SST Predictions with a Global Coupled GCM
contributed by Bohua Huang, Zhengxin Zhu, Edwin K. Schneider, and J. Shukla
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) and Huang et al.(1999), 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 MOM1 (Pacanowski et al., 1993). There are 20 levels in the vertical with 16 in the upper 400 m. The zonal resolution is 1.5o longitude and 0.5o latitude between 10oN and 10oS. 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 65oN to 70oS.
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 six 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 spinup 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 at 00Z of September 1, 1999 (solid curve), October 1, 1999 (long dashed curve), and November 1, 1999 (short dashed curve). Each curve spans 12 months after its initial time. The three predictions are qualitatively consistent with each other in forecasting a cooling trend of the NINO3 SSTA in January or February of 2000 that will peak at -1.5o to -2oC in late boreal spring (April or May). However, there are differences in details among the three runs during the earlier and later forecast periods that should be kept in mind in viewing the ensemble forecast. For instance, starting from weak cold anomalies, the coupled runs initialized in September and October, 1999 predict small SST anomalies during the rest of this year. However, the November prediction starting from a colder state (short dashed line) persists the cold anomaly near -1oC throughout the same period. After the peak season, two of the three predictions (September and November) indicate that the cold anomalies will weaken and the ocean will return to normal or slightly warm conditions at the end of boreal summer. However, the other suggests the cold anomaly will persist through boreal summer and may further intensify.
The ensemble average forecast of the SSTA structure from the winter of 1999 through the summer of 2000 in the tropical Pacific is shown in Fig.2. The forecast indicates moderate to relatively strong cold condition in the eastern equatorial Pacific in the boreal spring of 2000. During the coming boreal winter (Dec-Jan-Feb), weak cold SST anomalies appear at the eastern equatorial Pacific with a maximum of -1 oC at 90oW-110oW (the first panel). During the boreal spring of 2000 (Mar-Apr-May), the center of the cold anomalies intensifies to around -3oC and expands westward to include the region near 100oW-130oW. These anomalies weaken to around -1oC during the boreal summer (Jun-Jul-Aug, the third panel).
Acknowledgments: This work was supported under NOAA grant NA26-GP0149 and NA46-GP0217 and NSF grant ATM-93-21354.
References:
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Figure captions:
Fig. 1. Time series of the observed and predicted NINO3 SST index. The solid curve corresponds to the prediction initialized at 00Z, September 1, 1999, the long dashed curve corresponds to the prediction initialized at 00Z, October 1, 1999, and the short dashed curve corresponds to the prediction initialized at 00Z, November 1, 1999.
Fig. 2. The ensemble mean SSTA fields in the tropical Pacific from all three predictions. The top panel shows the ensemble mean averaged from December 1999 to February 2000. The middle panel shows the ensemble mean averaged from March to May 2000. The lower panel shows the ensemble mean averaged from June to August 2000.
