Transports and budgets of volume, heat, and salt from a global eddy-resolving ocean model

Abstract

The results from an integration of a global ocean circulation model have been condensed into an analysis of the volume, heat, and salt transports among the major ocean basins. Transports are also broken down between the model's Ekman, thermocline, and deep layers. Overall, the model does well. Horizontal exchanges of mass, heat, and salt between ocean basins have reasonable values; and the volume of North Atlantic Deep Water (NADW) transport is in general agreement with what limited observations exist. On a global basis the zonally integrated meridional heat transport is poleward at all latitudes except for the latitude band 30°S to 45°S. This anomalous transport is most likely a signature of the model's inability to form Antarctic Intermediate (AAIW) and Antarctic bottom water (AABW) properly. Eddy heat transport is strong at the equator where its convergence heats the equatorial Pacific about twice as much as it heats the equatorial Atlantic. The greater heating in the Pacific suggests that mesoscale eddies may be a vital mechanism for warming and maintaining an upwelling portion of the global conveyor-belt circulation. The model's fresh water transport compares well with observations. However, in the Atlantic there is an excessive southward transport of fresh water due to the absence of the Mediterranean outflow and weak northward flow of AAIW. Eddies in the mid-latitudes act to redistribute heat and salt down the mean gradients. Residual fluxes calculated from a sum of the computed advective (including eddies), forced, and stored fluxes of heat and salt represent transport mostly due to vertical sub-grid scale mixing processes. Perhaps the model's greatest weakness is the lack of strong AAIW and AABW circulation cells. Accurate thermohaline forcing in the North Atlantic (based on numerous hydrographic observations) helps the model adequately produce NADW. In contrast, the southern ocean is an area of sparse observation. Better thermohaline observations in this area may be needed if models such as this are to produce the deep convection that will achieve more accurate simulations of the global 3-dimensional circulation.