech. Rep. No. 2.

Reynolds, R.W., and T. M. Smith, 1995: A high resolution global sea surface temperature climatology. J. Climate, 8, 1571-1583.

Xue, Y., P. J. Sellers, J. L. Kinter III, and J. Shukla, 1991: A simple biosphere model for global climate studies. J. Climate, 4, 345-364.

Fig. 1. Time evolution of the Niño 3 SSTA forecast. The solid (dashed) [dotted] curve corresponds to the forecast initialized at the beginning of July (August) [September] of 1996.

Fig. 2. The ensemble mean SSTA. The top panel shows the predicted ensemble mean averaged over Sep-Oct-Nov 1996, the middle panel Dec-Jan-Feb 1996-97, and the bottom panel Mar-Apr-May 1997.