Observations of Submesoscale Variability and Frontal Subduction within the Mesoscale Eddy Field of the Tasman Sea

Abstract

Submesoscale lenses of water with anomalous hydrographic properties have previously been observed in the East Australian Current (EAC) system, embedded within the thermocline of mesoscale anticyclonic eddies. The waters within these lenses have high oxygen content and temperature–salinity properties that signify a surface origin. However, it is not known how these lenses form. This study presents field observations that provide insight into a possible generation mechanism via subduction at upper-ocean fronts. High-resolution hydrographic and velocity measurements of submesoscale activity were taken across a front between a mesoscale eddy dipole downstream of the EAC separation point. The front had O(1) Rossby number, strong vertical shear, and flow conducive to symmetric instability. Frontogenesis was measured in conjunction with subduction of an anticyclonic water parcel, indicative of intrathermocline eddy formation. Twenty-five years of satellite imagery reveals the existence of strong mesoscale strain coupled with strong temperature fronts in this region and indicates the conditions that led to frontal subduction observed here are a persistent feature. These processes impact the vertical export of tracers from the surface and dissipation of mesoscale kinetic energy, implicating their importance for understanding regional ocean circulation and biological productivity.

Transport variability over the Hawkesbury Shelf (31.5–34.5°S) driven by the East Australian Current

“…Long term SST data (25 years), shows the presence of a quasi-steady mesoscale eddy dipole centered at approximately 32.5 o S [ 52 ], immediately downstream of the separation. In addition, the eddy dipole was shown to be a driver of cross-shelf transport, which is a maximum at 33-34 o S and occurs more than 50% of the time [ 53 ].…”

“…Further north, off Coffs Harbour (30° S), Kerry et al [ 61 ] showed that a 750m resolution model provided improved representation of sub-mesoscale features inshore of the EAC (compared to the coarser EAC model). In this region, Archer et al [ 52 ] identified sub-mesoscale processes including frontogenesis and instabilities associated with the separation of the EAC jet and high strain between two counter rotating eddies. The HSM model could now be used to explore the dynamics of such features more fully and their role in vertical and horizontal exchange.…”

Vertical fluxes in the upper ocean

“…The vertical displacement associated with these features is on the order of 100 meters. Unlike some previous observations that report subduction of low potential vorticity (PV) water, these subducted features are associated with strong cyclonic vorticity and are moderately stratified (Beaird et al, 2016;Archer et al, 2020). We use a process study model to examine the role that unforced frontal dynamics might have played in these observed subduction events, elaborate on the dynamical mechanisms of subduction, and describe the role of along-front variability in subduction from the surface mixed layer to the interior.…”

“…When ocean circulation acts on these gradients, they become tracers of that motion. Observations of low PV water in the interior provide evidence of transport of surface waters (Spall, 1995;Pallàs-Sanz et al, 2010;Archer et al, 2020). Geochemical, biogeochemical, and biological tracers often have strong vertical gradients, as described above, so they can serve as natural tracers of vertical motion.…”

“…However, submesoscale dynamics also has an influence below the surface mixed layer. Symmetric instability mixes momentum and tracers along isopycnal surfaces, often reaching below the mixed layer (Thomas et al, 2013) and leading to exchange of tracers between the surface and pycnocline (Smith et al, 2016;Erickson and Thompson, 2018;Archer et al, 2020). Submesoscale mixed layer eddies can also barotropize and induce along-isopycnal stirring in the pycnocline (Badin et al, 2011).…”