Coupled ocean/sea-ice modelling in the Southern Ocean

The ocean/sea-ice interaction in the Southern Ocean is investigated using the Hamburg Ocean Primitive Equation Model (HOPE). The ocean model is three dimensional and prognostically calculates velocity, temperature, salinity and sea-surface elevation on a multi-level staggered Arakawa E-grid. Coupled to this is a thermodynamic model of sea-ice growth and melt, and a dynamic model with viscous-plastic rheology. Two versions have been formulated: a high-resolution re-entrant channel model of the East Antarctic coastline; and a medium-resolution southern hemisphere model. In the high-resolution model the mean annual oceanic heat flux (OHF) to the sea-ice (8 W m -2 ) is dominated by convection. There is considerable seasonal variability in the OHF, with areal-averaged values approaching 20 W m-2in winter, and falling below 5 W M-2 in summer. There is also considerable spatial variability in the OHF: near the sea-ice edge mean monthly values can be above 100 W m -2; within a coastal polynya, where the maximum annual in situ sea-ice growth is 15 m, values approach 50 W m-2; in the seasonal zone (62-64°S) an OHF in the range 5-20 W m -2 is common in the winter months; but for most of the year at most other locations the oceanic heat flux lies in the range of 0-5 W m-2 . Sensitivities to the magnitudes of sea-ice salinity and precipitation minus evaporation (P-E) are investigated. The model is found to be very sensitive to processes that affect the sea surface salinity, which determines the vertical stability of the ocean. In turn this controls the rate of convection, and consequently the sea-ice thickness. For the extreme case of zero P-E the model enters into a mode of temperature-driven deep convection, characterised by relatively warm sea surface temperature and a total collapse of the seasonal sea-ice cycle. The medium-resolution version is used to investigate the response of the coupled ocean/sea-ice system to a number of surface fresh water flux (SFWF) climatologies, and to changes in the mean surface air temperature in the southern hemisphere. As with the high-resolution model the seaice is very sensitive to the SFWF. In particular, the occurrence of a large scale polynya in the Weddell Sea is found to depend critically on there being a sufficient SFWF. The mean annual OHF for the Southern Ocean is found to be around 25 W m -2 , also with considerable seasonal and spatial variability. Increasing the SFWF by 10 cm a-1results in a decrease of 10% in the OHF, while increasing the surface air temperature by 2°C increases the OHF by 10%. The increase in surface air temperature results in a decrease in mean annual sea-ice extent and volume of around 20%, while the increase in SFWF results in an increase in mean annual sea-ice extent of around 5%, and an increase in mean annual sea-ice thickness of around 12%.