Monday 12 Sep 2016: Deciphering deep ocean circulation changes between the present and the last glacial
Prof. Malte Jansen - University of Chicago
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 by affecting the partitioning of carbon between the atmosphere and ocean. The mechanisms by which the deep ocean circulation changed, however, are still unclear and represent a major challenge to our understanding of past and future climates.
We address this question using a hierarchy of models of varying complexity, ranging from a highly idealized ocean-only model to coupled climate simulations from the Paleoclimate Modelling Intercomparison Project (PMIP). The results suggest 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 long-term changes to the deep ocean circulation 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 found to be highly non-monotonic, with the response on centennial time-scales differing qualitatively from the equilibrium results. This distinction is rarely observable in complex coupled climate models, which cannot be integrated for sufficiently long times.