SST Predictions with a Global Coupled GCM

contributed by Bohua Huang, Zhengxin Zhu, and 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)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 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^o longitude and 0.5^o latitude between 10^oN and 10^oS. 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 65^oN to 70^oS.

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 June 1, 2000 (solid curve), July 1, 2000 (long dashed curve), and August 1, 2000 (short dashed curve). Each curve spans 12 months after its initial time. The two earlier predictions forecast a warming up of the NINO3 SSTA starting from the boreal summer that will peak at 1.5^oC in late autumn or early winter (November or December 2000). However, the most recent forecast using the August 1 initial condition shows a weaker warming with a delayed initiation. According to this forecast, the warming will start at November 2000 and peak in February 2001 at about 1^oC. From August to November, however, the SSTA will be negligible.

The ensemble average forecast of the SSTA structure from the boreal autumn of 2000 through the spring of 2001 in the tropical Pacific is shown in Fig.2. It indicates a moderate warming in the eastern equatorial Pacific. During the coming autumn (SEP-OCT-NOV), warm SST anomalies appear at the equatorial Pacific with a maximum of 1^oC at 120^oW. The anomalies greater than 0.5^oC extend from the eastern coast to the dateline (the first panel). During the winter of 2000-01 (DEC-JAN-FEB), the center of the SST anomalies intensifies to 1.5^oC, while the extent of 0.5^oC contour shrinks slightly to 160^oW (the second panel). In the spring of 2001 (MAR-APR-MAY), the SST anomalies at the center weaken to 1^oC and the extent of 0.5^oC contour around the equator further moves to the east of approximately 140^oW (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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DeWitt, D. G. and E. K. Schneider, 1999: Simulation of the climate with a coupled ocean-atmosphere general circulation model: Seasonal cycle and adjustment to mean climate. Mon. Wea. Rev., 127, 381-395.

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Figure captions:

Fig. 1. Time series of the predicted NINO3 SST index. The solid curve corresponds to the prediction initialized at 00Z, June 1, 2000; the long dashed curve corresponds to the prediction initialized at 00Z, July 1, 2000; and the short dashed curve corresponds to the prediction initialized at 00Z, August 1, 2000.

Fig. 2. The ensemble mean SSTA fields in the tropical Pacific from all three predictions. The top panel shows the ensemble mean averaged from September to November 2000. The middle panel shows the ensemble mean averaged from December 2000 to February 2001. The lower panel shows the ensemble mean averaged from March to May 2001.