The impact of the representation of Antarctic freshwater fluxes on the Southern Ocean in the Energy Exascale Earth System Model (E3SM)

Tuesday, December 11, 2018 - 08:00
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In many ocean/sea ice model configurations, freshwater forcing from Antarctica is typically crudely represented as being prescribed near the coast at the surface, constant in space and time. In reality, this freshwater forcing is driven by two processes, ice shelf basal melting and iceberg calving, which interact with the ocean in ways very different from this simplified approximation. Basal melting has considerable variability between ice shelves and deposits meltwater at depth, and calved icebergs can transport freshwater far from the coast into the Southern Ocean. Simplified model approximations that deposit excessive cool freshwater around the Antarctic coast can lead to increased sea ice growth that blocks polynya formation and suppressing ocean overturning. We have performed global experiments with ocean/sea-ice configurations of the new Department of Energy's Energy Exascale Earth System Model (E3SM) that include ocean circulation beneath Antarctic ice shelf cavities, which allows us to directly model ice shelf basal melt rates. In addition, we use prescribe freshwater forcing due to iceberg calving using steady state iceberg climatology model data. We evaluate the impact of Antarctic freshwater forcing in E3SM's global ocean/sea-ice simulations with various combinations of simplified coastal runoff, modeled ice shelf basal melting, and iceberg freshwater flux data. Prescribed freshwater flux that is too large in magnitude or incorrectly partitioned between subshelf and open ocean locations can lead to unrealistically high modeled subshelf melt rates due to overly vigorous activation of the ice shelf cavity melt pump. Careful treatment of solid and liquid freshwater fluxes around Antarctica is critical for producing realistic Southern Ocean circulation, temperature, and salinity and associated subshelf melt rates in models.

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