SST Predictions with a Global Coupled GCM
contributed by Zhengxin Zhu, Bohua Huang, Edwin K. Schneider
Center for Ocean-Land-Atmosphere Studies (COLA), Calverton, Maryland
A system has been developed at COLA for making seasonal to interannual predictions of Tropical Pacific SST using a coupled atmosphere-ocean general circulation model that incorporates subsurface ocean measurements in the initial conditions. The ocean component of the prediction model has a nearly global domain, and the model uses no anomaly coupling or flux correction. Instead, the approach of anomaly initial conditions (Latif et al., 1993; Schneider et al., 1999) is used to reduce problems associated with climate drift and the shock of inserting initial conditions. Initial conditions for the ocean are obtained from a near-global ocean analysis produced by an in-house ocean data assimilation system.
The ocean data assimilation (ODA) is described in Huang and Kinter (1997), while the complete system is described in Schneider et al. (1997, 1998, 1999). The ODA uses variational optimal interpolation following Derber and Rosati (1989). The period of the analysis starts from January 1986. The ocean model for the assimilation and for the coupled model is a nearly global version of the GFDL ocean model MOM 1 (Pacanowski et al., 1993). There are 20 levels in the vertical with 16 in the upper 400 m. The zonal resolution is 1.5 degrees longitude 0.5 degrees latitude between 10N and 10S. This tropical resolution and vertical structure are the same as used in the COLA anomaly coupled forecast system (Kirtman et al., 1997). The zonal domain in the coupled system is extended to all longitudes, and the meridional domain is extended to 65N to 70S.
The atmospheric component of the coupled model is the COLA atmospheric GCM. The AGCM is a global spectral model with a state of the art suite of physical parameterizations, as described by DeWitt and Schneider (1999). The horizontal truncation is triangular at wave number 30, and there are 18 unevenly spaced levels in the vertical. The AGCM resolution is the same as used by the anomaly coupled forecast system, and the physics is the same except that the deep cumulus parameterization is the relaxed Arakawa-Schubert scheme of Moorthi and Suarez (1992), and the diagnostic cloud-radiative interaction scheme is modified following Kiehl et al. (1994, 1996).
The coupled model climatology is obtained from the last eight years of a 12 year coupled simulation starting from an ocean state generated by the ODA. The coupled model has a realistic annual cycle of SST at the equator, as well as vigorous interannual SST variability in the Tropical Pacific. However, the annual mean SST is too warm in the eastern equatorial Pacific, and the heat content is too low and the thermocline is too shallow in the western Tropical Pacific.
Ocean initial conditions for the forecasts are obtained by adding the anomalies of the ODA from its own climatology to the climatology of the coupled model. The atmospheric initial condition is obtained by a one-month spin-up with prescribed SST as the sum of the coupled model climate and the observed anomalies of the previous month. The predicted SST anomalies are deviations from the coupled model climate without correction for systematic error. Based on 48 hindcasts initialized at the end of January, April, June and September in 1986-1997, the correlations between the predicted and observed NINO3 SST anomalies (SSTA) are above 0.6 up to six months and above 0.5 up to 12 months lead time (Zhu et al., 1998).
Fig. 1 shows the NINO3 SSTA time series from three predictions initialized on Mar 1 (solid curve), April 1 (long dashed curve), and May 1 (short dashed curve), 2000. Each curve spans 12 months. The first two predictions are qualitatively consistent. Both of them predict transition from cold to normal NINO3 SST in this summer to fall. The prediction starting from May 1, however, forecasts a nomal to slightly warm condition in the eastern Pacific.
The seasonal means of the Pacific SSTA, which are from the ensemble average of the recent three forecasts, are shown in Fig. 2. In the boreal summer of 2000, the predicted SST in the equatorial Pacific is still slightly colder than normal. The SST returns to normal in the autumn, and becomes slighly warmer than normal in the winter.
Acknowledgments: This work was supported under NOAA grants NA26-GP0149 and NA46-GP0217 and NSF grants ATM-93-21354 and ATM-98-14295.
References:
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Figure captions:
Fig. 1. Time series of the predicted monthly mean NINO3 SST index. The solid curve corresponds to the prediction initialized from March 1, 2000. The long dashed curve corresponds to the prediction initialized from April 1, 2000. The short dashed curve corresponds to the prediction initialized from May 1, 2000.
Fig. 2. The ensemble mean SSTA fields in the tropical Pacific. The top panel shows the average from June through August 2000. The middle panel shows the average from September through November 2000. The lower panel shows the average from December 2000 through February 2001.