Eddy generation and propagation in the Southern Ocean diagnosed from Satellite Altimetry and an Ocean State Estimate
Event: 2017 Ocean Surface Topography Science Team Meeting
Session: Science III: Mesoscale and sub-mesoscale oceanography
Presentation type: Type Oral
The Southern Ocean features high eddy kinetic energy, identifiable in satellite altimetry along the entire length of the Antarctic Circumpolar Current (ACC). However, analysis of altimeter data and an ocean state estimate show that the generation of relatively large amplitude eddies is not a ubiquitous feature of the Southern Ocean but rather a phenomenon that is constrained to five isolated, well-defined "hotspots". These hotspots, identified using altimeter data, are located downstream of major topographic features, with their boundaries closely following f/H contours. Eddies generated in these locations tend to disperse along f/H contours rather than following lines of constant latitude (or f). Furthermore, eddies generated in these locations show no evidence of a bias in polarity and decay within the boundaries of the generation area rather than propagating long distances. An ocean state estimate shows enhanced values of both buoyancy and shear production inside the hotspots, with buoyancy production one order of magnitude larger than shear production. This is consistent with baroclinic instability being the main mechanism of eddy generation. The mean potential density field estimated from Argo floats shows that inside the hotspots, isopycnal slopes are steep, indicating availability of potential energy and providing further evidence of the main generation mechanism. The hotspots identified in this study overlap with previously identified regions of standing meanders. We provide evidence that hotspot locations can be explained by the combined effect of topographic features, standing meanders that enhance baroclinic instability, and availability of potential energy to generate eddies via baroclinic instabilities. The tracking of eddy trajectories using the altimeter shows that the Southern Ocean has two distinct eddy motion regimes. North and south of the ACC, eddies propagate westward with a mean meridional drift directed poleward for cyclonic eddies (CEs) and equatorward for anticyclonic eddies (AEs). Eddies spawned within the boundaries of the ACC have an effective eastward propagation with respect to the mean deep ACC flow, and the mean meridional drift is reversed, with warm-core AEs propagating poleward and cold-core CEs propagating equatorward. This reversed meridional circulation pattern of semi-coherent eddies in the ACC, drives down-gradient eddy heat transport, which could potentially transport a significant fraction of the net poleward ACC eddy heat flux (30 to 70 x 10^13 W, as inferred in previous studies).