Tropical
Pacific SST Forecasts Utilizing Multiple Coupled GCMs
Contributed by David G. DeWitt1, Edwin K.
Schneider2,3 and Daniel Paolino3
1International
Research Institute for Climate Prediction,
2George
3Center for
Ocean-Land-Atmosphere Studies
We describe forecasts for tropical Pacific sea surface
temperature (SST) made by combining the forecasts produced by multiple
independent coupled atmosphere-ocean general circulation models (CGCMs). This initial forecast framework uses two
CGCMs. The two CGCMs utilize the
same ocean component model and are initialized using the same ocean data
assimilation product. This work is an extension of the combined CGCM forecasts
described in Schneider et al. (2003).
Differences between that work and the current forecasts include use of
an extended period for developing prior statistics, use of fewer models, and
employment of a different technique for combining the SST forecasts from the
different models. This manuscript gives the real time forecast starting from
October 1, 2006. In a previous version of the ELLFB, retrospective forecast
skill for October 1 IC forecasts has been documented.
The
model descriptions have been given in previous versions of the Extended Long-Lead
Forecast Bulletin (ELLFB) and are skipped here for brevity. The atmospheric GCM (AGCM) component
models used are the ECHAM4.5 AGCM
of the Max Planck Institute for Meteorologie (Roeckner et al., 1996) and
the Center for Ocean-Land-Atmosphere (COLA) studies AGCM version 2 (Schneider,
2002). The ocean GCM (OGCM)
component model is version 3 of the Geophysical Fluid Dynamics Laboratory
(GFDL) modular ocean model (MOM) (Pacanowski and Griffes, 1998). The ocean
initial conditions are taken from an ocean data assimilation system produced at
GFDL using a variational optimal interpolation (Derber and Rosati, 1989).
Retrospective
forecasts using 7 ensemble members for the period January 1982 to present have
been made using the coupled system with ECHAM AGCM component.
Similar
retrospective forecasts using 5 ensemble members have been made using the
coupled system with COLA AGCM component.
The variance of SST anomalies differs between the two models and is also
not the same as found in nature.
The retrospective forecasts from both of the coupled systems have been
normalized by the observed variance.
At each point a bootstrap cross validation procedure is applied to bring
the forecast variance closer to observed. In this calculation, the forecast for
a particular year is not used to compute the mean or the standard deviation for
either the model or the observations.
The forecast anomaly for that year is taken as the difference from the
model climatology made without using that year multiplied by the ratio of
observed to model standard deviation also calculated without the data from the
year of the forecast. The final SST forecast is the simple arithmetic mean of the
normalized forecasts from the two coupled models.
The
current forecast for the Nino indices averaged SST anomalies made from October
1, 2006 is shown in Figure 1. The combined coupled forecast calls for
both Nino regions to have a continuation of warm conditions over the forecast period
with SST anomalies on the order of 1ºC over the period..
These
forecasts were made possible due to help from several institutions. Matt Harrison and Tony Rosati of GFDL developed
the ODA system and ran the 1980 to 1999 period. Ben Kirtman and Duhong Min of
COLA have ported the ODA to their system and run the 2000 to present
period. Bohua Huang of COLA
implemented the lower resolution of the OGCM used here. Max Planck has kindly provided the
ECHAM4.5 AGCM to the IRI. David
DeWitt was supported by a grant from the National Oceanic an Atmospheric
Administration: (NA07-GP0213). Support for Schneider and Hu was provided by the
National Science Foundation (ATM 98-14295,ATM01-22859), the National Oceanic
and Atmospheric Administration (NA 96-GP0056), and the National Aeronautics and
Space Administration (NAG 5-8202).
References
Balmaseda, M. A., M. K. Davey, and D.
L. T. Anderson, 1995: Decadal and
seasonal dependence of ENSO prediction skill. J. Climate, 8, 2705-2715.
Derber, J. and A. Rosati, 1989:
A global oceanic data assimilation system. J. Phys. Oceanogr., 19, 1333-1347.
Pacanowski, R. C., and S. M. Griffes,
1998: MOM 3.0 Manual,
NOAA/Geophysical Fluid Dynamics Laboratory,
Roeckner, E., and Coauthors, 1996: The atmospheric general circulation
model ECHAM4: Model description and
simulation of present day climate.
Max-Planck-Institut fur Meteorologie, Rep. 218, 90pp. [Available from
MPI fur Meteorlogie, Bundesstr. 55, 20146
Schneider, E. K., 2002: Understanding the differences between
the equatorial Pacific as simulated by two coupled GCMs. J. Climate, 15, 449-469.
Schneider, E. K., D. G. DeWitt, A.
Rosati, B. P. Kirtman, L. Ji, and J. J. Tribbia, 2003: Retrospective ENSO Forecasts: Sensitivity to atmospheric model and
ocean resolution. Mon. Wea. Rev., 131, 3038-3060.
Figure Captions
Figure 1. Combined coupled model forecast for the Nino3 and
Nino3.4 regions from October 1, 2006
initial conditions.