The Impact of Surface Flux Parameterizations on the Modeling of the North Atlantic Ocean

Abstract

The response of an ocean general circulation model to several distinct parameterizations of the surface heat and freshwater fluxes, which differ primarily by their representation of the ocean?atmosphere feedbacks, is investigated in a realistic configuration for the North Atlantic Ocean. The impact of explicitly introducing oceanic information (climatological sea-surface temperature) into the computation of the heat flux through a Haney-type restoring boundary condition, as opposed to the case in which the flux is based on atmosphere-only climatologies and is computed with the full bulk formulation, is considered. The strong similarity between these two approaches is demonstrated, and the sources of possible differences are discussed. When restoring boundary conditions are applied to the surface salinity, however, an unphysical feedback mechanism is being introduced. The model's response to this restoring is contrasted to the response to a flux boundary condition that prescribes the freshwater flux derived from evaporation, precipitation, and river runoff climatologies (and therefore does not allow any feedback), as well as to the more realistic case in terms of the feedback parameterization, in which the dependence of evaporation on the model sea surface temperature is explicitly represented. Limited-area models introduce a further complicating factor for the thermodynamic adjustment, namely the representation of the oceanic heat and freshwater fluxes at the lateral boundaries. The degree to which the model solution is influenced by such fluxes, in combination with the different surface parameterizations, is also assessed. In all cases, the various components of the model's thermodynamic adjustment are considered, and the interdependence between the surface fluxes and the simulated sea surface temperature and surface salinity, their combined effect upon the ventilation of subsurface layers and production of different water masses, and their effect upon the simulated meridional heat and freshwater transports are analyzed.