OPEN-OCEAN DEEP CONVECTION IN THE WEDDELL SEA - 2-DIMENSIONAL NUMERICAL EXPERIMENTS WITH A NONHYDROSTATIC MODEL

Numerical experiments with a two-dimensional nonhydrostatic model were carried out to investigate the generation processes of open-ocean dee p convection, deep water formation and ventilation around the Maud Ris e in the Weddell Sea, Antarctica. The thermobaric effect, i.e. an incr ease in the thermal expansion rate of sea water with pressure (water d epth), is essential for the onset of deep convection in this region an d for the overturning of the water column to occur abruptly. Thermal-l ike plumes induced by the thermobaric instability destroy the thermocl ine (halocline) and transport the cold and less-saline mixed layer wat er into the warm and more-saline underlying layer. Then the underlying water ascends to push the thermocline (halocline) up until it disappe ars at the sea surface. It takes only a few days for the thermocline ( halocline) to disappear without sea-ice cover. On the other hand, it t akes more than 30 days with a sea-ice cover because of the reduction o f the cooling rate. The stability of the water column around the Maud Rise in no-polynya winter 1986 was examined. The area over the rise is likely to overturn before the end of the cooling season while the mar ginal area is not. This suggests that the area over the rise may be on e of the source regions for the convective features observed throughou t the Weddell Sea. The rate of deep water formation due to this overtu rning and the associated upward heat flux are estimated at 0.91 x 10(5 ) m(3) s(-1) and 36 W m(-2) over one year, respectively, which are 2 s imilar to 3 times of those caused by the entrainment of the Weddell De ep Water into the mixed layer when no overturning occurs. No-overturni ng stations on the margin of the rise are classified into two types. A t one type of station, salinity in the relatively thick mixed layer is too low for the water column to be destabilized before the end of the cooling season; at the other type of station, the relatively thin mix ed layer with a relatively high salinity and the warm Weddell Deep Wat er inhibit an overturning through a previously described negative feed back process. Sensitivity of the model result to several physical para meters expressing turbulent mixing was also examined.