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 wave number 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 20°N and 20°S. Further details of the ocean model are provided in Huang and Schneider (1997).

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).

Figure 1 shows the NINO3 time series of the predicted SSTA for three forecasts initialized on, December 1, 1997, January 1, 1998 and February 1, 1998, respectively. Each forecast is run for 18 months. The evolution of all three forecasts are consistent. The model predicts a rapid decay of the NINO3 SSTA from the winter 1997-98 maximum to near normal conditions in the late spring early summer. During the fall of 1998 the temperatures continue to cool with NINO3 SSTA more than one standard deviation below normal. The cold anomalies peak during the winter of 1998-99 with strong La Nina conditions persisting through the spring of 1999.

The ensemble mean (average of all three forecasts) horizontal structure of the predicted SSTA for the spring, summer and fall of 1998, are shown in the three panels of Fig. 2, respectively. While the SSTA is rapidly decaying during the spring (MAM98), the SSTA remains relatively warm throughout most of the tropical Pacific. By the summer season (JJA98) the early development of cold anomalies in the far eastern Pacific can be detected. Cold anomalies dominate most of the tropical Pacific basin by fall (SON98) and mature La Nina conditions prevail during the winter of 1998-99 (not shown).

These latest forecasts are consistent with the forecasts shown in the previous issue of this bulletin and indicate that the 1997-98 El Niño will rapidly decay through the spring giving near normal conditions for the summer. In addition, these forecast, as well as the previous three forecasts, call for relatively cold conditions for the boreal winter of 1998-99.

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.

References: 

Huang, B., and J. Shukla, 1995: An examination of AGCM simulated surface stress and low level winds over the tropical Pacific ocean. Mon. Wea. Rev., 125, 985-998. 

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, 1996a: Multiseasonal predictions with a coupled tropical ocean global atmosphere system. Mon. Wea. Rev., 125, 789-808. 

Kirtman, B. P., and E. K. Schneider, 1996: Model based estimates of equatorial Pacific wind stress. J. Climate, 9, 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.

Figure Captions:

Figure 1: Time evolution of the NINO3 SSTA forecast. The solid curve corresponds to the forecast initialized in September 1997, the dashed curve corresponds to the October 1997 forecast and the dotted curve corresponds to the November 1997 forecast. 

Figure 2: The ensemble mean SSTA. The top panel shows the predicted ensemble mean averaged from December 1997 to February 1998. The middle panel shows the predicted ensemble mean SSTA averaged from March 1997 to May 1998. The bottom panel shows the ensemble mean averaged over June 1998 to August 1998.