EOS (Supplement), April 28, 1998, AGU 1998 Spring Meeting, p. S51.

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GEOMETRIC CONTROL OVER THE GLOBAL OCEAN THERMOHALINE CIRCULATION AND PALEOCIRCULATION

Seidov, D., Bice K., Pollard D., Poulsen C.

Earth System Science Center, Pennsylvania State University, University Park, PA 16802-2711, USA.

A strong sensitivity to small geometrical changes, especially to the position of gateways, has been found in numerical experiments aimed at understanding the major controls over ocean paleocirculation. In our idealized numerical experiments, we use GFDL-MOM model of ocean circulation with simplified land-ocean geometry and zonal mean forcing in order to isolate and clarify the impacts of small changes in land-ocean basin geometry. The model is driven at the surface by specified zonal and annual mean wind stress, sea-surface temperature and freshwater flux (converted to effective sea surface salinity). Wind stress and freshwater flux are from the GENESIS 2.0 atmospheric model, and sea-surface temperature is from the World Ocean Atlas (Levitus et al., 1994). All basins have flat-bottom bathymetry, except Drake Passage, which has a sill in some experiments. At present, Drake Passage is the only gateway providing a continuous circum-global flow, which is believed to have a profound effect on the earth's climate. However, in ancient oceans other passageways allowing circum-global flows existed. Their role is not quite clearly comprehended, and the impact of uncertainties in their exact position are rarely considered. We have performed a series of simulations in which the position, depth and width of Drake Passage are varied, corresponding to typical uncertainties in paleo-geographical reconstructions. We conclude that the most important factor affecting thermohaline circulation is the latitudinal position of the passageway. This study implies a strong sensitivity to small uncertainties in paleogeographical reconstructions, and calls for a reassessment of current practices in paleoceanographic modeling. These conclusions are illustrated by preliminary results of a recent numerical study of Cretaceous ocean currents, in which the position of a passageway, allowing tropical circum-global flow, is varied. Small latitudinal shifts in the passageway's location cause substantial changes in the ocean circulation and temperature and salinity fields at low latitudes.