The COLA Anomaly Coupled Model Global SST Predictions

contributed by Ben P. Kirtman^1,2 and Dughong Min²

¹George Mason University and ²Center for Ocean-Land-Atmosphere Studies

The anomaly coupled GCM (ACGCM) combines the COLA atmospheric GCM and the Geophysical Fluid Dynamics Laboratory (GFDL) Modular Ocean Model (MOM), version 3.0. Brief descriptions of these models and the coupling procedures are given below. Details of how well the model performs in long climate simulations is described in detail in Kirtman et al. (2002) and Kirtman and Shukla (2002). An in depth discussion of the hindcast skill is provided in Kirtman (2002) and Kirtman (2003).

a. Atmospheric Model

The dynamic core used in the National Center for Atmospheric Research (NCAR) Community Climate Model (CCM) version 3.0 has been adopted (Schneider, 2001). The dynamic core is spectral (triangular truncation at total wavenumber 42) with semi-Lagrangian transport. There are 18 unevenly spaced -coordinate vertical levels. The parameterization of the solar radiation is after Briegleb (1992) and terrestrial radiation follows Harshvardhan et al. (1987). The deep convection is an implementation of the Relaxed Arakawa-Schubert (RAS) scheme of Moorthi and Suarez (1992) described by DeWitt (1996). The convective cloud fraction follows the scheme used by the NCAR CCM (Kiehl et al., 1994; see DeWitt and Schneider, 1996 for additional details). The model includes a turbulent closure scheme for the subgrid scale exchange of heat, momentum, and moisture after Miyakoda and Sirutis (1977) and Mellor and Yamada (1982). Additional details regarding the AGCM physics can be found in Kinter et al. (1988) and DeWitt (1996). Model documentation is given in Kinter et al. (1997).

b. Ocean Model

The ocean model is version 3 of the GFDL MOM (Pacanowski and Griffies, 1998), a finite-difference treatment of the primitive equations of motion using the Boussinesq and hydrostatic approximations in spherical coordinates. The domain is that of the world ocean between 74^oS and 65^oN. The coastline and bottom topography are realistic except that ocean depths less than 100 m are set to 100 m and the maximum depth is set to 6000 m. The artificial high-latitude meridional boundaries are impermeable and insulating. The zonal resolution is 1.5^o. The meridional grid spacing is 0.5^o between 10^oS and 10^oN, gradually increasing to 1.5^o at 30^oN and 30^oS and fixed at 1.5^o in the extratropics. There are 25 levels in the vertical with 17 levels in the upper 450 m. The vertical mixing scheme is the non-local K-profile parameterization of Large et al. (1994). The horizontal mixing of tracers and momentum is Laplacian. The momentum mixing uses the space-time dependent scheme of Smagorinsky (1963) and the tracer mixing uses Redi (1982) diffusion along with Gent and McWilliams (1990) quasi-adiabatic stirring.

c. Coupling Strategy

The anomaly coupling strategy is described in detail in Kirtman et al. (1997) and in Kirtman et al. (2002). The main idea is that the ocean and atmosphere exchange predicted anomalies, which are computed relative to their own model climatologies, while the climatology upon which the anomalies are superimposed is specified from observations. The anomaly coupling strategy requires atmospheric model climatologies of momentum, heat and fresh water flux, and an ocean model SST climatology. Similarly, observed climatologies of momentum, heat and fresh water flux and SST are also required. The model climatologies are defined by separate uncoupled extended simulations of the ocean and atmospheric models. In the case of the atmosphere, the model climatology is computed from a 30 year (1961-1990) integration with observed specified SST and sea ice. This SST is also used to define the observed SST climatology. In the case of the ocean model SST climatology, an extended uncoupled ocean model simulation is made using 30 years of 1000 mb National Centers for Environmental Prediction (NCEP) reanalysis winds. As with the SST, this observed wind stress product is used to define the observed momentum flux climatology. The heat flux and the fresh water flux in this ocean-only simulation is parameterized using damping of SST and sea surface salinity to observed conditions with a 100 day time scale. The heat and fresh water flux "observed" climatologies are then calculated from the results of the extended ocean only simulation. The ocean and atmosphere model exchange daily mean fluxes and SST once a day.

d. Retrospective Forecast Experiments

In order to assess the potential predictive skill of the coupled model, a large sample of retrospective forecast experiments have been made and compared to available observations. The retrospective forecasts or hindcasts cover the period 1980-1999. A twelve-month hindcast is initialized each January, April, July and October during this 20-year period. For each initial month, an ensemble of six hindcasts is run, yielding a total of 480 retrospective forecasts to be verified. The ocean and atmosphere initial states and the method of generating the ensemble members are described below.

The ocean initial conditions were taken from a 1980-1999 ocean data assimilation produced at GFDL using variational optimal interpolation (Derber and Rosati, 1989). The GFDL ocean initial states were generated using a somewhat higher resolution ocean model than that used in the ocean component of the ACGCM with identical physics and parameter settings. In the forecast experiments presented here, the ocean initial states were interpolated to the lower resolution.

The atmospheric initial states are taken from an extended atmosphere-only simulation with observed prescribed SST. The atmospheric ensemble members were obtained by resetting the model calendar back one week and integrating the model forward one week with prescribed observed SST. In this way, it is possible to generate an unlimited sample of initial conditions that are synoptically independent (separated by one week) but have the same initial date.

e. Forecasts

Forecast for NINO3.4 SSTA initialized in June 2004 are presented in Fig. 1. All the forecasts indicate the same general trend for the first 2-3 months with weak warm condition tending to cool to near normal. By the third month of the forecast (August 2004) the most probable state is near normal, but there is considerable spread among the forecast. During the boreal winter there is an overall warming trend with 5 out of the 6 forecasts indicating temperature above 0.5 for December 2004. By the end of January 2005 the forecast plume indicates a rapid return to near normal.

References

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Figure Captions

Figure 1: NINO3.4 SSTA evolution for forecast initialized in June 2004. The ensemble mean is denoted by the dot-dashed curve. The individual ensemble members are denoted by the solid curves.