Abstract

Observations of dense off-slope flow with energetic tidal forcing

Hydrographic and velocity measurements of the dense shelf water exiting the Ross Sea during the Antarctic Slope (AnSlope) experiment in early 2003 are reported. For a series of stations on the upper continental slope near the head of the Drygalski Trough we have concurrent CTD, Lowered ADCP, and scalar microstructure data extending from the ice-covered surface to near the seabed. Two short-duration moorings deployed nearby provide time series of currents and scalar properties (T and S). These measurements reveal a thick (>100 m) seaward-flowing benthic layer of cold, dense water. Preliminary analyses suggest that the fate of this “plume” is modulated by the cross-slope barotropic diurnal tidal currents, whose speed can reach ~1 m s-1 at spring tides. Tides and other processes concentrated at the shelf break provide energy for mixing the plume with the ambient fluid; i.e., reducing the T-S anomaly of the plume and thereby reducing the depth at which the plume fluid will interleave into the deep ocean. We present microstructure evidence for energetic mixing at the top of the plume during spring tides. A non-geostrophic source of cross-slope flow (such as tides) also provides a mechanism to force dense water down across f/H contours to the point where further deepening with limited mixing can occur through thermobaric processes. The ultimate fate of dense shelf water involves complex interactions between mixing and advection, and may be sensitive to the timing of outflows with respect to the strong fortnightly spring-neap tidal cycle.