SENSITIVITY TO SURFACE FORCING AND BOUNDARY-LAYER MIXING IN A GLOBAL OCEAN MODEL - ANNUAL-MEAN CLIMATOLOGY

The effects of more realistic bulk forcing boundary conditions, a more physical subgrid-scale vertical mixing parameterization, and more acc urate bottom topography are investigated in a coarse-resolution, globa l oceanic general circulation model. In contrast to forcing with presc ribed fluxes. the bull; forcing utilizes the evolving model sea surfac e temperatures and monthly atmospheric fields based on reanalyses by t he National Centers for Environmental Prediction and on satellite data products. The vertical mixing in the oceanic boundary layer is govern ed by a nonlocal K-profile parameterization (KPP) and is matched to pa rameterizations of mixing in the interior The KPP scheme is designed t o represent well both convective and wind-driven entrainment The near- equilibrium solutions are compared to a baseline experiment in which t he surface tracers are strongly restored everywhere to climatology and the vertical mixing is conventional with constant coefficients, excep t where there is either convective or near-surface enhancement. The mo st profound effects are due to the bulk forcing boundary conditions, w hile KPP mixing has little effect on the annual-mean state of the ocea n model below the upper few hundred meters. Compared to restoring boun dary conditions, bulk forcing produces poleward heat and salt transpor ts 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 la rge temporal and spatial variability. In addition, it allows for more near-surface vertical shear, particularly in the equatorial regions, a nd results in enhanced large-scale surface divergence and convegence. Generally, topographic effects are confined locally, with some importa nt consequences. For example, realistic ocean bottom topography betwee n Greenland and Europe locks the position of the sinking branch of the Atlantic thermohaline circulation to the Icelandic Ridge. The model s olutions 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%.