Parameterising Eddy-Mediated Heat Transports Across the Weddell Sea Continental Slope

Abstract

The Weddell Sea is an Antarctic marginal sea featuring extensive continental shelf areas. Here, dense water is produced through interactions with ice shelves and sea ice and propagates down the continental slope where it forms the densest contribution to the Antarctic Bottom Water. The downflow of dense water creates an isopycnal connection between the continental shelf and slope allowing for the intrusion of warm Circumpolar Deep Water (CDW) onto the continental shelf. When reaching the ice shelf cavities, CDW has the potential to strongly increase the melt rate of the ice shelves with global implications such as sea level rise. Mesoscale eddies sourced from local baroclinic instability play a central role in the shoreward transport of CDW since they supply the momentum to overcome the vorticity gradient imposed by the continental slope. Capturing these eddies in ocean models is particularly challenging because the small Rossby radius of deformation at high latitudes requires a much higher horizontal resolution than currently available in state-of-the-art climate models. This invites the question as to how the shoreward heat flux can be parameterised at coarse resolution and motivates a process oriented modelling study. For this purpose we use the MIT general circulation model (MITgcm) in a configuration featuring idealised sloping topography and surface forcing and typical hydrographic fields representing the Weddell Sea continental shelf and slope. In this setup, a strong heat transport from the open ocean onto the continental shelf only emerges at a resolution of O(1km). At coarser resolution, shoreward heat transport is almost absent resulting in a cold bias on the continental shelf and the exported deep water. We then apply the classical schemes of Gent-McWilliams and Redi (GM/Redi) which parameterise the effect of eddies by introducing an advective tracer flux as a function of the isopycnal slopes and by aligning the diffusion operator with the local isopycnals. We show that using the GM/Redi parameterization shoreward heat transports can be represented so that the difference between the high and coarse resolution hydrographic fields strongly reduces. Advective heat transport dominates over the centre of the continental slope and is captured by the GM part of the parameterisation. The diffusive heat flux dominating over the continental shelf break on the other hand is reproduced by the Redi scheme. In light of potential future changes to the Weddell Sea system we further discuss different approaches to obtain the transfer coefficients needed for the GM/Redi parameterisation based on the resolved flow and sub-grid eddy kinetic energy.