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

Abstract

Like many western boundary currents, the East Australian Current (EAC) extension is projected to get stronger and warmer in the future. The CMIP5 multimodel mean (MMM) projection suggests up to 5°C of warming under an RCP85 scenario by 2100. Previous studies employed Sverdrup balance to associate a trend in basin wide zonally integrated wind stress curl (resulting from the multidecadal poleward intensification in the westerly winds over the Southern Ocean) with enhanced transport in the EAC extension. Possible regional drivers are yet to be considered. Here we introduce the NEMO‐OASIS‐WRF coupled regional climate model as a framework to improve our understanding of CMIP5 projections. We analyze a hierarchy of simulations in which the regional atmosphere and ocean circulations are allowed to freely evolve subject to boundary conditions that represent present‐day and CMIP5 RCP8.5 climate change anomalies. Evaluation of the historical simulation shows an EAC extension that is stronger than similar ocean‐only models and observations. This bias is not explained by a linear response to differences in wind stress. The climate change simulations show that regional atmospheric CMIP5 MMM anomalies drive 73% of the projected 12 Sv increase in EAC extension transport whereas the remote ocean boundary conditions and regional radiative forcing (greenhouse gases within the domain) play a smaller role. The importance of regional changes in wind stress curl in driving the enhanced EAC extension is consistent with linear theory where the NEMO‐OASIS‐WRF response is closer to linear transport estimates compared to the CMIP5 MMM.

The Rate of Coastal Temperature Rise Adjacent to a Warming Western Boundary Current is Nonuniform with Latitude

“…However, with most attention on changes in the separation latitude and the nature of eddies in the EAC extension region (Cetina‐Heredia et al., 2014; Oliver & Holbrook, 2014; Rykova & Oke, 2015), there has been little investigation into whether the effect of the spin‐up on the EAC is homogeneous. Of the many studies that have investigated a system‐wide temperature response to climate change, they mostly have global to basin‐scale focus (Bowen et al., 2017; Bull et al., 2020; Duran et al., 2020; Oliver & Holbrook 2014; Wu et al., 2012) with little investigation into latitudinal dependence. A knowledge of this latitudinal dependence is particularly important considering the societal benefit derived from coastal and shelf waters particularly fisheries (Suthers et al., 2011), the large range of dynamical regimes, and the tropicalization of ecosystems (Messer et al., 2020; Vergés et al., 2014) occurring in WBCs, e.g., in the EAC between 26°S and 40°S.…”

East Australian Cyclones and Air‐Sea Feedbacks

“…1 C E , with a bias exceeding  2 C E in the Tasman outflow, likely linked with a larger transport of the EAC extension and of the Tasman outflow in NOW-CTRL compared to observations (see the comparison of transports with observational estimates in Figure 2 of Bull et al, 2020).…”

“…This larger variability is probably linked with different eddy kinetic energy that modulates local heat transport and SST fluctuations. South of Australia, NOW-CTRL has also a warm bias larger than 1 • C, with a bias exceeding 2 • C in the Tasman outflow, likely linked with a larger transport of the EAC extension and of the Tasman outflow in NOW-CTRL compared to observations (see the comparison of transports with observational estimates in Figure 2 of Bull et al (2020)).…”

Future ice-sheet surface mass balance and melting in the Amundsen region, West Antarctica

“…Monthly anomalies are linearly interpolated to avoid discontinuity of 6-hourly boundary conditions. In the future simulation, we do not modify greenhouse gases concentrations in our regional domain, which is expected to have a minor effect because the dominant effect of global increase in greenhouse gases concentrations on our regional simulations comes from changes in sea surface and sea ice forcing as well as through increased humidity and temperature at the lateral boundaries (Krinner et al, 2014;Bull et al, 2020).…”