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Oceanographic controls on Arctic sea ice and its future evolution

The annual cycle of sea ice in the Arctic and marginal ice zones is strongly affected by the flux of heat from the ocean to sea ice. This flux is mediated by a number of processes:

1. During the summer, solar radiation can penetrate below the seasonal mixed layer. This is mediated by colored dissolved organic material, whose concentration is thought to be increasing in the Arctic, and by the presence of clear ice-melt layers. Neither of these processes is well-represented in the current generation of GFDL coupled climate models.
2. During the winter, this heat can be returned to the mixed layer by mixing, and additional heat is added from Atlantic waters entering the Arctic. The ease with which this occurs depends on the amount of freshwater stored in the mixed layer and the depth to which this water is mixed. Understanding the evolution of freshwater anomalies within the Arctic may therefore be important for predicting the future of Arctic sea ice.
3. Once it reaches the mixed layer, the heat must be transferred to the sea ice by turbulent exchange. The current version of the GFDL model parameterizes this exchange in a relatively crude fashion, using a heat transfer coefficient that is independent of the friction velocity.
Our proposal will carry on work currently being done at Johns Hopkins to look at all three processes.

Graduate student Grace Kim has recently developed a new parameterization of solar absorption which includes colored dissolved material (CDM), an important absorber of light in the open-ocean Arctic waters, and implemented this within the GFDL CM2Mc model (Galbraith et al., 2011). She finds that the inclusion of CDM produces significant regional changes in Arctic ice cover with a small overall increase. It is thus possible that increasing CDM in Arctic rivers and increasing chlorophyll in the Arctic interior will serve as a negative feedback on sea ice loss. We propose to expand this work to higher resolution GFDL models, particularly ESM2G, which has a very different mixed layer scheme.

Tom Haine’s group has worked extensively on the processes maintaining the freshwater anomalies in the Arctic. A particular question we wish to examine is whether changes in Arctic freshwater storage will modulate both the seasonal storage of heat and the supply of heat from warm Atlantic waters. This work will be done using high-resolution models of the Arctic previously used in Haine’s group. Postdoctoral support will be requested for this task.

Graduate student Eshwan Ramadu in Gnanadesikan’s group has been examining the impact of relaxing the assumption of a constant (relatively large) heat transfer coefficient within the GFDL ice model. This work has found that increasing this coefficient (as might be expected to occur with thinner sea ice) leads to a build up of freshwater in the Beaufort Sea and a reduction of sea ice in the marginal ice zone. We have also started simulations where the ice-ocean heat transfer coefficient is replaced with one that is dependent on the friction velocity. Support is requested to continue this work.

We anticipate that all of these projects will be conducted in collaboration with Gnanadesikan’s former colleagues at the Geophysical Fluid Dynamics Lab, particularly Bob Hallberg and John Dunne.

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