Role of variable form drag coefficients over sea ice for the ocean surface layer in polar regions

Sea ice features a variety of obstacles to the flow of air and ocean near its surface. The distribution of ridges on the ice bottom and top surfaces, as well as the edges of the floes, the ice surface roughness and the melt ponds edges interact with the exchanges of heat and momentum between the ice and ocean and atmosphere systems. In most climate models, the surface fluxes of heat and momentum are estimated with bulk formulas, using a drag coefficient that reflects the surface roughness of the interface. Drag coefficients over sea ice are usually set to constant, which do not account for the variability of the ice surface roughness. However, drag coefficients can be estimated as a function the sea ice state (Tsamados et al, 2013). To study the effect of variable drag coefficients on the sea ice and its subsequent impact on the ocean surface, we have implemented the formalism of Tsamados et al (2013). We performed a first simulation with constant drag coefficients and a second with variable drag coefficients, using NEMO-LIM3 and a prescribed atmospheric state of the past decades. The differences between the two simulations allow us to formulate an initial assessment of the importance of variable drag coefficients over sea ice for the ocean surface in the Arctic.