SST Predictions with a Global Coupled GCM

contributed by Bohua Huang, Zhengxin Zhu, David G. DeWitt, J. Shukla, 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., 1998b) 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, 1998b). 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 (1998). 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 after a three year spinup of the ocean with observed wind stress starting from Levitus (1982) January climatology. 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 (defined from the period of 1986-1996) to the climatology of the coupled model. The atmospheric initial condition is obtained by a one month AGCM spinup with SST prescribed 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. The predictions from this system appear to have skill out to 18 months lead time in the central and eastern part of the equatorial Pacific Ocean. Schneider et al. (1998a) have demonstrated that the correlations between the predicted and observed NINO3 SST anomalies (SSTA) are near 0.6 at six months lead and oscillates about this value for the rest of the period for July 1 initial conditions. More recently, additional hindcasts have been done using initial conditions from other months.

Fig. 1 shows the NINO3 SSTA time series from four predictions initialized at October 1, 1997, February 1, 1998, April 1, 1998 and May 1, 1998. All are made for 18 months lead time. These predictions are quite consistent with each other, showing that the warm NINO3 SSTA from the 1997-98 El Niño event decays continuously in early 1998 and becomes negative near July. The cooling continues until the largest negative anomaly is reached around October to November of 1998. A gradual recovery follows and SSTA is near zero by the spring or summer of 1999. This evolution is fairly consistent with observations that are available through July, 1998 (solid curve, Fig.1).

The spatial structure of the predicted SSTA from the fall 1998 to spring 1999 in the tropical Pacific is demonstrated by the seasonal means of the ensemble averaged fields from all four predictions (Fig.2). The strongest cold SST anomalies appear in the boreal fall (Sep-Oct-Nov) of 1998, centered at the equator and extended from the east coast to the date line (first panel). The maximum is located near 100^oW-120^oW with magnitude of -3^oC. These anomalies persist through the winter of 1998-99 with slightly reduced magnitude (second panel). They are further weakened during the spring of 1999 and contract to the eastern ocean. The predicted SSTA seems to be too narrowly confined to the equator except near the east coast, which has been noted to be a systematic error of the model.

Acknowledgments: This work was supported under NOAA grant NA26-GP0149 and NA46-GP0217 and NSF grant ATM-93-21354.

References:

Derber, J. and A. Rosati, 1989: A global oceanic data assimilation system, J. Phys. Oceanogr., 19, 1333-1347.

DeWitt, D. G. and E. K. Schneider, 1998: Simulation of the climate with a coupled ocean-atmosphere general circulation model: Seasonal cycle and adjustment to mean climate. Mon. Wea. Rev., accepted.

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.

Huang, B. and J. L. Kinter III, 1997: A global ocean analysis for 1986-1992. COLA Rep. 38, 62 .

Kiehl, J. T., J. J. Hack, and B. P. Briegleb, 1994: The simulated Earth radiation budget of the National Center for Atmospheric Research community climate model CCM2 and comparisons with the Earth Radiation Budget Experiment (ERBE). J. Geophys. Res., 99, 20815-20827.

Kiehl, J. T., B. Boville, B. Briegleb, J. Hack, P. Rasch, and D. Williamson, 1996: Description of the NCAR Community Climate Model (CCM3). NCAR Technical Note NCAR/TN-420+STR.

Kirtman, B. P., J. Shukla, B. Huang, Z. Zhu, and E. K. Schneider, 1997: Multiseasonal predictions with a coupled tropical ocean global atmosphere system. Mon. Wea. Rev., 125, 789-808.

Latif, M., A. Sterl, E. Maier-Reimer, and M. M. Junge, 1993: Structure and predictability of the El Niño/Southern Oscillation phenomenon in a coupled ocean-atmosphere general circulation model. J. Climate, 6, 700-708.

Levitus, S., 1982: Climatological Atlas of the World Ocean. NOAA Professional Paper 13. U. S. Dept. of Commerce, 173.

Moorthi, S. and M. J. Suarez, 1992: Relaxed Arakawa-Schubert: A parameterization of moist convection for general circulation models. Mon. Wea. Rev., 120, 978-1002.

Pacanowski, R. C., K. Dixon, and A. Rosati, 1993: The GFDL modular ocean model users guide, version 1.0. GFDL Ocean Group Tech. Rep. No. 2.

Schneider, E. K., Z. Zhu, D. G. DeWitt, B. Huang and B. P. Kirtman, 1997: ENSO Hindcasts with a Coupled GCM. COLA Report 39, 40.

Schneider, E. K., Z. Zhu, B. Huang, D. G. DeWitt, and J. Shukla, 1998a: SST predictions with a global coupled GCM. Experimental Long-Lead Forecast Bulletin, Vol. 7, No.2, 6-9.

Schneider, E. K., B. Huang, Z. Zhu, D. G. DeWitt, J. L. Kinter III, B. Kirtman, and J. Shukla, 1998b: Ocean data assimilation, initialization, and predictions of ENSO with a coupled GCM. Mon. Wea. Rev., accepted.

Figure captions:

Fig. 1. Time series of the observed and predicted NINO3 SST index. The solid curve corresponds to the observation. The dashed curve with solid circles corresponds to the prediction initialized in October 1, 1997, the dashed curve with open squares corresponds to the prediction initialized in February 1, 1998, the dashed curve with solid squares corresponds to the prediction initialized in April 1, 1998, the dashed curve with crosses corresponds to the prediction initialized in May 1, 1998.

Fig. 2. The ensemble mean SSTA fields in the tropical Pacific from all four predictions. The top panel shows the ensemble mean averaged from September 98 to November 98. The middle panel shows the ensemble mean averaged from December 98 to February 99. The lower panel shows the ensemble mean averaged from March 99 to May 99.