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Malte F. Jansen

Assistant Professor, Department of the Geophysical Sciences, The University of Chicago

Deciphering Deep Ocean Circulation Changes Between the Present and the Last Glacial Maximum

Room 811 AOSS, March 13, 2017, 3:30 PM

Abstract

The paleoclimate record indicates that the deep ocean circulation and water masses have undergone major rearrangements between glacial and interglacial climates, which have likely played an important role in the observed atmospheric carbon dioxide swings.The mechanisms governing circulation changes are addressed using a hierarchy of models of varying complexity. The results support the hypothesis that various inferred differences in the deep ocean circulation and stratification between glacial and interglacial climates can be attributed to increased Antarctic sea-ice formation in a colder world. Colder temperatures lead to thicker ice, which is exported by winds and causes enhanced freshwater loss around Antarctica. The increased freshwater export leads to saltier and denser Antarctic Bottom Water, consistent with high abyssal salinities inferred for the Last Glacial Maximum (LGM). The enhanced deep ocean stratification moreover results in a shoaling of the inter-hemispheric overturning circulation, again consistent with proxy evidence for the LGM. The results also allow us to speculate about deep ocean circulation changes in a warmer world. Idealized simulations of a warmer climate reveal circulation changes opposite to those inferred for the glacial climate. The disappearance of Antarctic sea ice leads to a shut-down of Antarctic bottom water formation, leaving the entire deep ocean filled with very weakly stratified water of North Atlantic origin. Finally, the results highlight the importance to distinguish between the equilibrium and transient response of the ocean circulation to climatic changes. The adjustment of the deep ocean circulation is highly non-monotonic, with the response on centennial time-scales differing qualitatively from the equilibrium results. This distinction is easily overlooked in complex coupled climate simulations, which cannot be integrated for sufficiently long times.



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