A Study on an Anticyclonic-Cyclonic Eddy Pair Off Fraser Island, Australia

Zhibing Li,Zhiqiang Liu,Fernando Pinheiro Andutta,Jianyu Hu,Xiaohua Wang

doi:10.3389/fmars.2020.594358

Abstract

This research examines a cyclonic-anticyclonic eddy (AE) pair off Fraser Island next to the eastern Australian coast in 2009 using the Bluelink Reanalysis data, where the local eddies are poorly understood. This eddy pair formed in July and dissipated in November. We detailed the horizontal and vertical structures of the eddy pair in terms of three-dimensional variations in relative vorticity, hydrographic properties, velocity, and dynamic structures, which presented notable scales of the eddy pair. The AE formed beside the meandering of the East Australian Current (EAC) at 24°S and had a tilting structure in the upper 1,000 m toward the EAC. A cyclonic eddy (CE) formed a month later and interacted with the AE, which had a tilting structure toward the AE in the upper 1,000 m. Heterogeneity in the AE and CE composing this eddy pair was observed in the horizontal and vertical planes. The AE had a stronger and more coherent dynamic structure than the CE. The AE and the EAC interacted in the generation stage when the EAC path shifted eastward, away from the coast. As the EAC subsequently swung back to the coastal area, the AE and the EAC separated. The AE then interacted with the surrounding eddy fields, propagated westward, before finally merging again with the EAC. The energy transfer during this process also indicated the interactions among the eddy pair, the surrounding eddy fields and the EAC. Baroclinic instability (BCI) was a main contributor to the AE in the generation stage. Barotropic instability (BTI) also contributed energy to the AE when it interacted with the EAC but accounted for a much smaller proportion. Both BCI and BTI contributed to the CE for most of its life cycle but to a much less extend than to the AE. The zonal heat and salt mass transported by the AE and CE were calculated based on a Lagrangian framework method, and these amounts were considerable compared with global zonal averaged heat and salt mass transported by other mesoscale eddies.

Highlights

Eddies alongside major western boundary currents have been widely studied for their structures, dynamics, and influences on large-scale ocean circulations, as well as biogeochemical processes (Glenn and Ebbesmeyer, 1994; Kawabe, 1995; Ridgway and Godfrey, 1997; Qiu and Chen, 2004; Meijers et al, 2007; Eddy Pair Off Fraser IslandSuthers et al, 2011)
In addition to the previously discussed motion in the zonal direction, this study found that the anticyclonic eddy (AE) propagated poleward, whereas the cyclonic eddy (CE) moved equatorward
This study has helped identifying mesoscale eddies offshore from Fraser Island, which have rarely been explored in previous studies

Summary

Introduction

Eddies alongside major western boundary currents (such as the Kuroshio, the Gulf Stream, and the East Australian Current) have been widely studied for their structures, dynamics, and influences on large-scale ocean circulations, as well as biogeochemical processes (Glenn and Ebbesmeyer, 1994; Kawabe, 1995; Ridgway and Godfrey, 1997; Qiu and Chen, 2004; Meijers et al, 2007; Eddy Pair Off Fraser IslandSuthers et al, 2011). The East Australian Current (EAC) is the major western boundary current of the South Pacific subtropical gyre, originating in the southern Coral Sea (Ridgway and Dunn, 2003; Cetina-Heredia et al, 2014). As a bifurcation of the South Equatorial Current that flows southward along the eastern Australian coast (Boland and Church, 1981; Ridgway and Godfrey, 1994), the EAC is associated with strong eddy activities and plays a predominant role in the local hydrodynamic circulation (Shevenell et al, 2004; Ridgway, 2007). The EAC is described as a continuous current while it is attached to the coast. It varies with time and feeds the offshore eddy fields when it separates from the coast (Oke et al, 2019). The mechanisms behind the variation of the EAC, especially its separation from the coast, are intricate and not conclusive (Oke et al, 2019)

Objectives

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Conclusion