High-frequency ice motion and divergence in the Weddell Sea

We describe the spatial variability of high frequency ice velocity in the W eddell Sea using satellite-tracked ice-mounted buoys. Ice motion is analyze d separately for "diurnal" (1/36-1/18 cph) and "semidiurnal" (1/18-1/6 cph) bands. Ice motion in both bands is caused by a combination of ocean tidal currents and wind stress. Monthly mean diurnal band ice speeds over the dee p basin range from 2 to 4 cm s(-1) depending on wind stress variance and ic e concentration (C-ice). Higher speeds (similar to 10 cm s(-1) are found in the semidiurnal band in regions of low C-ice, notably the northern Weddell Sea, where the ice velocity is dominated by the inertial response to wind stress variations. Monthly mean tidal band ice speeds over the continental slope and shelves often exceed 10 cm s(-1). We use comparisons between buoy velocities, moored current meter data, and an ocean tidal model to demonst rate that ice motion is frequently a good indicator of ocean tidal currents in strongly tidal regions. The standard deviation of the divergence of oce an tidal currents estimated from an ocean-only tidal model is small (<0.1x1 0(-6) s(-1)) over most of the Weddell Sea but has values of 1-5x10(-6) s(-1 ) along the Ronne Ice Front and the continental shelf break. High frequency ice divergence is dominated by ice response to wind stress rather than by tides except along the shelf break and ice fronts. In these tidally dominat ed regions the periodic divergence maintains a mean lead (open water) area of similar to 2-5%. This increased lead fraction implies an increase in are a-averaged winter ocean-to-atmosphere heat exchange rate of similar to 4-10 W m(-2) and an increase in salt flux into the upper ocean as new ice forms in the leads.