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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.

Wagner, TJW, Stern AA, Dell RW, Eisenman I.  2017.  On the representation of capsizing in iceberg models. Ocean Modelling. 117:88-96.   10.1016/j.ocemod.2017.07.003   AbstractWebsite

Although iceberg models have been used for decades, they have received far more widespread attention in recent years, due in part to efforts to explicitly represent icebergs in climate models. This calls for increased scrutiny of all aspects of typical iceberg models. An important component of iceberg models is the representation of iceberg capsizing, or rolling. Rolling occurs spontaneously when the ratio of iceberg width to height falls below a critical threshold. Here we examine previously proposed representations of this threshold, and we find that there have been crucial flaws in the representation of rolling in many modeling studies to date. We correct these errors and identify an accurate model representation of iceberg rolling. Next, we assess how iceberg rolling influences simulation results in a hierarchy of models. Rolling is found to substantially prolong the lifespan of individual icebergs and allow them to drift farther offshore. However, rolling occurs only after large icebergs have lost most of their initial volume, and it thus has a relatively small impact on the large-scale freshwater distribution in comprehensive model simulations. The results suggest that accurate representations of iceberg rolling may be of particular importance for operational forecast models of iceberg drift, as well as for regional changes in high-resolution climate model simulations. (C) 2017 Elsevier Ltd. All rights reserved.

Wagner, TJW, Eisenman I.  2017.  How climate model biases skew the distribution of iceberg meltwater. Geophysical Research Letters.   10.1002/2016GL071645   Abstract

The discharge of icebergs into the polar oceans is expected to increase over the coming century, which raises the importance of accurate representations of icebergs in global climate models (GCMs) used for future projections. Here we analyze the prospects for interactive icebergs in GCMs by forcing an iceberg drift and decay model with circulation and temperature fields from (i) state-of-the-art GCM output and (ii) an observational state estimate. The spread of meltwater is found to be smaller for the GCM than for the observational state estimate, despite a substantial high wind bias in the GCM—a bias that is similar to most current GCMs. We argue that this large-scale reduction in the spread of meltwater occurs primarily due to localized differences in ocean currents, which may be related to the coarseness of the horizontal resolution in the GCM. The high wind bias in the GCM is shown to have relatively little impact on the meltwater distribution, despite Arctic iceberg drift typically being dominated by the wind forcing. We find that this is due to compensating effects between faster drift under stronger winds and larger wind-driven wave erosion. These results may have implications for future changes in the Atlantic meridional overturning circulation simulated with iceberg-enabled GCMs.