The Southeast Indian Ridge: A Hotspot for Cross-Frontal Exchange

Cross-frontal exchange facilitated by mesoscale eddies in the lee of major topographic features of the Southern Ocean is fundamental to the global overturning circulation. Despite the outsize importance for meridional heat flux, we lack an accurate estimation of fluxes across the Antarctic Circumpolar Current (ACC) due to the challenges of observing mesoscale eddy fluctuations on the temporal and spatial scales required. Here, 12 years of Argo data are used to observe patterns of cross-frontal exchange in the Southeast Indian Ridge system, a relatively underobserved region, known to be a hotspot of exchange. Spice variance along ACC streamlines is used as a proxy for cross-frontal exchange. Elevated exchange is observed downstream of the ridge system in nearly every streamline and is particularly prominent in the core of the ACC. Notably, exchange peaks progressively downstream at each poleward streamline suggesting a systematic north-to-south handoff across nearly the full breadth of the ACC. Employing a mixing length framework, lateral stirring is parameterized as an eddy diffusivity on the isopycnal of peak exchange. 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. Spatially, this diffusivity pattern correlates with an along-stream increase in eddy kinetic energy. Along-stream vertical wavenumber spectra of spice anomaly profiles indicate that the vertical scales of intrusions, which are initially large (approximately 800 m), rapidly evolve downstream to smaller wavenumbers (100–300 m) presumably in response to intense vertical shear and filamentation.

Duke Scholars

Author Kathleen Donohue Biology