Report 4:  Behavior of an Ocean General Circulation Model at Four Different Horizontal Resolutions

Covey, Curt
August 1992, 30 pp. 

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A global ocean general circulation model is used to simulate the present-day ocean climate using four different latitude/longitude grid spacings. Horizontal resolution varies from l/2° x l/2°--the highest attained to date in an ocean circulation model with global coverage--to a coarse 4° x 4° grid traditionally used in climate modeling. This study addresses the question of whether resolution of smaller scale circulations is necessary in order to simulate the large scale ocean climate correctly. Results indicate that large scale circulation is rather sluggish at 4° x 4° resolution but surprisingly insensitive to grid spacing for resolutions finer than 2° x 2°, even considering the constraints imposed on the model by surface boundary conditions and by "robust diagnostic" forcing near the poles and below the thermocline. Simulated transport of heat from the warm tropics to cooler higher latitudes is not overly sensitive to grid spacing for resolutions finer than 2° x 2°. Furthermore, heat transport does not increase at l/2° x 1/2° resolution when sub-gridscale mixing of heat and momentum are altered so that mesoscale eddies appear in the model.

These results support inferences from earlier studies(based on simplified, limited-domain circulation models) that mesoscale eddies make little net contribution to poleward heat transport by the oceans. They suggest that for global climate modeling, the substantial computer resources required to explicitly resolve ocean mesoscale eddies might be better spent on improving simulations of other components of the climate system. Making this conclusion definite, however, requires a global ocean simulation that is fully eddy-resolving and that relaxes the artificial constraints of the present simulation, which tend to force the results toward present ocean climatology. (pdf file)

UCRL-MI-123395