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

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 the 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 quantified by comparisons with data from tide gauges at South Georgia, with satellite-derived sea surface temperatures, and with an extensive CTD dataset collected during January–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 processes such as larval transport and retention. The comparison with CTD data reveals mean and root mean square errors in temperature (salinity) of –0.27 °C (−0.07) and 0.64 °C (0.23), respectively. Vertical profiles of potential temperature and salinity on the shelf agree acceptably well with observations, but there is a tendency for the model to under-predict the density contrast between surface and bottom waters. The main limitation on model accuracy is found to be the large-scale forcing. 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. Significantly, the model suggests this to be a unidirectional link, from South Georgia to Shag Rocks, with possible significance for fisheries management. The implications of these results in the context of the South Georgia ecosystem are discussed briefly, demonstrating the usefulness of this new tool for interdisciplinary studies of the region.

Details