LOCAL AND CIRCUMGLOBAL EFFECTS OF EDDY-INITIATED CROSS-FRONTAL EXCHANGE IN THE ANTARCTIC CIRCUMPOLAR CURRENT

Mesoscale eddy processes initiate a large fraction of the exchange of water properties and heat across the Antarctic Circumpolar Current. Previous studies show cross-frontal exchanges are punctuated at different geographic longitudes for individual ACC fronts. For a particular front, hotspots of exchange occur downstream of crossing topographic ridges, where the front's path exhibits a standing meander. In these meanders, energetic meander variability is driven by baroclinic instability. Foppert et al. (2017, doi:10.1002/2017JC012837) used satellite sea surface height standard deviation as an index of divergent eddy heat flux, estimating the magnitude of alongstream warming O(0.1C).

In this study, ARGO float profiles are binned by longitude and by geopotential anomaly to analyze the Polar Front in stream-coordinates. Spice is calculated from the [TS] profiles as a neutral tracer. Depths greater than 150m are considered to minimize direct atmospheric diapycnal influences. Heightened cross-front exchange produces: increased [TS] variance; increased spice variance; filamentation of the exchanged parcels as they undergo sheared advection; and altered watermass properties where filaments bearing [TS] biases are stirred and mixed alongstream. Filamentation is revealed via vertical wavenumber spectra of the spice anomalies, which evolve from whole pycnocline vertical scales of ~800m near the stationary meanders to much thinner scales within 300-500km downstream. Case studies reveal that, along isopycnals, water-mass mean temperatures can warm by O(0.4C) within downstream distances of ~300km.

Along the PF path the mean temperature on isopycnal surfaces is warmed in four main steps: in the lee of crossing topography at Kerguelan Plateau, the Southeast Indian Rise, Udintsev Fracture Zone, and a broad crossing from Drake Passage through the Argentine Basin. These balance an otherwise gradual along-path trend of cooling.