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

Abstract

Mesoscale eddies are ubiquitous in the ocean, transporting semi‐isolated water masses as well as advecting tracers and biota. The extent to which eddies impact the environment depends on the time they retain water parcels. Here we quantify retention times of mesoscale eddies in a (1/10)° model of the East Australian Current and its extension along the southeast coast of Australia. We find that retention times vary widely, between 3 and 357 days, but peak around 24 and 27 days for anticyclones and cyclones, respectively. Changes in eddy shape, though not in eddy size, relate to water exchange between the eddy and the background flow. An increase in eccentricity (eddy elongation) often leads to water leakage, while a decrease is associated with water retention. Thus, the change in eddy eccentricity can be used as a diagnostic of the eddy's likelihood to exchange water with its surrounding. We find that water within a region of the eddy that is close to uniform rotation and rotating faster than uniform vorticity is more likely to be retained. Typical retention times are long enough for eddies to transport water across regions of contrasting hydrographic properties, develop a biogeochemical response, and influence connectivity patterns.

Quantifying Eulerian Eddy Leakiness in An Idealized Model

“…Comparing with SSH eddies, the mean RCLV radius (35.5 km) is approximately about half of the mean SSH eddy radius (72.4 km). Similar to this result, Cetina-Heredia et al (2019) showed that the solid body rotation radius is 0.6 and 0.5 the radius of anticyclonic and cyclonic eddies detected from velocity fields, respectively. In contrast with Abernathey and Haller (2018), no significant latitude dependence or relationship with deformation radius is visible in either RCLVs or SSH eddies.…”

Quantifying Eulerian Eddy Leakiness in an Idealized Model

“…Comparing with SSH eddies, the mean RCLV radius (35.5 km) is approximately about half of the mean SSH eddy radius (72.4 km). Similar to this result, Cetina‐Heredia et al () showed that the solid body rotation radius is 0.6 and 0.5 the radius of anticyclonic and cyclonic eddies detected from velocity fields, respectively. In contrast with Abernathey and Haller (), no significant latitude dependence or relationship with deformation radius is visible in either RCLVs or SSH eddies.…”

“…However, Abernathey and Haller () concluded that RCLVs uniformly live shorter than SSH eddies and they only identified 1 RCLV with lifetime of 270 days in the eastern Pacific over more 20 years, against the number of 2,076 in the SSH eddy data set (Chelton et al ()). Also, Cetina‐Heredia et al () found eddies retain water for several months most commonly in the east Australian current system, some also retain water over a year but these are less frequent (their Figure ). In this study, therefore, we excluded the long‐lived eddies and focus on coherent eddies with lifetime shorter than 90 days.…”

“…However, several works (e.g., Wang et al (), Abernathey and Haller ()) using Lagrangian frameworks indicate these Eulerian methods strongly overestimate the degree of eddy material transport. Additionally, Cetina‐Heredia et al () adopted Eulerian and Lagrangian approaches to highlight the potential of eddies to impact the environment depends upon whether they maintain water inside2 or leak it. In this study, we explored the relationship between eddy detection methods and coherent material transport and quantitatively calculated the leakiness of Eulerian eddies.…”

“…Similarly, the work of Beron‐Vera et al (), Froyland et al ()) and Wang et al (, ) examined the ability of Agulhas rings to coherently carry Agulhas leakage water using Lagrangian methods, finding considerably smaller transport estimates than previous Eulerian‐based methods. Most recently, Cetina‐Heredia et al () quantified the time that mesoscale eddies along the southeast coast of Australia retain water and pointed out the change in eddy shape relates to water exchange between the eddy interior and the background flow. Though Eulerian eddies can temporarily trap and transport water parcels and tracers (Gaube et al (); Condie and Condie ()), under the effects of stretching and filamentation, such eddies may lose coherence and experience leakage and deformation in the turbulent flow(Haller (); Early et al ().…”