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EOS, Transactions, American Geophysical Union (Suppl.), 1998, Vol. 79, No. 1, OS52A-5
OCEAN THERMOHALINE CONVEYOR AT PRESENT AND IN RECENT GEOLOGICAL PAST
Earth System Science Center, Pennsylvania State University, University Park, PA
Understanding of the global ocean thermohaline conveyor may be advanced by studying scenarios of extreme glacial-to-interglacial transits of the earth climate. The last glacial maximum (LGM) is one of the most severe changes to ocean surface conditions during the Late Quaternary and has the best data coverage of any glacial time-slice to support numerical experiments. Here, the global ocean thermohaline conveyor at present, at the last glacial maximum, and at a subsequent meltwater event (MWE) is simulated using a combination of a global ocean circulation model and a Lagrangian trajectory tracing technique (Seidov and Haupt, 1997). Trajectories of water volumes illustrate the true three-dimensional circulation in a way that cannot be obtained using only conventional ocean circulation models. This technique reveals the global inter-ocean connections and helps to understand the deep ocean ventilation regime. The model results do not support the idea that a global conveyor, either now or in the past, directly connected the high-latitudinal North Atlantic and North Pacific. Even at present, the two oceans are only weakly connected via a series of basin-scale horizontal gyres. The simulated trajectories indicate strong vertical displacement of water parcels traveling within these gyres, which implies that both deep and intermediate circulation maintains inter-basin communications. This interpretation contradicts the traditional view of these remote basins being directly connected by global-scale abyssal and deep-water flow. Instead, the trajectories indicate that the deep water that escapes from the ACC does not penetrate as far northward as might be thought on the basis of the global conveyor paradigm. The primary mechanism of trans-oceanic linkages is deep convection, allowing water parcels to be tunneled upward or downward to different depths and thereby escape circumpolar motion. So, changes in the production of NADW might still have had a major impact over the entire ocean circulation at deep to intermediate depths, in spite of the inability of the deep ocean conveyor alone to connect the most remote parts of the world ocean. Although the modeled glacial conveyor is noticeably weaker then today, as many previous studies imply, even larger changes in the global deep ocean conveyor occurred at the MWE. These changes include a reversal of the Indian-Atlantic branch of the deep conveyor due to a cessation of North Atlantic Deep Water production, caused by capping of convection by a localized meltwater impact.