A numerical investigation into Southern Ocean sea ice using CICE version 4.0

Numerical simulations of Southern Ocean sea ice were conducted using the Los Alamos numerical sea ice model CICE version 4 (CICE4) which was configured in stand-alone mode on a moderately high-resolution Southern Ocean grid. The atmospheric forcing was derived from the hemispheric forecasting model Polar Limited Area Prediction Systems and ocean forcing from the global ocean general circulation model Australian Climate Ocean Model. An eleven-year simulation was carried out for the interval from 1998 to 2008. Results show that the meridional position of the sea ice edge follows an annual cycle of northward (southward) movement during ice growth (decay). This cycle is largely driven by thermodynamics but ice advection also contributes. Once a mature winter ice pack has formed, ice advection exceeds thermodynamic expansion in providing impetus for northward ice edge movement, most prominently in regions such as north of Prydz Bay and the western Weddell Sea. These zones are characterized by persistent southerly winds. With progressing seasons there is an increase in the size and number of longitudinal zones where ice advection out-paces the thermodynamic advance of the ice edge. This is caused by a slowing in the thermodynamically-driven expansion, while ice advection remains fairly constant over monthly intervals. Eventually there is a time interval when the zonal average northward ice velocity is greater than the zonal average northward expansion of the ice edge and so ice advection is dominant in determining the ice edge position. This is not sustainable due to a lack of thermodynamic growth in austral spring, consequently the ice retreats poleward. We examined the relationship between total ice area and total ice volume. There exists a correlation which is best in February. This correlation deteriorates as the year progresses so that July fails at 90% confidence. During November and December the correlation becomes significant at 95% confidence level. Simulations were also conducted using forcing adjusted by amounts that are compatible with the A1B scenario for IPCC Fourth Assessment Report projected changes in climate for the year 2100. These simulations showed a dramatic reduction in summer total ice area (to about 2% of the control simulation) and total ice volume (to about 5% of the control simulation) but a less dramatic reduction from April to October (to about 80% for total ice area and about 70% for total ice volume compared to the control simulation). In summer most ice was confined to the southern Weddell Sea and to the up-stream sides of the East Antarctic coastal protrusions, especially the fast ice over the Ninnis Bank. Even in climates warmer than that of the A1B projections ice remained east of the Ninnis Bank because the large thickness of ice here does not have time to melt before freezing conditions return. Whether fast ice over the Bank could remain trapped by the many grounded icebergs is such a warm climate is uncertain.