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, 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, December 1, 2000, January 1, 2001 and February 1, 2001, respectively. Each forecast is run for 18 months. All three prediction start near normal or slightly cold and rapidly cool giving fairly strong cold conditions for the boreal spring and summer of 2001 through the boreal fall of 2001. These forecast are fairly consistent with the previous three forecasts, except that the maximum cold anomaly is shifted to later in the forecast period.
The ensemble mean (average of all three forecasts) horizontal structure of the predicted SSTA for the boreal spring 2001, the boreal summer of 2001 and the boreal fall of 2001 are shown in the three panels of Fig. 2. The ensemble mean forecast for MAM01 indicates relatively strong cold conditions. The cold anomalies continue to amplify through JJA01 and begin to decay during SON01.
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.
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Figure 1: Time evolution of the NINO3 SSTA forecast. The solid curve corresponds to the forecast initialized in December 2000, the dashed curve corresponds to the January 2001 forecast and the dotted curve corresponds to the February 2001 forecast.
Figure 2: The ensemble mean SSTA. The top panel shows the predicted ensemble mean averaged from March 2001 to May 2001. The middle panel shows the predicted ensemble mean SSTA averaged from June 2001 to August 2001. The bottom panel shows the ensemble mean averaged over September 2001 to October 2001.