Lagrangian and Eulerian characterization of two counter‐rotating submesoscale eddies in a western boundary current

Abstract

In recent decades, high‐spatial resolution ocean radar and satellite imagery measurements have revealed a complex tangle of submesoscale filaments and eddies, in the surface velocity, temperature, and chlorophyll a fields. We use a suite of high‐resolution data to characterize two counter‐rotating, short‐lived eddies formed at the front between the warm East Australian Current (EAC) and temperate coastal waters (30°S, Eastern Australia). In this region, submesoscale filaments and short‐lived eddies are dynamically generated and decay at time scales of hours to days. Dominant cyclonic filaments of O(1) Rossby number formed along frontal jets and eddy boundaries, generating localized ageostrophic circulations at the submesoscale. Measurements of over‐ocean wind direction and surface currents from high‐frequency radars reveal the influence of the short‐term, small‐scale wind forcing on the surface circulation, enhancement of the horizontal shear, frontal jet destabilization, and the generation and decay of the cyclonic eddy. By contrast, the anticyclonic eddy formation was most likely associated with EAC mesoscale instability and anticyclonic vorticity. Lagrangian tracks show that surface particles can be temporarily trapped in the eddies and frontal convergent zones, limiting their transport. Mixing between EAC‐derived and coastal waters was increased along the frontal regions, and particles starting at the divergent regions around the eddies experienced significant dispersion at submesoscales. The cyclonic cold‐core eddy entrained high chlorophyll a shelf waters on its convergent side, suggesting spiral eddy cyclogenesis.

On the Variability of the East Australian Current: Jet Structure, Meandering, and Influence on Shelf Circulation

“…At 30.27°S, the mean cross‐jet profile of the two coordinate frames are compared, which shows the effect of meandering on the geographical time mean; it produces much larger variance, with a mean jet core reduced by 40% (–0.8 m s −1 versus −1.35 m s −1 ), and significantly weaker shear (Figures c and d). In both coordinate frames, cross‐jet shear is twice as strong on the cyclonic shoreward flank of the jet compared to offshore, as has been shown in other western boundary currents (Archer et al, ; Johns et al, ; Rossby & Gottlieb, ) and was seen in the submesoscale instabilities investigated by Schaeffer et al () and Mantovanelli et al ().…”

“…In the submesoscale range, cyclonic frontal eddies have been observed in high frequency (HF) ocean radar maps, propagating poleward inshore of the EAC, irregularly timed but on average one per week, with inshore radii of ∼10 km (Schaeffer et al, ). These small‐scale eddies are associated with Rossby numbers of order 1 and intense, asymmetrical vorticity and divergence fields (Mantovanelli et al, ), and high productivity (Roughan et al, ).…”

“…To answer these questions, we analyze a 4 year data set of high‐resolution ocean surface currents obtained by HF radar. In 2012, the Australian Integrated Marine Observing System (IMOS) deployed two radars at 30°S–31°S to provide sustained long‐term monitoring of the EAC in unprecedented spatiotemporal resolution (Mantovanelli et al, ; Schaeffer et al, ). This data set offers new insights into the EAC spatial structure at the surface and its variability.…”

Assessment of Surface Currents Measured With High-Frequency Phased-Array Radars in Two Regions of Complex Circulation

“…The existence and importance of topographically-induced secondary circulations, small scale eddies and upwelling around reef islands in the GBR has been extensively documented (e.g. White and Wolanski [42]) while submesoscale features (3-10 km) are relevant in the Coffs Harbour region (Mantovanelli et al [43], Schaeffer et al [44]). …”

The Kinematic Similarity of Two Western Boundary Currents Revealed by Sustained High‐Resolution Observations

“…Whereas a mooring array typically resolves O(10) grid points in a cross-jet direction, HF radar resolves O(1)-km scale grid points over a range of 100 km in both cross-jet and along-jet directions. This high spatial resolution allows us to accurately track the jets as they meander across the continental slope and resolve frontal eddies and other instabilities in 2-D (e.g., Archer, Shay, Jaimes, et al, 2015;Mantovanelli et al, 2017;Shay et al, 1998;Schaeffer et al, 2017). The two study regions are as follows:…”