Sensitivity to Surface Forcing and Boundary Layer Mixing in a Global Ocean Model: Annual-Mean Climatology

Abstract

The effects of more realistic bulk forcing boundary conditions, a more physical subgrid-scale vertical mixing parameterization, and more accurate bottom topography are investigated in a coarse-resolution, global oceanic general circulation model. In contrast to forcing with prescribed fluxes, the bulk forcing utilizes the evolving model sea surface temperatures and monthly atmospheric fields based on reanalyses by the National Centers for Environmental Prediction and on satellite data products. The vertical mixing in the oceanic boundary layer is governed by a nonlocal K-profile parameterization (KPP) and is matched to parameterizations of mixing in the interior. The KPP scheme is designed to represent well both convective and wind-driven entrainment. The near- equilibrium solutions are compared to a baseline experiment in which the surface tracers are strongly restored everywhere to climatology and the vertical mixing is conventional with constant coefficients, except where there is either convective or near-surface enhancement. The most profound effects are due to the bulk forcing boundary conditions, while KPP mixing has little effect on the annual-mean state of the ocean model below the upper few hundred meters. Compared to restoring boundary conditions, bulk forcing produces poleward heat and salt transports in better agreement with most oceanographic estimates and maintains the abyssal salinity and temperature closer to observations. The KPP scheme produces mixed layers and boundary layers with realistically large temporal and spatial variability. In addition, it allows for more near-surface vertical shear, particularly in the equatorial regions, and results in enhanced large-scale surface divergence and convergence. Generally, topographic effects are confined locally, with some important consequences. For example, realistic ocean bottom topography between Greenland and Europe locks the position of the sinking branch of the Atlantic thermohaline circulation to the Icelandic Ridge. The model solutions are especially sensitive to the under-ice boundary conditions where model tracers are strongly restored to climatology in all cases. In particular, a factor of 4 reduction in the strength of under-ice restoring diminishes the abyssal salinity improvements by about 30%.