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

Abstract

Given the importance of western boundary currents over a wide range of scales in the ocean, it is crucial that we understand their dynamics to accurately predict future changes. For this, we need detailed knowledge of their structure and variability. Here we investigate the jet structure of the East Australian Current (EAC), using observations from HF radars and moorings deployed at 30°S–31°S. Meandering, core velocity, width, and eddy kinetic energy (EKE) are quantified from 4 years of hourly 1.5 km resolution surface current maps (2012–2016), to obtain the most detailed representation of the surface EAC jet to date. The EAC flows predominantly over the ∼1,500 m isobath 50 km offshore but makes large amplitude displacements eastward every 65–100 days—the time scale associated with mesoscale eddy shedding at the EAC separation. Smaller‐amplitude, higher‐frequency meanders occur every 20–45 days. Using a coordinate frame that follows the jet, we show core velocity and EKE exhibit seasonality in both magnitude and variance, being maximum in summer (1.55 m s−1 mean core velocity), minimum in winter (0.8 m s−1). However, it is the eddy‐shedding time scale that dominates jet variability. As the EAC moves shoreward, shelf temperature and along‐stream velocity vary linearly with jet movement, within ∼35 km of the core. The EAC is within this range 75% of the time, demonstrating its importance to the shelf circulation. Temperature and velocity fluctuations at the 70 m (100 m) isobath are more influenced by wind (EAC encroachment), with the strongest response occurring when wind and EAC act constructively.

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

“…From a global survey of mesoscale wavenumber spectra, Xu and Fu (2011) showed that the steepest slopes are over WBC regions. Increased summer EKE has been observed before in WBCs (e.g., Archer, Roughan, et al, 2017;Qiu & Chen, 2004;Ridgway & Godfrey, 1997), in phase with summer intensification of WBC jet speeds (Archer, Roughan, et al, 2017). Results from a global analysis of drifter pair separation by Corrado et al (2017) also found steep slopes down into the marginal submesoscale range (5 km).…”

“…Remote wind forcing, communicated via first mode baroclinic Rossby waves, has been shown to affect the annual cycle in volume transport (Domingues et al, 2016), but for the data presented here at 25-26°N, the Bahamas island chain blocks most of this direct mid-ocean influence (Archer, Shay, et al, 2017). At 30-31°S, the EAC has a mean core speed between 0.6 and 1.35 m s À1 (Archer, Roughan, et al, 2017;Mata et al, 2000;Schaeffer et al, 2017) and a volume transport of~22 Sv with an STD of~5-7 Sv (Mata et al, 2000;Sloyan et al, 2016). The EAC closes the South Pacific subtropical gyre, flowing poleward along SE Australia (Figure 1b).…”

“…At 30°S the EAC flows over the continental slope about 30 km offshore. The amplitude of EAC meandering is larger than the FC, with a lateral displacement from the mean over 80 km and STD of 20 km (Archer, Roughan, et al, 2017). Instead, the separation point ranges from 28 to 37°S, but 30-32°S about 50% of the time (Cetina-Heredia et al, 2014).…”

“…Each radial velocity data set is combined using the unweighted least squares method of Gurgel (1994) to a Cartesian grid with 1.5-km resolution at 10-min intervals smoothed with a 17-point (3 hr) Hann window. Full details are presented in Archer, Roughan, et al (2017). Full details are presented in Archer, Roughan, et al (2017).…”

“…2. The jet speed and EKE undergo a seasonal cycle with an austral summer maximum (Archer, Roughan, et al, 2017;Ridgway & Godfrey, 1997). At 30°S the EAC flows over the continental slope about 30 km offshore.…”