Deep boundary current disintegration in Drake Passage

Abstract

The fate of a deep boundary current that originates in the Southeast Pacific and flows southward along the continental slope of South America is elucidated. The current transports poorly ventilated water of low salinity (a type of Pacific Deep Water, PDW), into Drake Passage. East of Drake Passage, the boundary current breaks into fresh anticyclonic eddies, nine examples of which were observed in mooring data from December 2009 to March 2012. The observed eddies appear to originate mainly from a topographic separation point close to 60°W, have typical diameters of 20-60 km and accompanying Rossby numbers of 0.1-0.3. These features are likely to be responsible for transporting PDW meridionally across the Antarctic Circumpolar Current, explaining the near homogenization of Circumpolar Deep Water properties downstream of Drake Passage. This mechanism of boundary current breakdown may constitute an important process in the Southern Ocean overturning circulation.

Circulation and Stirring in the Southeast Pacific Ocean and the Scotia Sea Sectors of the Antarctic Circumpolar Current

“…The complexity of topography in this region can create flow structure in the deeper layers significantly different from that in the flow above, leading to strong vertical motion and currents that (locally) cross the core of the ACC. One example of this is the generation of middepth vortices from the flow along the northern boundary of the Scotia Sea that move southward, across the major fronts (Brearley et al 2014). Another example is seen in the floats that continued east in the Scotia Sea, instead of crossing over the North Scotia Ridge into the Argentine basin (Fig.…”

Modification of turbulent dissipation rates by a deep Southern Ocean eddy

“…Acoustically tracked floats and mooring data in the Drake Passage region indicate that middepth anticyclonic eddies occur regularly and play a major role in the disintegration of the deep boundary current that flows along the southwestern continental slope of South America [ Brearley et al , ]. This study suggests that these persistent, ubiquitous features also significantly modify the local small‐scale turbulence field along their path through some flavor of wave‐mean flow interactions, contributing to the patchy nature of oceanic turbulent dissipation and diapycnal mixing.…”

“…Full details of the analysis used to compute these parameters, alongside CTD, LADCP, and VMP processing and accuracies, are given in Sheen et al [2013]. Brearley et al [2014] used these CTD and LADCP data to show that a middepth anticyclonic eddy was captured on the western part of the grid survey (stations 1-6, collected on 7-10 December 2010). The eddy was distinguished by ∼1 km thick intensification in LADCP current speeds centered at 2000 m ( Figure 2); pronounced isopycnal separation between neutral density surfaces n = 27.85 kg m −3 and n = 28.00 kg m −3 ( Figure 2); a local minimum in potential vorticity; pronounced cooling and freshening (0.1 • C and 0.02, respectively) along isopycnal surfaces; and an oxygen concentration minimum.…”

“…Similarly, a constant buoyancy frequency-depth profile, N(z), is employed, and a Coriolis parameter, f, of 1.2 × 10 −4 rad s −1 is assumed. In this simple model we choose to ignore the relative vorticity of the eddy (typically 0.3 × 10 −4 rad s −1 [Brearley et al, 2014]), which locally is significantly smaller than f. Two models are run, which differ only by the background U(z) and N(z) profiles, and are chosen to be representative of eddy and noneddy scenarios (Figures 5a and 5b). The profiles are computed from averaged LADCP and CTD data for eddy (stations 3-6) and noneddy (stations 14-20) times.…”

“…The best known examples are the lenses of warm, salty Mediterranean Water (MW) known as Meddies, whose long lifetimes enable the transport of MW as far west as the Bahamas, modifying the water mass properties of the subtropical North Atlantic [ McDowell and Rossby , ; Richardson et al , ]. More recently, a number of anticyclonic middepth eddies were identified in the Scotia Sea sector of the Southern Ocean [ Brearley et al , ]. The stability of these eddies, which are advected into the highly energetic Antarctic Circumpolar Current (ACC), may explain why the distinct T / S and oxygen fingerprint of the Pacific Deep Water is traceable for thousands of kilometers downstream of Drake Passage [ Well et al , ].…”