Abstract
Abstract. Leads are linear-like structures of open water within the sea ice cover that develop as the result of fracturing due to divergence or shear. Through leads, air and water come into contact and directly exchange latent and sensible heat through convective processes driven by the large temperature and moisture differences between them. In the central Arctic, leads only cover 1 to 2% of the ocean during winter, but account for more than 70% of the upward heat fluxes. Furthermore, narrow leads (several meters) are more than twice as efficient at transmitting turbulent heat than larger ones (several hundreds of meters). We show that lead widths are power law distributed, P(X)~X−a with a>1, down to very small spatial scales (20 m or below). This implies that the open water fraction is by far dominated by very small leads. Using two classical formulations, which provide first order turbulence closure for the fetch-dependence of heat fluxes, we find that the mean heat fluxes (sensible and latent) over open water are up to 55% larger when considering the lead-width distribution obtained from a SPOT satellite image of the ice cover, compared to the situation where the open water fraction constitutes one unique large lead and the rest of the area is covered by ice, as it is usually considered in climate models at the grid scale. This difference may be even larger if we assume that the power law scaling of lead widths extends down to smaller (~1 m) scales. Such estimations may be a first step towards a subgrid scale parameterization of the spatial distribution of open water for heat fluxes calculations in ocean/sea ice coupled models.
Highlights
Sea ice is a fundamental component of the climate of polar regions
Using two classical formulations, which provide first order turbulence closure for the fetch-dependence of heat fluxes, we find that the mean heat fluxes over open water are up to 55 % larger when considering the lead-width distribution obtained from a SPOT satellite image of the ice cover, compared to the situation where the open water fraction constitutes one unique large lead and the rest of the area is covered by ice, as it is usually considered in climate models at the grid scale
The positive trends in Arctic sea ice velocity and strain rates reported by Rampal et al (2009), which related to increasing sea ice fracturing, may imply an increasing role of sea ice leads in heat transfer over the Arctic Ocean in the future
Summary
Sea ice is a fundamental component of the climate of polar regions. In the Arctic, the sea ice cover extends from about 5 × 106 km at the end of summer to 14 × 106 km in winter. Weiss: Influence of leads widths distribution on turbulent heat transfer under clear-sky conditions during polar night They have confirmed that the upward heat fluxes over leads are almost balanced by downward heat fluxes over the snow on nearby sea ice, as first shown by in situ data from the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment (Overland et al, 2000). Maslanik and Key (1995) calculated the sensitivity of turbulent flux estimates to changes in lead-width distribution using the heat flux parameterization of Andreas and Murphy (1986), and assuming that the distribution of leads followed an exponential distribution function They argued that parameterizing these fluxes in a sea ice model can be done effectively using a single representative lead width (e.g. the mean lead width) rather than requiring a full distribution of lead widths. We will analyze the sensitivity of the fluxes to the two parameters defining a power law distribution of lead widths, i.e. the exponent a and the lower cut-off L0
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