OCEAN
CONDITIONS AND PROCESSES BENEATH
THE RONNE ICE SHELF
Daniel Feltham and Paul Holland
Ice shelves exist where ice flows off mainland Antarctica into the sea and are floating expanses of ice that may have horizontal dimensions of several hundred kilometres and depths of over a kilometre. Processes beneath ice shelves, particularly the Filchner-Ronne ice shelf (FRIS) in the Weddell Sea, make a significant contribution to the conversion of water masses over the Antarctic continental shelf, producing deep and bottom waters that drive global oceanic circulation.
Observations and models suggest that as sea ice forms off the edge of FRIS, dense brine sinks and runs back to the grounding line of the ice shelf due to the topography of the sea bed (figure 1). As a result of the decrease in the freezing temperature with depth (pressure), this high-salinity and relatively warm water starts to melt the base of the ice shelf, releasing fresh water. This fresh water (known as Ice Shelf Water, ISW) starts to ascend along the underside of the ice shelf due to buoyancy but, as it does so, becomes supercooled. Supercooling of this turbulent plume results in the formation of tiny crystals of ice known as frazil. Frazil ice affects the net buoyancy of the ISW and, where the ISW slows, frazil may deposit onto the underside of the ice shelf to form marine ice.
Figure 1: Schematic diagram of the flow and thermodynamics beneath an ice shelf.
Due to their inaccessibility, the ocean cavities beneath ice shelves are
some of the least explored areas of the planet. This project will use the
most complete set of data ever obtained beneath an ice shelf. This unique
data set will be collected by Autosub,
an automated submarine that will conduct cruises beneath the Ronne ice shelf,
taking a range of measurements including temperature, salinity, and turbulence
(figures 2 and 3).
Figure 2: Autosub is released on an autonomous mission under sea ice.
Figure 3: Proposed Autosub mission routes
The project will develop an existing model of the interaction of an ice shelf with the underlying ocean and use the data sets from Autosub for calibration and validation purposes. It will also utilise data from an array of moorings maintained by collaborators at the British Antarctic Survey and draw on their expertise and modelling experience. One aim of the project is to support their fieldwork by roughly predicting the distribution of frazil ice clouds underneath FRIS in order to help safely guide the Autosub missions to areas of interest.
By developing the physics of the plume model, we also hope to further the scientific understanding of ISW dynamics and the deposition of frazil onto FRIS. This will assist researchers in parameterising fluxes at the ice shelf-ocean boundary for use in regional models, such as the sub-ice shelf versions of MICOM and the GFDL model.
A hierarchy of specific
ISW plume models of increasing complexity has been developed over the last
decade or so, and it is our intention to develop this series further. The
most advanced of current models are one-dimensional in the along-shelf direction
(perpendicular to the ice front) and couple dynamical and scalar transport
predictions to a frazil ice model which allows crystals to grow and interact.
In this project, we hope to make the following developments:
- Examination of the behaviour of a multiple-size-class frazil model in simplified test cases representing a sub-ice shelf cavity.
- Incorporation of depth-dependence into the full ISW plume model.
- Incorporation of Coriolis forces into the full ISW plume model.
The latter two objectives will utilise the computational fluid dynamics package PHOENICS in the prediction of plume dynamics.
For
more information on this project please contact Daniel
Feltham.