The separation of the East Australian Current: A Lagrangian approach to potential vorticity and upstream control

Abstract

The East Australian Current (EAC) is the western boundary current flowing along the east coast of Australia separating from the coast at approximately 34°S. After the separation two main pathways can be distinguished, the eastward flowing Tasman Front and the extension of the EAC flowing southward. The area south of the separation latitude is eddy‐rich and the separation latitude of the EAC is variable. Little is known of the properties of the water masses that separate at the bifurcation of the EAC. This paper presents new insights from the Lagrangian perspective, where the water masses that veer east and those that continue south are tracked in an eddy‐permitting numerical model. The transport along the two pathways is computed, and a 1:3 ratio between transport in the EAC extension and transport in the Tasman Front is found. The results show that the “fate” of the particles is to first order already determined by the particle distribution within the EAC current upstream of the separation latitude, where 85% of the particles following the EAC extension originate from below 460 m and 90% of the particles following the Tasman Front originate from the top 460 m depth at 28°S. The separation and pathways are controlled by the structure of the isopycnals in this region. Analysis of anomalies in potential vorticity show that in the region where the two water masses overlap, the fate of the water depends on the presence of anticyclonic eddies that push isopycnals down and therefore enable particles to travel further south.

The Role of the New Zealand Plateau in the Tasman Sea Circulation and Separation of the East Australian Current

“…For example, there are sharper SSH gradients in CTRL both on the south-eastern side of New Zealand and across the Tasman Sea in the Tasman Front. The bias across the Tasman Sea results in a more focused eastward flow and is typical of other modeling studies in the region (Oliver & Holbrook, 2014;Ypma et al, 2016). The Tasman Front's observed transport is highly variable and at times westward (Sutton & Bowen, 2014).…”

“…The variability of sea surface height in CTRL is also in good agreement with AVISO in many areas but underestimated in the EAC (Figures 2c-2e). This is typical of 1/48 eddy-permitting resolution models (e.g., Ypma et al, 2016). The standard deviation bias (Figure 2e) south-east of New Zealand may be related to the documented semipermanent eddies in the region (see , Figure 13), who note that some altimetry data sets do not resolve these features) and/or the strong fronts associated with the confluence of subtropical and subantarctic waters in the region (Fernandez et al, 2014).…”

“…The separation latitude of the EAC shown in Figures 4a-4d is calculated from each daily average snapshot. The separation latitude is found following the method described by Cetina-Heredia et al (2014) and Ypma et al (2016), namely, we first find the upstream core of the EAC, defined here as the maximum southward geostrophic surface velocity at 288S. The SSH contour at the location of the maximum velocity is then followed south and the first location at which the contour turns more than 308 east of south is recorded as the (Figure 4a) represents the growth of the EAC retroflection south, then at 358S-378S the detected EAC separation latitude retracts abruptly north to 338S (where the black line segment has a discontinuity).…”

“…Prior to separation, the upstream EAC has a range of observed and modeled transports from past studies; observed estimates include 27.4 Sv (Ridgway & Godfrey, 1994) and 25.8 Sv (derived from CARS; Ridgway et al, 2002). Modeling estimates of EAC transport range between 20.4 and 30 Sv (Oliver & Holbrook, 2014;Wang et al, 2013;Ypma et al, 2016). In our study, the modeled CTRL upstream EAC transport to 1,945 m across 1548E-1568E at 288S is 24.3 Sv (Figure 3), and therefore within both modeled and observed estimates of EAC transport, and also consistent with the 22.1 6 7.5 Sv directly observed transport at 278S (Sloyan et al 2016).…”

The physical oceanography of the transport of floating marine debris

“…Drijfhout et al 1996, Blanke and Raynaud 1997, Döös et al 2008 and ocean dynamics (e.g. van Sebille et al 2012b, Ypma et al 2015. It is also the most commonly used framework to compute the pathways and distributions of plastic particles in the ocean (e.g.…”