Physics of early summer ice/ocean exchanges in the western Weddell Sea during ISPOL

Early summer exchanges of momentum, heat, and salt between a multiyear ice floe and the underlying upper ocean in the western Weddell Sea were investigated during the Ice Station Polarstern (ISPOL) drift. Instrumentation clusters including three-dimensional acoustic current meters, temperature and conductivity meters were deployed at up to three levels beneath two different sites on the ISPOL floe, at depths ranging from 3 to 10 m below the ice/ocean interface. ISPOL presented a complex physical environment with the floe comprising a mixture of ice types, responding to relatively strong tidal and internal ice forcing, drifting in a matrix of rapidly shifting open-water areas. Turbulence measured during the drift was generally moderately energetic during spring tides early and late in the drift, with maximum interfacial stress approaching 0.2 Pa. Measured turbulent heat flux at the instrument levels, which varied from about 20 upward to 70 W m−2 downward, responded to both solar radiative and horizontal advective effects, therefore did not necessarily correspond to interface heat flux values. Currents measured with an acoustic Doppler profiler showed consistent Ekman turning in the water column between 10 and 30 m. A novel application of modeling based on temperature and salinity profiles plus relative current measured at 20 m provided an estimate of the representative undersurface hydraulic roughness for the floe: z0=0.04 m. By solving the numerical model used to establish z0 for all 3-h average segments when relative current at 20 m exceeded 0.03 m s−1, interface conditions were estimated for the entire drift. Average values evaluated at the ice/water interface included turbulent stress of about 0.05 Pa; turbulent heat flux of about 15 W m−2 (implying a maximum bottom ablation of around 15 cm); and salinity flux of about 1.5×10−6 psu m s−1.