Covey, Curt
June 1994, 33 pp. 

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To determine whether resolution of smaller scales is necessary to simulate large-scale ocean climate correctly, I examine results from a global ocean GCM run with horizontal grid spacings. The horizontal grid spacings span a range from coarse resolutions traditionally used in climate modeling to nearly the highest resolution attained with today's computers. This work represents an extension (to lower resolutions) of earlier studies by Semtner and Chervin. The experiments include four cases employing 4°ƒ, 2°ƒ, 1°ƒ and 1/2ƒ° spacing in latitude and longitude, which were run with minimal differences among them, i.e., in a controlled experiment. Two additional cases?? 1/2°ƒ spacing with a more scale-selective sub-gridscale mixing of heat and momentum, and approximate 1/4°ƒ spacing?are also included. The 1/4ƒ° run resolves most of the observed mesoscale eddy energy in the ocean.

Several artificial constraints on the model tend to minimize differences among the different resolution cases. Nevertheless, for quantities of interest to global climate studies the simulations show significant changes as resolution increases. These changes generally but not always bring the model into better agreement with observations. Differences are typically more noticeable when comparing the 4ƒ° and 2ƒ° runs than when comparing the 2°ƒ and 1°ƒ runs or the 1°ƒ and 1/2ƒ° runs. A reasonable conclusion to draw for current studies with coupled ocean-atmosphere GCMs is that the ocean grid spacing could be set to about 1°ƒ to accrue the benefits of enhanced resolution without paying an excessively steep price in computer-time cost.

The model's poleward heat transport at 1/2°ƒ grid spacing peaks at about 1 x 10^15 W in the Northern Hemisphere and 0.5 x 10^15 W in the Southern Hemisphere. These values are significantly below observations, a problem typical of ocean GCMs even when they are less constrained than in the present study (e.g., when they are coupled to an interactive atmosphere). This problem is alleviated somewhat in the 1/4° run. In this case, however, the eddies resolved by the model generally act to counter rather than to reinforce the heat transport of the mean flow. Improved heat transport may result less from enhanced resolution than from other changes made in this version of the model, such as more accurate wind forcing. (pdf file)

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