Seeking the missing mixing in Southern Ocean GLOBEC: High-resolution modeling of tides and wind forcing in Marguerite Bay

L. Padman, S.L. Howard, R.D. Muench, J. Klinck, and M.S. Dinniman

Previous observational studies in the SO GLOBEC study region have failed to identify the sources of the vertical mixing that is required to maintain the large-scale advection-diffusion balance for intrusions of Upper Circumpolar Deep Water (UCDW) onto the western Antarctic Peninsula (wAP) shelf. In this study we use a three-dimensional, primitive-equation numerical model to investigate the hypothesis that most turbulent transport of water properties including nutrients occurs through the interaction of wind-forced and tidal currents over shallow banks and at the coastline, regions which were not sampled during the SO GLOBEC cruises in 2001 and 2002. The model is run at high lateral resolution (~1 km grid spacing) with realistic initial stratification, and is forced by (i) tides, (ii) idealized wind fields, and (iii) idealized surface buoyancy fluxes, and combinations of these factors. Model results indicate that the parameterized mixing is energetic across sloping topography when the water depth is fairly shallow (<100 m) and currents are sufficiently energetic. This water depth is comparable to the depth of the winter mixed layer found over deeper water. A combination of intermittent and tide-forced upwelling over steeply sloping bathymetry, and the higher mixing rates expected in these regions, leads to ventilation of the UCDW layer by lateral stirring and shear dispersion of mixing products, and the associated upward flux of UCDW properties including nutrients into the surface mixed layer.