Improving Ocean-ice Interactions: A Dynamic-Thermodynamic Iceberg Model to Simulate High-Latitude Freshwater Forcing

Monday, May 12, 2014 - 07:00
Add to Calendar

A sophisticated dynamic-thermodynamic iceberg model has been developed with funding from DoE SciDAC DE-FOA-0000452 to simulate the coupling between the cryosphere and the ocean. Icebergs are treated as lagrangian particles with horizontal motion derived from the sum of all drag forces (wind, ocean, waves, sea ice, Coriolis force and pressure gradient force) acting on the iceberg. The model employs a multilevel drag scheme that takes into account changes in horizontal ocean velocity in the vertical acting on the iceberg. Icebergs can also assume a variety of shapes, including both tabular and pinnacle, and a keel and sail model are used to prescribe more authentic ocean and air drags beneath and above the waterline. Icebergs surrounded by high concentrations of sea ice drift with the pack ice while grounded icebergs remain stationary until they melt sufficiently to start drifting again. Iceberg melt occurs due to buoyant vertical convection, sensible heat exchange, wave erosion, and radiative heating. Erosion at the waterline leads to overhanging slabs of ice that eventually become unstable and calve into the ocean. Freshwater from melting ice freshens the ocean while heat used to melt the ice locally cools it. Freshwater released from individual icebergs can be tracked in the ocean using passive dye tracers, allowing ocean freshwater pathways to be identified even after an iceberg has melted. The model is coded in Fortran to run efficiently on parallel super computing facilities, while faster integration times can also be achieved by treating several icebergs as one individual lagrangian particle, allowing hundreds-of-thousands of icebergs to be simulated at any one time. Coupling the iceberg model to a 1/6 deg. resolution ocean model produces realistic iceberg trajectories and melt rates. Icebergs calved from the Greenland Ice Sheet are confined to the East and West Greenland Currents and the Labrador Current, and melt at the Grand Banks. Multidecadal model integrations are now investigating AMOC sensitivity to IPCC projected increases in freshwater and iceberg discharge from the Greenland Ice Sheet. Future research plans include using the iceberg model to examine the AMOC response to a collapse of the West Antarctic Ice Sheet (WAIS) and collaborating with COSIM to advance iceberg model development for CESM to enhance DOE efforts to understand ice-ocean interactions and AMOC response to freshwater forcing.