EFFECT OF DRAKE AND PANAMANIAN GATEWAYS ON THE CIRCULATION OF AN OCEAN MODEL

Geologic studies indicate that prior to approximately 40 Ma the Drake Passage was closed and the Central American Isthmus was open. The effe ct of these changes has been examined in an ocean general circulation model. Several sensitivity experiments were conducted, all with atmosp heric forcing and other boundary conditions from the present climate, but with different combinations of closed and open gateways. In the fi rst experiment, the only change involved closure of the Drake Passage. In agreement with earlier studies the barrier modified the geostrophi c balance that now maintains the circumpolar flow in the southern ocea n, with the net effect being decreased transport of the Antarctic Curr ent and an approximate fourfold increase in outflow of Antarctic deep- bottom waters. The very large increase in Antarctic outflow suppresses North Atlantic Deep Water (NADW) formation. In addition to corroborat ion of results from earlier studies, our simulations provide several n ew insights into the role of a closed Drake Passage. A more geological ly realistic closed Drake/open central American isthmus experiment pro duces essentially the same pattern of deepwater circulation from the f irst experiment, except that Antarctic outflow is about 20% less than the first experiment. The resultant unipolar deepwater circulation. pa ttern for the second experiment is consistent with paleoceanographic o bservations from the early Cenozoic. A third experiment involved an op en Drake and open central American isthmus. In this experiment, Antarc tic outflow is diminished to slightly above present levels but NADW pr oduction is still low due to free exchange of low-salinity surface wat er between the North Pacific and North Atlantic. The low level of ther mohaline overturn should have reduced oceanic productivity in the Olig ocene (approximately 30 Ma), a result in agreement with geologic obser vations. Finally, simulations with an energy balance model demonstrate that the changes in surface heat flux south of 60-degrees-S due to br eaching of the Drake barrier do not result in temperature changes larg e enough to have triggered Antarctic glaciation. This last result sugg ests that some other factor (CO2?) may be required for Antarctic ice s heet expansion in the Oligocene (approximately 30-34 Ma). Our results lend further support to the concept that even in the absence of changi ng boundary conditions due to ice sheet growth, variations in the geom etry of the ocean basins can significantly influence ocean circulation patterns and the sediment record. The results also suggest that the p rimary polarities of the Cenozoic deepwater circulation may have been controlled by opening and closing of these two gateways.