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., 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, 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 60^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 December 1, 1998 (solid curve), January 1, 1999 (long dashed curve), and February 1, 1999 (short dashed curve). Each curve spans for 12 months after its initial time. In general, these predictions are qualitatively consistent with each other. They indicate that the cold NINO3 SSTA present since late 1998 will persist through the boreal spring season of 1999 and become colder in the summer (June to August). Although the cold anomalies will be gradually weakened through the rest of 1999, there will still be a NINO3 SSTA around -1^oC at the end of this year.

The spatial structure of the predicted SSTA from the spring to fall 1999 in the tropical Pacific is demonstrated by the seasonal means of the ensemble averaged fields from all three predictions (Fig.2). During boreal spring (Mar-Apr-May), cold SST anomalies are centered at the equator with a maximum of -3^oC extending from the date line to central Pacific (the first panel). During the following season (Jun-Jul-Aug), the center of the cold anomalies is shifted eastward to the region near 90^oW-120^oW. Its magnitude is strengthened to -4^oC (the second panel). These anomalies persist through the autumn of 1999 with slightly reduced magnitude (Sep-Oct-Nov, the third panel).

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, 1999: Simulation of the climate with a coupled ocean-atmosphere general circulation model: Seasonal cycle and adjustment to mean climate. Mon. Wea. Rev., in press.

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 pp.

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 Nino/Southern Oscillation phenomenon in a coupled ocean-atmosphere general circulation model. J. Climate, 6, 700-708.

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 pp.

Schneider, E. K., B. Huang, Z. Zhu, D. G. DeWitt, J. L. Kinter III, B. Kirtman, and J. Shukla, 1999: Ocean data assimilation, initialization, and predictions of ENSO with a coupled GCM. Mon. Wea. Rev., in press.

Zhu, Z., B. Huang, D.G. DeWitt, J. Shukla, and E.K. Schneider, 1998: SST predictions with a global coupled GCM. Experimental Long-Lead Forecast Bulletin, Vol. 7, No.4 14-17.

Fig. 1. Time series of the observed and predicted NINO3 SST index. The solid curve corresponds to the prediction initialized at 00Z, December 1, 1998, the long dashed curve corresponds to the prediction initialized at 00Z, January 1, 1999, and the short dashed curve corresponds to the prediction initialized at 00Z, February 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 March 99 to May 99. The middle panel shows the ensemble mean averaged from June 99 to August 99. The lower panel shows the ensemble mean averaged from September 99 to November 99.