Under-ice Boundary Layer

Sea ice is in almost constant motion in response to wind, ocean currents, and forces transmitted within the ice cover itself, thus there is nearly always a zone of sheared Sow between the ice and underlying, undisturbed ocean where turbulence transports momentum, heat, salt, and other contaminants vertically. The zone in which these turbulent Suxes occur, which can span from a few to hundreds of meters, is the under-ice boundary layer (UBL). This article describes general characteristics of the UBL, with emphasis on the physics of vertical turbulent transfer, speciRcally turbulent mixing length and eddy diffusivity. Extensive measurements of turbulence in the UBL, not available elsewhere, have not only made these ideas concrete, but have also provided quantitative guidance on how external forcing controls the efRciency of vertical exchange. Here we stress features that the UBL has in common with ocean boundary layers everywhere. The article on ice}ocean interaction emphasizes unique aspects of the interaction between sea ice and the ocean (see Ice+Ocean Interaction). While largely responsible for the relative paucity of oceanographic data from polar regions, sea ice also serves as an exceptionally stable platform, often moving with the maximum velocity in the water column. In effect, it provides a rotating geophysical laboratory with unique opportunities for directly measuring turbulent Suxes of momentum, heat, and salt at multiple levels in the oceanic boundary layer } measurements that are extremely difRcult in the open ocean. Examples of important oceanographic boundary-layer processes Rrst observed from sea ice include: (1) the Ekman spiral of velocity with depth; (2) Reynolds stress through the entire boundary layer, and its associated spiral with depth; (3) direct measurements of turbulent heat Sux and salinity Sux; (4) direct measurements of eddy viscosity and diffusivity in the ocean boundary layer; (5) the impact of surface buoyancy, both negative and positive, on boundary layer turbulence, and (6) internal wave drag (‘dead water’) as an important factor in the surface momentum and energy budgets. The UBL differs from temperate open ocean boundary layers by the absence of strong diurnal forcing and of high frequency, wind-driven surface waves. It thus lacks the near surface zone of intense turbulence and dissipation associated with wave breaking, and organized Langmuir circulation due to the nonlinear interaction between waves and currents (e.g., the interaction of Stokes drift with near surface vorticity) (see Langmuir Circulation and Instability). On the other hand, quasi-organized roll structures associated with sheared convective cells have been observed under freezing ice, and are apparently a ubiquitous feature of freezing leads and polynyas. Large inertial-period oscillations in UBL horizontal velocity are observed routinely, especially in summer when the ice pack is relaxed. The annual cycle of buoyancy Sux from freezing and melting mimics in some respects the diurnal cycle of heating and cooling, as well as the annual evolution of temperate ocean boundary layers. The range of surface forcing, with observations of surface stress ranging up to 1 Pa, and buoyancy Sux magnitudes as high as 10~ W kg~, is comparable to that encountered in open oceans. All these factors suggest that similarities between the UBL and the open ocean boundary layer far outweigh the differences.