High-resolution modelling of the shelf and open ocean adjacent to South Georgia, Southern Ocean

Emma F. Young, Michael P. Meredith, Eugene J. Murphy, and Gary R. Carvalho

The marine ecosystem on the shelf and open ocean adjacent to South Georgia is extraordinarily rich, with a history of commercial exploitation. Although much progress has been made, attempts at modelling (and hence better understanding) this system have consistently been hampered by the poor representation of key physical processes in global or regional ocean general circulation models. Here we present the development of a high resolution 3D hydrodynamic model of the South Georgia shelf and adjacent open ocean, including a novel method for prescribing freshwater fluxes. The ability of the model to reproduce the observed oceanography of the region is demonstrated by comparisons with data from tide gauges at South Georgia, and with an extensive CTD dataset collected during January to April 1995. Predicted cotidal charts for the diurnal tides O1 and K1 show a periodic amplification in both the current and elevation fields at the shelf edge, suggesting the presence of a diurnally-forced continental shelf wave. This could have important implications for larval transport and retention. The comparison with CTD data reveals mean and RMS errors in temperature (salinity) of *0.29 ºC (-0.07) and 0.64 ºC (0.23) respectively. Although vertical salinity gradients are marginally weaker than observed due to limitations in the method used to impose freshwater fluxes at the South Georgia coast, vertical profiles of potential temperature and salinity on the shelf agree well with observations. These comparisons demonstrate that the model is a useful tool for studies of physical and biological interactions in the region. By releasing a passive tracer into the model, transport and retention pathways are identified, including a prevalence for tracer export from the shelf to the west of South Georgia, and a transport pathway linking South Georgia and Shag Rocks. The implications of these results in the context of the biogeochemistry of the South Georgia region are discussed briefly, demonstrating the usefulness of this new tool for interdisciplinary studies of the region.