Deep-water properties and surface buoyancy flux as simulated by a Z-coordinate model including eddy-induced advection

Abstract

A parameterization for the adiabatic transport effect of eddies is introduced into a World Ocean model based on the Bryan-Cox code. The model topography is only lightly smoothed and retains realistic sill depths and representation of continental shelves commensurate with the model's 1.6° latitude by 2.8° longitude resolution. The parameterization allows the model to be run without horizontal diffusivity (though only under certain conditions as discussed). A first (control) run features a typical horizontal diffusivity and no eddy-induced transport. A second run features the eddy-induced transport scheme and zero horizontal diffusivity. Substantial changes occur between the runs in subsurface temperature, salinity, and density throughout the model ocean. The most profound changes occur in the deep ocean and feature a marked increase in density in the second run associated with a substantial decline in temperature (especially in the Atlantic) and an increase in salinity (especially in the Southern, Indian, and Pacific Oceans). The large changes in deep-water properties reflect a marked change in the relationship between dense sill/shelf overflow water and the water that reaches the deep ocean. Deep water in the second run much more closely resembles the source overflow water, because of elimination of local and remote effects of horizontal diffusivity, and because of reduced convection and isopycnal slope near downslope flows resulting from direct action by the transport scheme. The deep-water properties in the second run are clearly more realistic than in the first. The greater density in the second run is achieved despite substantial reductions in polar surface heat and salt fluxes from those in the first run. In particular, surface fluxes near Antarctica are generally small and smoothly varying in the second run. Water mass interactions important in the formation of model deep water are examined. The realistically high density of Circumpolar Deep Water in the second run prevents the occurrence of widespread deep convection near Antarctica, which, in the first run, seriously depletes the salinity of this water mass and consequently leads to marked salinity deficiencies in the deep Indian and Pacific Oceans. This convection also severely distorts the surface flux patterns near Antarctica. Practical implications of the marked reduction in Antarctic convection are discussed. Finally, a third run shows that much of the benefit of the eddy-induced transport scheme accrues upon its introduction even when the standard horizontal diffusivity is retained. This last result supports the use of the scheme even in models where the resolution is too coarse to allow for complete elimination of horizontal diffusivity.