EOS, Transactions, AGU, 2000 Ocean Science
Meeting, Vol. 80, No. 49, December 07, 1999, page OS52-OS53
Glacial
and Meltwater Deep-Ocean Conveyor: The Appraisal of Water Transport Alterations
in a Numerical Model
1Earth System Science Center,
Pennsylvania State University, University Park, PA 16802-2711
2Environmental Change Research
Centre, Department of Geography, University College London, 26 Bedford Way, London
WC1H 0AP, United Kingdom
Present-day ocean thermohaline
circulation is driven by production of the North Atlantic Deep Water (NADW)
forming a presumed global conveyor-type system, which provides a deep-ocean
connection between all the most remote regions of the World Ocean. Data analyses
and numerical models indicate that this conveyor has undergone substantial changes
during glacial cycles of Pleistocene. The most dramatic changes are attributed
to freshwater impacts characteristic of Heinrich events, when meltwater from
melting icebergs suppresses the glacial deep convection in the northern North
Atlantic. It is a widespread opinion that the conveyor was diminished at glacial
and completely shut-off or even reversed during Heinrich events. We have tested
the state of the global conveyor; at present, during glacial, and during meltwater
events, using MOM-2 ocean circulation model with the tracer visualization technique
that allows us to track actual deepwater motion. The results of these simulations
challenge the idea of strong connections between northern parts of the North
Atlantic and North Pacific Ocean. They favor the so-called "cold path"
hypothesis implying the return of surface water necessary to replace the NADW
via Drake Passage by the Antarctic Circumpolar Current and further to Benguela
Current, rather than via Indonesian Throughflow and western Indian Ocean (an
alternative "warm path" hypothesis). Also, our experiments indicate
that the apparent asynchrony of Antarctic and Greenland climate records during
the last glacial cycle of Pleistocene can be explained by rebounds of the deep-ocean
conveyor during Heinrich and Dansgaard-Oeschger meltwater episodes. On the basis
of numerical modeling of several generic meltwater scenarios, we conclude that
the deep-ocean conveyor can alone cause and sustain the bipolar asynchrony of
the surface ocean climate. Since the centuries are required to warm the respective
high latitudinal waters due to the deepwater-driven oscillating system, the
observed climate millennial-scale leads and lags between the Hemispheres can
be attributed to the change in the oceanic heat transport that results from
the conveyor rebounds.
