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Tropical Pacific SST Predictions with a Coupled GCM
contributed by Ben Kirtman, Bohua Huang, 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 day of December 1996, and January and February 1997, respectively. Each forecast is run for
18 months. The forecast initialized in December starts with relatively cold conditions, but begins
to warm in January 1997 with near normal temperatures by April and a peak warm anomaly in
October 1997. The forecasts initialized in January and February show similar behavior starting
from somewhat warmer conditions and giving weaker warm anomalies in the boreal fall of 1997.
The ensemble mean (average of all three forecasts) horizontal structure of the predicted SSTA for
the boreal spring, summer and fall of 1997 are shown in the three panels of Fig. 2. During spring
the model predicts near normal conditions throughout the tropical Pacific with weak positive
anomalies in the western and central Pacific. There is a steady warming trend through the boreal
summer and fall of 1997 with anomalies greater than 0.6 developing throughout much of the
equatorial Pacific. During the fall the model predicts an east-west dipole pattern of positive
anomalies along the equator with stronger positive anomalies in the eastern Pacific.
These latest forecasts are consistent with the forecasts shown in the previous (December 1996)
issue of this Bulletin.
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, 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, 1997: Multiseasonal predictions
with a coupled tropical ocean global atmosphere system. Mon. Wea. Rev., 125, in press.
Kirtman, B. P. and E. K. Schneider 1996: Model based estimates of equatorial Pacific wind stress.
J. Climate, 124, 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.
Fig. 1. Time evolution of the Niño 3 SSTA forecast. The solid (dashed) [dotted] curve
corresponds to the forecast initialized at the beginning of December 1996 (January 1997)
[February 1997].
Fig. 2. The ensemble mean SSTA. The top panel shows the predicted 3-member ensemble mean
averaged over Mar-Apr-May 1997, the middle panel Jun-Jul-Aug 1997, and the bottom panel
Sep-Oct-Nov 1997.
