One of the major objectives of the Surface Heat Budget of the Arctic (SHEBA) experiment is to quantify the process of the ice-albedo feedback. By this process the radiation absorbed at the surface of an ice-covered sea tends to decrease the albedo of the surface and increase the amount of radiation absorbed. The feedback is positive. An important determinant of ice-albedo feedback is the ratio of ice floe bottom melting to floe edge melting by ocean heat flux. The former occurs when radiative heating in open leads is mixed down into the mixed layer and then up under the ice. It has no immediate effect on albedo except for the thinnest ice floes. Edge melting occurs when strong stratification is maintained in the leads of melt water flux and the heat is trapped above the level of the bottom of the ice. This increases ice-albedo feedback.
In an effort to quantify the ratio between bottom and edge melting, we will be measuring the horizontal variability of vertical heat and salt flux in the ocean at SHEBA. We will be doing this using Autonomous Underwater Vehicles (AUV) run beneath the sea ice at various depths. The method was first developed using the Autonomous Conductivity Temperature Vehicle (ACTV) to study winter lead convection during the 1992 Lead Experiment (LeadEx). The ACTV is a small, autonomous vehicle that can traverse a programmed, dead reckoned course carrying a Sea-Bird Sea-Cat CTD. ACTV temperature and salinity spectra show a k-5/3 dependence in agreement with the fixed turbulence measurements. In conjunction with fixed turbulence measurements, the temperature and salinity spectra can be used to estimate vertical heat and salt flux along horizontal transects using an inertial dissipation method. LeadEx results also show that turbulent vertical velocity perturbations can be estimated from the vertical motion of the ACTV, and with such estimates, horizontal profiles of vertical salt and heat flux can be computed by a direct eddy correlation method. The method has been developed further with a Kalman filtering approach. A new instrument, the Autonomous Microconductivity and Temperature Vehicle (AMTV), has been built to exploit these new approaches.
At SHEBA we will be operating the AMTV and ACTV to measure the turbulent heat and salt flux at various depths in and around summer leads. With the results of these measurements, we hope to present preliminary budgets of heat and salt, and estimates of the amount of heat going to side versus bottom melting. We will also present observations of the effect on the ice made with a small, Remotely Operated Vehicle (ROV) and an upward looking sonar on the AMTV.