Long‐term trends in the East Australian Current separation latitude and eddy driven transport

Abstract

An observed warming of the Tasman Sea in recent decades has been linked to a poleward shift of the maximum wind stress curl, and a strengthening of the poleward flow along the coast of southeastern Australia. However, changes in the East Australian Current (EAC) separation latitude, as well as in the contribution of the EAC, the EAC extension and its eddy field to the total southward transport due to such a strengthening remain unknown. This study uses 30 years of the Ocean Forecast for the Earth Simulator (OFES) sea surface height and velocity outputs to obtain a three decade long-time series of (i) the EAC separation latitude, (ii) the southward transport along the coast of southeastern Australia (28 S-39 S), and (iii) the southward transport across the EAC separation latitude. A Lagrangian approach is implemented and the spin parameter X is used to provide a quantitative distinction between the transports occurring outside and inside (cyclonic and anticyclonic) eddies. Significant positive trends of the low pass southward transports indicate that the intensification of the poleward flow has occurred both within the EAC and in the EAC extension. In addition, a significant increase in southward transport inside and outside eddies is found. Importantly, the contribution of eddy driven transport has a large temporal variability and shows a sharp increase from 2005 onward. Finally our results show that the EAC has not penetrated further south but it has separated more frequently at the southernmost latitudes within the region where it typically turns eastward.

The East Australian Current and Property Transport at 27°S from 2012 to 2013

“…The EAC strengthens as it flows southward between 278 to 338S from 25 to 37 Sverdrups (Sv; 1 Sv [ 10 6 m 3 s 21 ; Ridgway and Dunn 2003). The EAC typically separates from the western boundary at 318-358S, forming the broad eastwardflowing Tasman Front and a residual poleward flow at the western boundary, the EAC extension (Ridgway and Dunn 2003;Cetina-Heredia et al 2014). The Tasman Front extends across the Tasman Sea to the northernmost tip of New Zealand, forming the East Auckland Current and a sequence of semipermanent eddies along the east coast of the New Zealand islands (Tilburg et al 2001).…”

Physical and biogeochemical spatial scales of variability in the East Australian Current separation from shelf glider measurements

“…The EAC is the western branch of the subtropical gyre in the South Pacific. It is a warm and dynamic poleward flowing current, encroaching on the continental shelf of southeastern Australia between around 18 • S (Ridgeway and Godfrey, 1994) and usually 30.7-32.4 • S (Cetina-Heredia et al, 2014) where it bifurcates eastward, forming the Tasman Front. Further south, eddies are shed (Everett et al, 2012), leading to high variability in the velocity field and water masses on the shelf (Schaeffer et al, 2014b;Schaeffer and Roughan, 2015).…”

Assessing the Use of Area- and Time-Averaging Based on Known De-correlation Scales to Provide Satellite Derived Sea Surface Temperatures in Coastal Areas

“…Although the EAC separation zone can occur anywhere between 28 • and 38 • S, it predominately occurs at 30. the temporal variability in the EAC separation zone, with a southward progression as eddies develop and then an abrupt retraction in latitude after the detachment of an eddy (Cetina Heredia et al, 2014). These eddies may be warm core (anticylonic) or cold core (cyclonic), with deep or shallow mixed layer depths respectively.…”

“…Such areas may be of high biological or ecological importance (Scales et al, 2014), thus the suitability of using SST as a proxy for in-water temperature will depend on localized conditions and specific temperature value of interest (e.g., weekly SST versus identifying "frontal" systems). For example, satellite derived measurements (regardless of whether they are areaand/or time-averaged) will not be able to capture the full variability in the ranges of temperatures experienced over short time scales (of hours rather than days) within the dynamic EAC separation zone, where, as previously discussed, warm-and coldcore eddies regularly detach from the EAC poleward flowing jet (Cetina Heredia et al, 2014). Likewise, satellites will not detect upwellings or marine heatwaves that do not reach the surface.…”

“…The East Australian Current (EAC) is the WBC in the South Pacific subtropical gyre. Formation of the EAC occurs in the Coral Sea (∼10-15 • S) and results in oligotrophic tropical waters flowing southward along the continental shelf break until it separates from the coast, typically at ∼31-32 • S (Cetina Heredia et al, 2014). Here, it bifurcates into an eastern flow across the Tasman Sea and a southward mesoscale eddy field that flows poleward.…”