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

Christopher Y S Bull,Nicolas C Jourdain,Alejandro Di Luca,Guillaume Sérazin,Alex Sen Gupta,Andrew E Kiss,Daniel Argüeso

doi:10.1029/2019jc015889

Abstract

AbstractLike 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.

Highlights

The poleward shift of western boundary currents (WBCs) or the intensification of their extensions has led to regional surface warming rates two to three times faster than the global average (Wu et al, 2012; Yang et al, 2016)
The climate change simulations show that regional atmospheric CMIP5 multimodel mean (MMM) anomalies drive 73% of the projected 12 Sv increase in East Australian Current (EAC) extension transport whereas the remote ocean boundary conditions and regional radiative forcing 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

Summary

Introduction

The poleward shift of western boundary currents (WBCs) or the intensification of their extensions has led to regional surface warming rates two to three times faster than the global average (Wu et al, 2012; Yang et al, 2016). The EAC forms the poleward flowing branch of the South Pacific Subtropical Gyre. It transports 22.1 Sv southward at 27°S (Sloyan et al, 2016) with a mean EAC surface core speed of 1.35 m s−1 at 30°S (Archer et al, 2017). A diminished EAC flows eastward forming the Tasman Front (or the “eastern extension of the EAC”; Oke, Pilo, et al, 2019) while the shed eddies migrate south‐west toward Tasmania (Everett et al, 2012). Some EACx eddies propagate around Tasmania into the Indian Ocean forming the Tasman Leakage (Baird & Ridgway, 2012; Pilo et al, 2015; Speich et al, 2002). For a recent review of the Tasman Sea circulation, see Oke, Roughan, et al (2019)

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