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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 day of March, April, and May 1997, respectively. Each forecast is run for 18 months. The
evolution of all three forecasts is fairly consistent. The model predicts steady warming through
boreal summer and fall of 1997 with the strongest anomalies (approximately 1K) during boreal
winter of 1997-98. After that winter the Niño 3 anomaly in all three forecasts decays to near
normal conditions by summer of 1998.
The ensemble mean (average of all three forecasts) horizontal structure of the predicted SSTA for
the boreal summer of 1997, fall of 1997 and boreal winter of 1997-98 are shown in the three
panels of Fig. 2. The ensemble mean forecast for JJA 97 calls for relatively warm SSTA
throughout much of the equatorial central and eastern Pacific. As is typical of this model, there
are two maxima: one in the central Pacific and one in the eastern Pacific. Given the typical ENSO
evolution in this model, the current forecast for JJA 1997 is quite warm. The SSTA continues to
warm through boreal fall with maxima well over 1K. By DJF 1997-98 much of the tropical Pacific
is over 1K warmer than normal, with a sizable region in the central Pacific over 1.5K.
These latest forecasts are consistent with the forecasts shown in the previous (March 1997) issue
of this Bulletin, indicating that peak El Niño conditions can be expected for the winter of
1997-98.
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., 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.
Kirtman, B.P. and E.K. Schneider 1996: Model based estimates of equatorial Pacific wind
stress.J. Climate, 9, 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 March 1997 (April 1997) [May 1997].
Fig. 2. The spatial field of ensemble mean SSTA from Mar-Apr-May 1997 initial conditions. The
top panel shows the predicted 3-member ensemble mean averaged over Jun-Jul-Aug 1997, the
middle panel Sep-Oct-Nov 1997, and the bottom panel Dec-Jan-Feb 1997-98.
