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

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.

Global Mapping of Mesoscale Eddy Vertical Tilt

“…In high latitudes (e.g., 45°N–60°N), the sloping bottom may contribute to the large eddy TD (Figure 3) via topographic β effect (Zhang et al., 2016). Besides, the nonlinear eddy‐eddy or eddy‐mean flow interaction (e.g., Li, Wang, et al., 2020) also take an effect. In reality, eddy tilt is very complex, and all of the factors can work together.…”

“…The observed tilted eddies can also be attributed to baroclinic instability when the eddy is in its growth stage (Sun et al., 2022). Besides, the lateral eddy‐advection background temperature gradient, the mean flow advection, eddy‐eddy or eddy‐mean flow interaction also contribute to eddy vertical tilt (e.g., Bonnici & Billant, 2020; Gaube et al., 2015; Li, Wang, et al., 2020).…”

Strain Evolution and Instability of an Anticyclonic Eddy From a Laboratory Experiment

“…Using eq.10, we can estimate the relative pressure field (Figure 12a). The cyclonic eddies and the anticyclonic eddies show an asymmetric pattern (Gallet et al, 2014;Liu and Andutta, 2020;Yang et al, 2020). The distance between C 0 and A 0 decreases (Figure 12b) resulting in the pressure gradient between them increases (Figure 12c).…”

A Multi‐Layer Linear Rossby Wave Dispersion Relation for Vertical Tilt of Mesoscale Eddies

“…Several mechanisms have been proposed to explain eddy vertical tilt, including wind‐eddy interaction, internal barotropic instability, baroclinic instability, lateral eddy advection of background temperature gradient, eddy baroclinic effect, topographic β ‐effect, stratification, eddy‐current or eddy‐eddy interactions, as well as nonlinear advection (Gaube et al., 2015; Hausmann & Czaja, 2012; Hu et al., 2011; Hua, 1988; Li et al., 2020; Liao et al., 2019; Morel & McWilliams, 2001; Stern, 1965; Sun et al., 2022; Tang et al., 2020; Yang et al., 2020; Zhang et al., 2016). Stern (1965) pointed out that the nonlinear interaction of the Ekman flow induced by stationary surface wind and geostrophic eddies tends to tilt the vertical axis of eddies.…”

“…These observed tilted eddies have various tilting directions (east‐west, north‐south) and tilting distance (TD; ∼6 km–∼150 km) from surface to deep ocean (Amores et al., 2017; Belkin et al., 2020; Bernstein & White, 1981; Kurczyn et al., 2013; Roemmich & Gilson, 2001; Sun et al., 2022; Tang et al., 2020; Zhang et al., 2016). Many numerical studies also produced vertical tilt of eddies (e.g., He et al., 2015; Li et al., 2020; Liao et al., 2019; Lin et al., 2015; Mason et al., 2019; Yang et al., 2020). A summary list of previous eddies tilting studies is given in Table 1.…”