Tropical Pacific SST Predictions with a Coupled GCM

contributed by Ben P. Kirtman and J. Shukla

Center for Ocean-Land-Atmosphere Studies

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 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 degrees longitude and 0.5 degrees latitude between 10N and 10S. 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).

Fig. 1 shows the NINO3 time series of the predicted SSTA for three forecasts initialized on, June 1, 1998, July 1, 1998 and August 1, 1998, respectively. Each forecast is run for 18 months. The evolution of all three forecasts are fairly consistent, although the forecast initialized in June does not get as cold as the July and August cases. The forecast initialized on June 1, 1998 has near normal NINO3 SSTA for the June monthly mean. The SSTA then decline rapidly until January 1999 where the SSTA remains well below normal (-1.7C) through May 1999. The July and August forecast start out almost a degree colder than the June forecast. These latter two forecasts cool slightly faster than the June forecast reaching peak La Niña (-2.5C) conditions during February-March 1999. All three forecast rapidly return to near normal conditions by September 1999.

The ensemble mean (average of all three forecasts) horizontal structure of the predicted SSTA for fall 1998, winter 1998-99 and spring 1999 are shown in the three panels of Fig. 2, respectively. During the fall season (SON98) cold anomalies dominate most of the tropical Pacific basin. These mature La Niña conditions also prevail during the winter of 1998-99 and the Spring of 1999.

These latest forecasts are consistent with the forecasts shown in the previous issue of this bulletin and indicate that strong La Niña conditions will develop through the Fall of 1998 and persist through the spring of 1999.

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 June 1998, the dashed curve corresponds to the July 1998 forecast and the dotted curve corresponds to the August 1998 forecast.

Figure 2: The ensemble mean SSTA. The top panel shows the predicted ensemble mean averaged from September 1998 to November 1998. The middle panel shows the predicted ensemble mean SSTA averaged from December 1998 to February 1999. The bottom panel shows the ensemble mean averaged over March 1998 to May 1999.