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

Abstract

The East Australian Current (EAC) plays a major role in regional climate, circulation, and ecosystems, but predicting future changes is hampered by limited understanding of the factors controlling EAC separation. While there has been speculation that the presence of New Zealand may be important for the EAC separation, the prevailing view is that the time‐mean partial separation is set by the ocean's response to gradients in the wind stress curl. This study focuses on the role of New Zealand, and the associated adjacent bathymetry, in the partial separation of the EAC and ocean circulation in the Tasman Sea. Here utilizing an eddy‐permitting ocean model (NEMO), we find that the complete removal of the New Zealand plateau leads to a smaller fraction of EAC transport heading east and more heading south, with the mean separation latitude shifting >100 km southward. To examine the underlying dynamics, we remove New Zealand with two linear models: the Sverdrup/Godfrey Island Rule and NEMO in linear mode. We find that linear processes and deep bathymetry play a major role in the mean Tasman Front position, whereas nonlinear processes are crucial for the extent of the EAC retroflection. Contrary to past work, we find that meridional gradients in the basin‐wide wind stress curl are not the sole factor determining the latitude of EAC separation. We suggest that the Tasman Front location is set by either the maximum meridional gradient in the wind stress curl or the northern tip of New Zealand, whichever is furthest north.

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

“…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.…”

Regional Versus Remote Atmosphere‐Ocean Drivers of the Rapid Projected Intensification of the East Australian Current

“…Here NEMO uses 75 vertical levels with partial cells, where there are 24 levels in the first 100 m and 22 levels between 100 and 1,000 m to realistically represent coastlines and continental shelves. The NEMO configuration here is similar to the one used by Bull et al (2017, 2018) (same domain, model version, present‐day ERA‐Interim (ERAI) forcing, parameterizations, ocean bathymetry, and model mesh) but differs in that Bull et al (2017) was ocean‐only, forced directly with fluxes, with absolute winds, and Bull et al (2018) used bulk formulas. An additional difference is that both Bull et al (2017, 2018) used 5‐day mean for the ocean boundary conditions whereas NOW uses daily.…”

“…The NEMO configuration here is similar to the one used by Bull et al (2017, 2018) (same domain, model version, present‐day ERA‐Interim (ERAI) forcing, parameterizations, ocean bathymetry, and model mesh) but differs in that Bull et al (2017) was ocean‐only, forced directly with fluxes, with absolute winds, and Bull et al (2018) used bulk formulas. An additional difference is that both Bull et al (2017, 2018) used 5‐day mean for the ocean boundary conditions whereas NOW uses daily. BULL‐2017 and BULL‐2018 will hereafter refer to historical NEMO simulations from Bull et al (2017, 2018), respectively.…”

“…An additional difference is that both Bull et al (2017, 2018) used 5‐day mean for the ocean boundary conditions whereas NOW uses daily. BULL‐2017 and BULL‐2018 will hereafter refer to historical NEMO simulations from Bull et al (2017, 2018), respectively.…”

“…Specifically, 6‐hourly varying lateral boundary conditions for atmospheric fields (wind velocity, potential temperature, specific humidity, geopotential height) from the ERAI reanalysis (Dee et al, 2011) and daily varying lateral boundary conditions for oceanic fields (ocean velocity, potential temperature, practical salinity) from the ORCA025‐L75‐MJM95 global simulation (Barnier et al, 2011) which is itself forced by ERAI. ERAI and ORCA025‐L75‐MJM95 were used as they have been successfully used for regional modeling studies around Australia (e.g., Bull et al, 2017, 2018; Di Virgilio et al, 2019; Di Luca et al, 2016) and show relatively small biases for the region. For example, Dussin et al (2012) show the absolute SST bias for ORCA025‐L75‐MJM95 is less than 0.5°C in the Indo‐Pacific region (their Figure 18) for the 2000–2009 period, which is small given that no data assimilation is used.…”