5.4: Coupled Models and Climate Projections Peter R. Gent National Center for Atmospheric Research, Boulder, Colorado, USA. gent@ucar.edu 5.4.1. Formulation of Coupled Models Coupled climate models consist of atmosphere, land and sea ice components, as well as the ocean component. The atmosphere component is similar to the ocean in that it solves the primitive equations of motion, usually with hydrostatic balance, the continuity equation and predictive equations for temperature and specific humidity. The vertical coordinate is usually pressure, or terrain-following, or a combination of both. A comprehensive review of atmosphere components can be found in Randall et al. (2007). The land surface component usually solves equations in the vertical direction for heat and water that include many complex interactions at the land surface and soil layers below. It also calculates water runoff, which is then routed by a realistic horizontal pattern of river basins to create the river runoff field that is sent to the ocean component. The sea ice component solves a complicated equation for the ice rheology and thermodynamic equations for the heat balance of the sea ice. When the ice melts, fresh water is sent to the ocean component, and the ocean receives a brine rejection flux when the sea ice is formed. The formulation of the ocean component of coupled models has been described in detail earlier in Chapter 5.1. In that chapter it is conjectured, and I concur, that ocean model simulations depend much more strongly on the parameterizations of mixing and the effects of unresolved scales than on details of the numerical discretization of the equations (Chassignet et al., 1996). The parameterization of vertical mixing has been discussed earlier in Chapter 3.3, and the parameterization of unresolved lateral transport in Chapter 3.4. The first climate model that used realistic geometry was assembled by Syukuro Manabe, Kirk Bryan and co- workers at the Geophysical Fluid Dynamics Laboratory (GFDL), and the results were published in two landmark papers; Manabe et al. (1975) and Bryan et al. (1975). The horizontal grid spacing was 5° x 5°, and there were nine vertical levels in the atmosphere component and five levels in the ocean component. Since then, the components have become much more sophisticated and complex, and the horizontal resolution has increased to about 1° x 1° or finer in present day climate models that use about 30-60 vertical levels in the atmosphere and ocean components. 5.4.2. Flux Adjustments Through most of the 1990s, present day control runs of all climate models would quite quickly drift away from the realistic initial conditions with which they were initialized. This drift was usually corrected by the use of flux adjustments described in Sausen et al. (1988), which could be calculated in two different ways. The heat and fresh water fluxes between the atmosphere and ocean components were diagnosed when the atmosphere component was run using observed sea surface temperatures (SSTs) and the ocean component was run using observed wind stresses and atmosphere surface variables. These flux diagnoses were quite different, and their differences were the flux adjustments. Alternatively, the coupled model was run with strong relaxation back to observations of SST and sea surface salinity, and these relaxation terms were used as the flux adjustments. The flux adjustments were added to the heat and fresh water fluxes between the atmosphere and ocean in a present day coupled control run at each time step. This method corrected most of the model drift, but was very unsatisfactory because flux adjustments are completely unphysical, mask deficiencies in the atmosphere and ocean components that require their use, and likely give an unrealistic response to large perturbations of the climate system. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43