The heat and salt balances of the upper ocean beneath a spatially variable melting sea ice cover

Thesis (Ph. D.)--University of Washington, 2003The aim of this study is to paint a picture of the evolution of the horizontally variable ice-ocean boundary layer throughout summer. Observations were made during the drifting Surface HEat Balance of the Arctic (SHEBA) experiment in the summer of 1998. The ice-ocean boundary layer near leads is studied with an Autonomous Underwater Vehicle (AUV) and a novel technique to use vehicle motion data to calculate turbulent vertical water velocity along the vehicle path. Vertical fluxes are obtained and extend from the energy-containing wavenumber range and continue into the inertial subrange. This study is the first to measure horizontal profiles of turbulent fluxes in the ice-ocean boundary layer. AUV data are used in conjunction with fixed-mast turbulent fluxes at discrete levels in the boundary layer, Conductivity-Temperature-Depth data (vertical casts and lead surveys), and a suite of measurements made by other investigators.The results indicate that scalars and their fluxes, as well as vertical stability, varied in the horizontal direction. AUV run-averaged turbulent stress in the boundary layer agrees well with the free-drift estimate. In early summer, fluxes were weak as ice velocity was low, and fresh meltwater was trapped at the upper ice surface. Also, surface melt was directed into leads rather than entering the ocean uniformly, resulting in a highly stable fresh layer. Near the end of July, a storm flushed leads, and the mixed layer freshened and deepened. The AUV observed strong fluxes under and downstream of rough, ridged ice. After the storm, heat and salt fluxes were strongest under leads.The results are simulated with 1-D and 2-D time-varying numerical models. The 1-D model produces a shallow, overly fresh mixed layer during the storm period. Simulations from the 2-D model suggest that both mechanical forcing from ice topography and a dynamic instability near downstream lead edges may enhance vertical mixing. AUV data agree well with the 2-D model after the storm and suggest mechanical forcing is important. The timing and strength of meltwater flux and the horizontal variability in interfacial fluxes have implications on large scales through the seasonal cycle of mixed layer depth and the surface heat budget