Circulation, mixing and interactions in the ocean near the Ross Ice Shelf, Antarctica

Abstract

This work explores the complex and interacting processes across a range of scales that can occur in the immediate vicinity of an ice shelf front, and which are important influences on regional sea ice growth, bottom water production and ice shelf maintenance. McMurdo Sound and Haskell Strait, Antarctica, form the setting for this study and can be considered a natural laboratory for these processes.

The interannual stability of regional oceanographic processes was scrutinised via the downstream response to the perturbation caused by two massive tabular icebergs that calved from the Ross Ice Shelf. This unique natural experiment demonstrated connectivity between the sea ice and ice shelf regimes of the Ross Sea and confirmed McMurdo Sound as a conduit for signal transfer between them. The icebergs each had distinct and separable influences on oceanic circulation and water mass production. Blocking by iceberg B-15a significantly reduced normal summer inflow of warm Antarctic Surface Water to the ice shelf cavity for four years. Iceberg C-19 interrupted normal operation of the Ross Sea Polynya, causing a reduction in HSSW production for the single winter that it moved through the area, although recovery to the disturbance took several years.

Generally cyclonic circulation, due to opposing northward and southward flows on either side of the sound, was revealed in a latitudinal transect conducted during November 2007. These flows form a sloping vertical gradient, where northward-flowing ISW cuts through the reservoir of HSSW, dividing oceanographic east from west in McMurdo Sound. The resulting density structure supports geostrophic velocities that are consistent with the direction of regional water mass flows through the sound. Seasonal evolution of water column density stratification, in response to the annual cycle of Ross Sea Polynya activity, controls the extent of turbulent overturns that can exist, and allowed internal waves of approximately 100 m amplitude to develop - a result of interaction between tidal flows and rough local topography.

Substantial layers of platelet ice were observed beneath sea ice on both sides of the sound, accompanied by near-surface supercooled water. In the east, a 1.5 m thick, mobile layer produced velocity shear in the ocean boundary layer with a roughness scale of approximately 3.0 m - indicative of an effective roughness well beyond the overall morphology of the platelet layer. In the west, individual platelets of up to 25 cm diameter were recovered from a layer estimated to be 3 m thick. Growth of individual ice crystals in water supercooled by as much as 50 mK released brine at a rate sufficient to create a continual density instability at the ice-ocean interface, and buoyancy fluxes equivalent to about 1/2 m day-1 of normal congelation growth. Resulting convective mixing created vertical velocities estimated to lie in the range 40 - 120 mm s-1, contributing to the active mixing of the homogeneous upper ocean layer which extended 200 m below the ice. Each of these platelet-induced processes has implications for the operation of marine ice bands beneath the large ice shelves, which are believed to possess a similar fine-structure to the platelet layers observed in this study.