Continental shelf Bottom water overflow Gyre Slope Current

Circulation and water-mass transformation over the continental shelf

Warm Deep Water (WDW), found below the cold surface layer in the gyre sector, is converted to HSSW and mCDW on the continental shelf, which flow directly into the ice shelf cavities guided by troughs cross-cutting the continental shelf. However, the processes controlling the transformation and circulation of water-masses are not well observed and, therefore, still largely unclear.

We propose to uncover the basin-scale circulation in the Filchner Depression and the associated surface and deep forcing, using sea-ice capable and rapidly profiling floats. Sound source moorings will be deployed for the floats to be able to locate themselves by triangulation under winter sea-ice. Additional mooring will be deployed at key location on the shelf to complement the view of the circulation described by the floats. 

Argo Mooring Sound source

Bottom water plume and its export toward the global ocean.

The return pathway on the continental shelf, going from the ice cavities to the open ocean, ultimately constitutes the core of the world’s ocean bottom water. In the escape pathway from the continental shelf to the large-scale open ocean, dense water sinks along the continental slope via a process termed “overflow”. The overflow ultimately sets the characteristic of the world ocean’s abyssal water: as waters descend they are significantly modified by entrainment of water from the ambient ocean. However, this downslope transport may occur via a range of different mechanisms, which are not well observed by the current observation network.

We propose to push back the limits of observations by providing the first-ever direct Lagrangian observation of entrainment of along a sinking bottom water plume. We will develop and deploy bottom following floats rated to 4000 m. The floats will not only enable to measure the impact of entrainment on temperature and salinity, but also have direct estimate of velocity and the fine-structure intensity in the bottom boundary layer.

Deep bottom following float

The subpolar gyre system

Large gyre systems are prominent features of the Southern Ocean, and the Weddell Gyre is the largest one of these, which provides a pathway between the World ocean basins and the main global production sites of abyssal water. Despite their key control on the global overturning circulation, the dynamical causes and the climate consequences of the intensity and variability of the gyre are still largely unclear.

We will investigate water-mass conversion and potential vorticity (PV) budget of the Weddell sea, which is what ultimately drives the circulation. Historical observations from a combination of international public databases will be put together for this investigation. We will make use of the spatial and temporal complementarily of ship-base observations, seals observations, and autonomous profiling floats observations.

Argo Elephant seal Ship

The Antarctic continental shelf break

The Antarctic shelf break hosts an intense and energetic current, which forms a dynamical barrier for water-mass exchanges across the shelf break. The dynamics allowing to cross the dynamical barrier and allowing the exchanges of water-masses are not well understood. Recent theoretical and idealized studies suggest the importance of 10 - 50 km eddy structures for this exchange. However, it is still unclear how these novel results would hold in a realistic model. The role of complex topography, for instance, is not well understood. Far from being minor dynamical side issues, these scientific questions defy our current conceptualization of the global ocean deep overturning.

We will use a state of the art realistic eddy-permitting model to explore how water cross toward and from Antarctica shelf. As a point of comparison, we will also run the model in a version where eddies will be damped. The behaviour of the different numerical simulations will be confronted to observations made during the project. 

Deep water formed around the Antarctic continent drives the world ocean circulation. More than half of this deep water is formed within only about 10% of the Antarctic circumpolar band: the Weddell Sea. Subtle changes in the circulation of the Weddell Sea can lead to major changes in floating ice-shelves, with critical implications for global sea-level, the production of deep water, and the global ocean overturning circulation. Despite these critical climate implications, the Antarctic shelf circulation remains poorly understood.

Water-mass transformation and Pathways In The Weddell Sea: uncovering the dynamics of a global climate chokepoint from In-situ measurements. 

Wapiti is a European funded project aimed at exploring the regional water-mass pathways in the Weddell Sea: an unchartered cornerstone for understanding the polar ocean circulation and its links to global climate.