SST Predictions with a Global Coupled GCM

contributed by Edwin K. Schneider, Bohua Huang, Zhengxin Zhu, 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), 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 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 eight 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 spin-up 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 on March 1, April 1, and May 1, 1999. The prediction from March 1 calls for continued cold SST through 1999, while the forecasts with April and May initialization indicate that the cold anomaly will decay and for a weak warm anomaly to develop after August, 1999.

The spatial structure of the predicted SSTA from the summer to winter 1999 in the tropical Pacific is shown by the seasonal means of the ensemble averaged fields from the three predictions (Fig.2). During boreal summer (Jun-Jul-Aug), cold SST anomalies are predicted on the equator from 160E eastward to the South American coast, with a maximum of -1.5^oC near 160W. During the follow-ing seasons (Sep-Oct-Nov and Dec-Jan-Feb), the ensemble mean forecast is for tropical Pacific SST to be close to normal.

Acknowledgments: This work was supported under NOAA grants NA26-GP0149 and NA46-GP0217 and NSF grants ATM-93-21354 and ATM-98-14295.

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., 381-395.

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 Niño/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 three 12 month predictions of the NINO3 SST anomaly. The solid curve corresponds to March 1, 1999 initial conditions, the long dashed curve to the April 1, 1999 initial condition, and the short dashed curve is for May 1, 1999 initial conditions.

Fig. 2. The ensemble mean SSTA in the tropical Pacific from the three predictions described in Fig. 1. The top panel shows the average from June 1999 to August 1999. The September 1999 to November 1999 average is shown in the middle panel, and the December 1999 to February 2000 average in the bottom panel.