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. (1996b) 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 1.5o longitude and 0.5 o latitude between 20oN and 20 oS. Further details of the ocean model are provided in Huang and Schneider (1996).
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, that improves initial conditions for coupled forecasts (Kirtman et al. 1996a).
The Niño 3 SSTA root mean squared error (RMSE) and correlation as a function of forecast lead time was shown in the March 1995 issue of this Bulletin. The RMSE and the correlation are computed with respect to the observed SSTA. The correlation in the Nino 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 time series of the predicted SSTA in the Niño 3 region for the three forecasts initialized on December 1, 1995, January 1 and February 1, 1996. Each forecast is run through February 1997. All three forecast show a consistent evolution with relatively cool temperatures in the boreal winter of 1995-96 and a fairly rapid transition to relatively warm temperatures in the boreal winter of 1996-97. The predicted SSTA anomalies are near normal during summer 1996. In the forecasts initialized in December 1995 and February 1996, the warm anomalies plateau during the boreal winter of 1996-97 while the forecast initialized in January 1996 shows continued warming. The warming trend seen in all three of these predictions is consistent with forecasts initialized in September 1995 and November 1995 shown in the December 1995 issue of this Bulletin.
The ensemble mean (average of all three forecasts) horizontal structure of the predicted SSTA for the boreal summer of 1996, fall of 1996 and winter of 1996-97 are shown in the three panels of Fig. 2. The ensemble averaged forecast indicates warm SSTA beginning in the boreal fall of 1996 with anomalies in excess of 1C over much of the central Pacific between 5S-5N during winter 1996-97. During summer 1996 the conditions are near normal with weak cold anomalies in the western Pacific.
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, 1996: An examination of AGCM simulated surface stress and low level winds over the tropical Pacific ocean. Mon. Wea. Rev., 124, 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, 1996a: Multiseasonal predictions with a coupled tropical ocean global atmosphere system. Mon. Wea. Rev., 124, in press.
Kirtman, B. P., E. K. Schneider and B. Kirtman, 1996b: Model based estimates of equatorial Pacific wind stress. J. Climate, 124, in press.
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.
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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 1. Time evolution of the Nino 3 SSTA forecast. The solid (dashed) [dotted] curve corresponds to the forecast initialized in December 1995 (January 1996) [February 1996].
Figure 2. The ensemble mean SSTA. The top panel shows the predicted ensemble mean averaged over Jun-Jul-Aug 1996, the middle panel averaged over Sep-Oct-Nov 1996, and the bottom panel Dec-Jan-Feb 1996-97.