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Wagner, TJW, Dell RW, Eisenman I, Keeling RF, Padman L, Severinghaus JP.  2018.  Wave inhibition by sea ice enables trans-Atlantic ice rafting of debris during Heinrich events. Earth and Planetary Science Letters. 495:157-163.   10.1016/j.epsl.2018.05.006   AbstractWebsite

The last glacial period was punctuated by episodes of massive iceberg calving from the Laurentide Ice Sheet, called Heinrich events, which are identified by layers of ice-rafted debris (IRD) in ocean sediment cores from the North Atlantic. The thickness of these IRD layers declines more gradually with distance from the iceberg sources than would be expected based on present-day iceberg drift and decay. Here we model icebergs as passive Lagrangian particles driven by ocean currents, winds, and sea surface temperatures. The icebergs are released in a comprehensive climate model simulation of the last glacial maximum (LGM), as well as a simulation of the modern climate. The two simulated climates result in qualitatively similar distributions of iceberg meltwater and hence debris, with the colder temperatures of the LGM having only a relatively small effect on meltwater spread. In both scenarios, meltwater flux falls off rapidly with zonal distance from the source, in contrast with the more uniform spread of IRD in sediment cores. To address this discrepancy, we propose a physical mechanism that could have prolonged the lifetime of icebergs during Heinrich events. The mechanism involves a surface layer of cold and fresh meltwater formed from, and retained around, large densely packed armadas of icebergs. This leads to wintertime sea ice formation even in relatively low latitudes. The sea ice in turn shields the icebergs from wave erosion, which is the main source of iceberg ablation. We find that sea ice could plausibly have formed around the icebergs during four months each winter. Allowing for four months of sea ice in the model results in a simulated IRD distribution which approximately agrees with the distribution of IRD in sediment cores. (C) 2018 Elsevier B.V. All rights reserved.

Sun, ST, Eisenman I, Stewart AL.  2018.  Does Southern Ocean surface forcing shape the global ocean overturning circulation? Geophysical Research Letters. 45:2413-2423.   10.1002/2017gl076437   AbstractWebsite

Paleoclimate proxy data suggest that the Atlantic Meridional Overturning Circulation (AMOC) was shallower at the Last Glacial Maximum (LGM) than its preindustrial (PI) depth. Previous studies have suggested that this shoaling necessarily accompanies Antarctic sea ice expansion at the LGM. Here the influence of Southern Ocean surface forcing on the AMOC depth is investigated using ocean-only simulations from a state-of-the-art climate model with surface forcing specified from the output of previous coupled PI and LGM simulations. In contrast to previous expectations, we find that applying LGM surface forcing in the Southern Ocean and PI surface forcing elsewhere causes the AMOC to shoal only about half as much as when LGM surface forcing is applied globally. We show that this occurs because diapycnal mixing renders the Southern Ocean overturning circulation more diabatic than previously assumed, which diminishes the influence of Southern Ocean surface buoyancy forcing on the depth of the AMOC.

Sun, S, Eisenman I, Stewart AL.  2016.  The influence of Southern Ocean surface buoyancy forcing on glacial-interglacial changes in the global deep ocean stratification. Geophysical Research Letters. 43:8124-8132.   10.1002/2016gl070058   AbstractWebsite

Previous studies have suggested that the global ocean density stratification below approximate to 3000m is approximately set by its direct connection to the Southern Ocean surface density, which in turn is constrained by the atmosphere. Here the role of Southern Ocean surface forcing in glacial-interglacial stratification changes is investigated using a comprehensive climate model and an idealized conceptual model. Southern Ocean surface forcing is found to control the global deep ocean stratification up to approximate to 2000m, which is much shallower than previously thought and contrary to the expectation that the North Atlantic surface forcing should strongly influence the ocean at intermediate depths. We show that this is due to the approximately fixed surface freshwater fluxes, rather than a fixed surface density distribution in the Southern Ocean as was previously assumed. These results suggest that Southern Ocean surface freshwater forcing controls glacial-interglacial stratification changes in much of the deep ocean.