SST Predictions with a Global Coupled GCM

contributed by Zeng-Zhen Hu, Bohua Huang, 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 1.5o 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 (Kirtmanse 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 the three latest predictions. Each curve spans 12 months after its initial time. The three curves consistently predict the decay of the warm event in the first half of 2003. The predicted maximum Nino3 SSTA is in September, October, and December 2002 for the initial conditions of June, 1, July 1, and August 1, 2002, respectively. The predicted maximum magnitude of the SSTA is between 1 oC and 1.5 oC.

Fig. 2 is the ensemble mean of the predicted SSTA evolution from the autumn of 2002 to the spring of 2003 in the tropical Pacific. This evolution is consistent with that shown in the previous issue of the ELLFB of this model. It indicates that the warm event reaches its mature phase in the autumn of 2002, and the amplitude of the SSTA is larger than 2 oC in the eastern tropical Pacific. In the winter there will be decay of the warm event, and in the spring the tropical Pacific returns to near normal conditions.

Acknowledgments: This work was supported under NOAA grant NA26-GP0149 and NA46-GP0217 and NSF grant ATM-93-21354. We would like to thank M. Hamilton of NODC/NOAA for providing the real-time measurements of temperature profiles for our ocean analysis. Temperature data of TOGA TAO moorings from PMEL and weekly global SST analyses from CPC are also used in the analysis.

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., 127, 381-395.

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. Huang, B., J. L. Kinter III, and P. S. Schopf, 1999: An ocean data assimilation system with intermittent analyses and continuous model error correction. COLA Rep. 72, 60pp.

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 Ni=F1o/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., Z. Zhu, B. Huang, D. G. DeWitt, and J. Shukla, 1998: 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, 1999: Ocean data assimilation, initialization, and predictions of ENSO with a coupled GCM. Mon. Wea. Rev., 127, 1187-1207.

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.

Figure captions:

Fig. 1. Time series of the predicted NINO3 SST index. The solid curve, long dashed curve, and short dashed curve correspond to the predictions initialized at 00Z, June 1, 2002 00Z, July 1, 2002, and 00Z, August 1, 2002, respectively.

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 2002 to November 2002. The middle panel shows the ensemble mean averaged from December 2002 to February 2003. The lower panel shows the ensemble mean averaged from March 2003 to May 2003.