A tale of two eddies: The biophysical characteristics of two contrasting cyclonic eddies in the East Australian Current System

Abstract

Mesoscale cyclonic eddies are known to be highly productive. Less well‐known are the dynamics and productivity of smaller cyclonic eddies, known as frontal eddies, that form on the landward side of western boundary currents. In this study, we investigate the physical and biogeochemical properties of two contrasting cyclonic eddies in the East Australian Current (EAC). The first (“Murphy”), a mesoscale cyclonic eddy that formed at ∼28°S with a diameter of ∼160 km and high surface chlorophyll‐a concentrations, which lived ∼47 days. The second (“Freddy”), a smaller frontal eddy (∼35 km diameter) that formed from a shelf water billow ∼7 days prior to sampling at ∼31.5°S and was advected off the shelf along the EAC front (from ∼200 m to 4000 m of water). Both eddies were at least 1000 m deep with a similar steric height anomaly. We introduce and employ “the method of closest approach” using shipboard ADCP velocities to estimate the eddy centers, which reveals significant tilting through the water column. We estimate rotation rates of 4–10 days and 1–9 days and Rossby numbers 0.25–0.1 and 0.6–0.1, from the surface to 600 m for Murphy and Freddy, respectively. High‐resolution altimetry measurements from the SARAL/AltiKA satellite provide estimates of the ageostrophic component of rotation. Our results show that the frontal eddy is significantly more ageostrophic, energetic, and productive than the mesoscale cyclone, despite its small size and short life (∼4 weeks). We suggest that frontal eddies have potential to contribute significantly to the net productivity of the Tasman Sea region.

Retention and Leakage of Water by Mesoscale Eddies in the East Australian Current System

“…The vorticity, radius, and distribution of mesoscale eddies diagnosed with weekly merged altimetry fields (Everett et al, ; Pilo, Mata, & Azevedo, ) are similar to those identified in our study. Individual mesoscale eddies in the region studied with models (Macdonald et al, , Oke & Griffin, ) and more recently observed (Roughan et al, ) also lie within the realm of eddies presented here.…”

“…This study is based on a numerical simulation with a spatial resolution of (1/10)°. Submesoscale and small mesoscale features that are not resolved at this spatial resolution may modify the retention time; nevertheless, the median retention times we find are the same order of magnitude than those obtained from observations of two‐dimensional current fields, specifically from drifter releases inside large and small eddies in the region (~163 and ~35 km in diameter, respectively, Roughan et al, ).…”

“…Along southeast Australia, the circulation is dominated by the poleward flowing East Australian Current (EAC), the EAC extension, and mesoscale eddies (e.g., Ridgway & Dunn, ; Figure ). Patterns of connectivity (Roughan et al, ), productivity (Roughan et al, ), and larval supply to the continental platform (Cetina‐Heredia et al, ) in this region have been shown to be influenced by mesoscale eddies. Cetina‐Heredia et al () showed that the contribution to poleward transport by eddies has increased in the last decade; moreover, eddies are projected to become more stable under a climate change scenario (A1B, Matear et al, ; Oliver et al, ).…”

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

“…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, ).…”

Patterns of Vertical Velocity Induced by Eddy Distortion in an Ocean Model

“…In the East Australian Current (EAC), the WBC of the South Pacific, there has been several studies on the properties and dynamics of eddies (e.g., Roughan et al, ; Rykova et al, ; Rykova & Oke, ), the interaction of eddies with other eddies (Cresswell & Legeckis, ), the mean flow (e.g., Mata et al, ; Nilsson & Cresswell, ), and the ocean bottom (e.g., Oke & Griffin, ), and the impact of eddies on productivity (e.g., Everett et al, ) and marine biota (e.g., Suthers et al, and references therein). However, only one study describes the vertical circulation within an eddy in the EAC region.…”