Summertime oceanic fluxes at SHEBA: observations and steady 2-D modelling

Horizontal profiles of temperature, conductivity, and vertical velocity deviations were gathered in and around a lead at the SHEBA site during the summer of 1998. The temperature and conductivity data were collected by probes on the Autonomous Micro-conductivity Temperature Vehicle (AMTV), and the vertical water velocity was calculated from vehicle depth, pitch, and pitch rate using a Kalman smoothing algorithm. In summer, fresh meltwater enters the ocean from bottom melting and from the ice surface through leads while solar heating in leads is strong, and wind and ice motion stir the upper ocean. AMTV data from this period indicate strong downward heat flux and upward salinity flux below the lead, while under the ice, fluxes are very small. In addition, the turbulent vertical water velocity appears to increase in total energy and in characteristic wavelength beyond approximately 100 m downstream of the lead edge. Horizontal profiles of temperature and salinity show the warmest, freshest water at a given depth in the boundary layer are found near the lead edge. The interplay of vertical shear and this horizontal density gradient may contribute to the horizontal variability in turbulent energy and length scales. The internal transition between flow affected by the lead surface and flow influenced by the ice underside (internal boundary layer) may also play a role in enhancing mixing at the downstream lead edge, both kinematically, and through a similar interplay of vertical shear and a horizontal density gradient.

The surface heat budget depends heavily on the redistribution of the incoming radiative flux (to lateral melting, basal melting, atmospheric loss, or oceanic storage) which in turn relies on the vertical heat flux in the oceanic boundary layer. A 2-D steady-state model helps answer questions raised by observations: the sign and magnitude of fluxes as a function of depth and distance downstream of the lead, the potential for a horizontal density gradient, and the effects of the internal boundary layer.