Tropical Pacific SST Predictions with a Coupled GCM
contributed by Ben P. Kirtman and J. Shukla
Center for Ocean-Land-Atmosphere Studies 4041 Powder Mill Road, Suite 302, Calverton, MD 20705
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 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, 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).
Fig. 1 shows the NINO3 time series of the predicted SSTA for three forecasts initialized on, June 1, 1999, July 1, 1999 and August 1, 1999, respectively. Each forecast is run for 18 months. The evolution of all three forecasts are fairly consistent in calling for continued cooling in the eastern Pacific through the fall 1999 and winter 1999-2000. Moderate to strong La Niña conditions are forecasted for winter 1999-2000. The SSTA returns to normal during summer 2000. Compared to the previous three forecasts, the maximum cold anomaly has reduced in strength and the return to normal conditions occurs later in 2000.
The ensemble mean (average of all three forecasts) horizontal structure of the predicted SSTA for fall 1999, winter 1999-00 and spring 2000 are shown in the three panels of Fig. 2, respectively. In all three seasons cold anomalies dominate throughout the tropical Pacific basin with mature La Niña conditions during the winter of 1999-2000 lasting through spring 2000.
In summary, the forecast is for cold conditions to continue to strengthen through winter 1999-2000. The model is predicting moderate to strong La Niña conditions for winter 1999-2000 returning to normal during summer 2000.
Acknowledgments: This work was supported under NOAA grant NA76GP0258, NAF grant ATM9321354 and NASA grant NAG54977.
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, 985-998.
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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, 789-808.
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Figure 1: Time evolution of the NINO3 SSTA forecast. The solid curve corresponds to the forecast initialized in June 1999, the dashed curve corresponds to the July 1999 forecast and the dotted curve corresponds to the August 1999 forecast.
Figure 2: The ensemble mean SSTA. The top panel shows the predicted ensemble mean averaged from September 1999 to November 1999. The middle panel shows the predicted ensemble mean SSTA averaged from December 1999 to February 2000. The bottom panel shows the ensemble mean averaged over March 2000 to May 2000.