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Tropical Pacific SST Predictions with a Coupled GCM
contributed by Ben Kirtman, J. Shukla and Zhengxin Zhu
Center for Ocean-Land-Atmosphere Studies, Calverton, Maryland

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 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 1.5 longitude and 0.5 latitude between 20N and 20S. 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).

The Niño 3 SSTA root mean squared error (RMSE) and correlation as a function of forecast lead time were shown in the March 1995 issue of this Bulletin. These two verification measures are computed with respect to the observed SSTA. The correlation in the Niño 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 Niño 3 time series of the predicted SSTA for three forecasts initialized on the first of June, July and August of 1997, respectively. Each forecast is run for 18 months The evolution of all three forecasts are consistent. The model predicts steady anomalously warm conditions (2K) in the eastern Pacific from the initial state (boreal summer 1997) through the boreal winter of 1997-98. After winter 1997-98, the Niño 3 anomalies in all three forecasts decay rapidly to near normal conditions by the boreal summer of 1998 with rather strong cold conditions by winter 1998-99.

The ensemble mean (average of all three forecasts) horizontal structure of the predicted SSTA for the boreal fall of 1997, winter 1997-98 and spring 1998 are shown in the three panels of Fig. 2. The ensemble mean forecast for Sep-Oct-Nov 1997 calls for relatively warm SSTA throughout much of the equatorial central and eastern Pacific. There is a small region of cold anomalies in the western Pacific centered along 5N. By the winter of 1997-98, the anomaly in the central Pacific has amplified and there is some narrowing of the anomaly in the far eastern Pacific. The region of cold anomalies in western Pacific has expanded farther to the west. Given the behavior of this model in simulating the SSTA in the past, the warm anomaly forecast for Dec-Jan-Feb 1997-98 is over 3 standard deviations above normal, reflecting the strength if this current ENSO event. By spring 1998 the rapid cooling seen in Fig. 1 can be detected. Throughout the central and eastern Pacific the warm anomaly is less than half that forecast for the previous winter.

These latest forecasts are consistent with the forecasts shown in the previous issue of this Bulletin and indicate that peak El Niño conditions can be expected for the winter of 1997-98. These most recent forecasts, however, give somewhat stronger warm anomalies than the previously forecast, and a more rapid decay of the warm event.

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, 1997: 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. and E.K. Schneider 1996: Model based estimates of equatorial Pacific wind stress.J. Climate, 9, 1077-1091.

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.

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.

Fig. 1. Time evolution of the Niño 3 SSTA forecast. The solid (dashed) [dotted] curve corresponds to the forecast initialized at the beginning of June 1997 (July 1997) [August 1997].

Fig. 2. The spatial field of ensemble mean SSTA from Jun-Jul-Aug 1997 initial conditions. The top panel shows the predicted 3-member ensemble mean averaged over Sep-Oct-Nov 1997, the middle panel Dec-Jan-Feb 1997-98, and the bottom panel Mar-Apr-May 1998.
 
 

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