Mesoscale Eddy Transport Across the Polar Front: a Model-Observation Comparison

As part of the Meridional Overturning Circulation, deep waters cross the strong fronts of the Antarctic Circumpolar Current (ACC) to upwell near the Antarctic coast, where they take up large amounts of excess heat and anthropogenic carbon from the atmosphere. Until recently, studies quantifying cross-frontal transport in the Southern Ocean have been mostly carried out with numerical models as observational data were too sparse in this region. Modelling studies have shown that mesoscale eddies are the main agents for the southward mass transport across the fronts of the ACC above major topographic sills. Recently, the number of measurements collected by the Argo array of profiling floats has become sufficient to allow reconstruction of eddy transport across the ACC. This study examines the transport of mass across the Polar Front, calculated between the surface and around 2 km depth from control simulations of a suite of climate models differing solely in the resolution of the ocean component (1${}^{\circ }$, 0.25${}^{\circ }$, 0.1${}^{\circ }$). These transports are compared to transport estimates from Argo float measurements. All models underestimate the eddy transport across the Polar Front, relative to the observation-based results. The mesoscale eddy transport parameterization used in the 1${}^{\circ }$ model is found to compensate for the weak resolved eddy transport so that the magnitude of the resulting eddy transport is close to that of its higher resolution counterparts. In both the 0.25${}^{\circ }$ and 0.1${}^{\circ }$ models, eddy transport is confined to regions associated with sharp topography in agreement with observations. In contrast, the 1${}^{\circ }$ model does not simulate enhanced eddy transport at topography. The 0.1${}^{\circ }$ model is found to be the most realistic model within the suite in terms of magnitude and spatial variability.