Wednesday February 12, DEOS will be hosting Ali Johnson Exley, Postdoctoral Inverstigator at WHOI to discuss “Patterns and drivers of cross-frontal exchange diagnosed in the Southeast Indian Ridge sector of the Southern Ocean“. The seminar will take place in SMAST East 101-103 from 12:30 to 1:30 or can be joined via Zoom!

 

Abstract:

The Southern Ocean plays a fundamental role in the meridional overturning circulation by ventilating deep, carbon-rich waters to the surface ocean, and through the conversion to bottom and intermediate waters, largely compensates sinking in the subpolar North Atlantic. Central to the Southern Ocean overturning circulation is the role of mesoscale eddies which are the primary mechanism by which heat, carbon, nutrients and other properties are transported poleward across fronts of the Antarctic Circumpolar Current (ACC), effectively balancing the wind-induced equatorward transport. Satellite altimetry has revealed distinct spatial patterns in eddy kinetic energy around the ACC, suggesting mesoscale eddy fields are largely confined to relatively small regions downstream from topographic ridge systems. Despite the outsize importance for meridional heat transport however, we lack an accurate estimate of fluxes across the ACC due to the challenges of observing mesoscale eddy fluctuations on the temporal and spatial scales required. Additionally, the physical mechanisms responsible for initiating and maintaining these mixing regimes remains poorly constrained. Here, observations from Argo are used together with high-resolution numerical model output to investigate horizontal patterns of eddy diffusivity and to diagnose eddy-mean field interactions in the Southeast Indian Ridge system, a relatively under-observed region known to be a hot spot of exchange in the Southern Ocean. We find a highly localized pattern of diffusivity, peaking between the crest and trough of the first standing meander in the lee of the ridge system, which correlates with an along-stream increase in eddy kinetic energy. Additionally, by adopting a wave-activity flux framework traditionally employed for atmospheric storm track studies, we decompose the role of barotropic and baroclinic instabilities as well as the ageostrophic fluxes which sustain the growth, flux and decay of energy in the along-stream direction. This work is an early step towards quantifying the contribution of these processes on the large-scale overturning circulation which ultimately will be essential for a comprehensive understanding of the system and how it might respond to future change.