Wind Forced Variability in Eddy Formation, Eddy Shedding, and the Separation of the East Australian Current

Abstract

The East Australian Current (EAC), like many other subtropical western boundary currents, is believed to be penetrating further poleward in recent decades. Previous observational and model studies have used steady state dynamics to relate changes in the westerly winds to changes in the separation behavior of the EAC. As yet, little work has been undertaken on the impact of forcing variability on the EAC and Tasman Sea circulation. Here using an eddy‐permitting regional ocean model, we present a suite of simulations forced by the same time‐mean fields, but with different atmospheric and remote ocean variability. These eddy‐permitting results demonstrate the nonlinear response of the EAC to variable, nonstationary inhomogeneous forcing. These simulations show an EAC with high intrinsic variability and stochastic eddy shedding. We show that wind stress variability on time scales shorter than 56 days leads to increases in eddy shedding rates and southward eddy propagation, producing an increased transport and southward reach of the mean EAC extension. We adopt an energetics framework that shows the EAC extension changes to be coincident with an increase in offshore, upstream eddy variance (via increased barotropic instability) and increase in subsurface mean kinetic energy along the length of the EAC. The response of EAC separation to regional variable wind stress has important implications for both past and future climate change studies.

Meridional Oceanic Heat Transport Influences Marine Heatwaves in the Tasman Sea on Interannual to Decadal Timescales

“…The EAC is the largest source of heat advection into the Tasman Sea along the TN section, while the remaining currents, in particular the TF, remove heat from the Tasman Sea. Time averaged volume (and heat transports) for the EAC of −23.3 ± 4 Sv (−1273 ± 232 PW) are in the range of results provided in earlier studies Sloyan and O'Kane, 2015;Ypma et al, 2016;Bull et al, 2017). Mean transports for the TF of 19.8 ± 3.6 Sv (888 ± 134 PW) are about twice as large.…”

“…The coefficients are in the order of r = ± 0.25 over most of the region. In this regard, the positive heat transport anomalies coinciding with positive wind stress curl anomalies between 30 • S and 20 • S suggest an acceleration of the western boundary current (i.e., the EAC), according to the Sverdrup balance (Sverdrup, 1947), the Island Rule (Godfrey, 1989) and recent studies built on them (Cai, 2006;Hill et al, 2008;Bull et al, 2017Bull et al, , 2018. Without additional wind stress curl changes over the Tasman Sea, both the TF and EAC-Extension heat and volume transports will increase to balance the EAC.…”

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

“…Because the EAC is known to shed anticyclonic eddies consistently (Bull et al, 2017;Cetina-Heredia et al, 2014), it is not surprising that there are more particle trajectories entering anticyclones than cyclones. Nevertheless, the largest proportion of particles entering anticyclones is also determined by the larger proportion of anticyclones relative to cyclones in the study region (Oliver et al, 2015;Pilo, Mata, & Azevedo, 2015).…”

Eddy‐Driven Cross‐Shelf Transport in the East Australian Current Separation Zone

“…Between 30 • S and 32 • S, the EAC usually separates from the coast (Cetina-Heredia et al, 2014), splitting into the eddy-dominated southern and eastern extensions (Oke et al, 2019). At the separation point, anticyclonic eddies are shed approximately every 100 days via a mixed baroclinic/barotropic instability mechanism (Bowen et al, 2005;Bull et al, 2017;Mata et al, 2006).…”