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. (1997) 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 1997). 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. 1997).

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 October, November and December of 1996, respectively. Each forecast is run for 18 months. All three forecasts indicate relatively cool conditions for October 1996 through April 1997. The Niño 3 SSTA steadily increases in all three forecasts, indicating warm conditions in the eastern tropical Pacific for boreal fall 1997 and winter 1997-98.

The horizontal structure of the ensemble mean (average of all three forecasts) of the predicted SSTA for the boreal winter of 1996-97 as well as the spring and summer of 1997 are shown in the three panels of Fig. 2. During winter of 1996-97 the model predicts relatively cool conditions throughout the tropical Pacific. There is a steady warming trend through spring to near normal conditions. The warming continues through boreal summer 1997, indicating relatively warm conditions in a narrow region along the equator.

These latest three forecasts should be compared to the forecast initialized in April - September 1996 (see the June and September 1996 issues of this Bulletin). The forecasts initialized in April 1996 - July 1996 were quite similar to each other and called for relatively warm conditions for boreal winter 1996-97. The forecasts initialized in August and September shifted the peak warming to considerably later during 1997, giving predictions that were somewhat more consistent with the current three forecasts. The current forecasts give peak warming during October 1997, whereas the forecast initialized in April-June 1996 gave peak warm conditions in January 1997. The April-June 1996 forecasts are consistent with several hindcast experiments with this model where there was a tendency to predict the onset of El Niño too early during the transition from cold to near normal conditions. This problem is currently being investigated in a detailed predictability study.

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, 1997: An examination of AGCM simulated surface stress and low level winds over the tropical Pacific ocean. Mon. Wea. Rev., 125, 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, 1997: Multiseasonal predictions with a coupled tropical ocean global atmosphere system. Mon. Wea. Rev., 125, 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 October (November) [December] of 1996.

Fig. 2. The ensemble mean SSTA. The top panel shows the predicted 3-member ensemble mean averaged over Dec-Jan-Feb 1996-97, the middle panel Mar-Apr-May 1997, and the bottom panel Jun-Jul-Aug 1997.

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