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Southern Australia Current System based on a gridded hydrography and a high-resolution model

Duran, Earl R., Phillips, Helen E., Furue, Ryo, Spence, Paul, Bindoff, Nathaniel L.
Progress in oceanography 2020 v.181 pp. 102254
autumn, hydrology, models, summer, wind, Tasmania
We quantify the annual-mean structure, transport budget and coupling between surface and deep currents along the southern Australian shelves, that we propose be collectively known as the Southern Australia Current System. The system contains eastward Shelf Break Currents (SBC) including the Leeuwin Current Extension, the South Australian Current and the Zeehan Current; and a counter-flowing westward Flinders Current (FC). In this work, we use a high resolution climatological hydrography and a high resolution ocean model forced by an atmospheric climatology.The westward FC is dual structured with (1) the slope-FC, an undercurrent to the SBC trapped near 600 m depth; and (2) the offshore-FC, a large deep-reaching flow located offshore of the SBC. Along the slanted shelf (not zonally oriented), onshore flows feeding into the FC are weak, and the FC is disconnected from the Tasman Leakage south of Tasmania. Along the zonal shelf, where nearly 80% of total onshore flows occur, the FC intensifies resulting in 12.3 Sv transport at Cape Leeuwin.The eastward SBC are continuous flows in the upper 250 m. Over steep slopes, downwelling occurs from the SBC into the slope-FC and may be important in maintaining the slope-FC. The Leeuwin Current Extension transport decreases from 1.1 Sv at Cape Leeuwin to 0.1 Sv near the Great Australian Bight due to strong downwelling and offshore exports around Cape Leeuwin. The South Australian Current is weak and steady, and the Zeehan Current increases to 0.4 Sv due to onshore flows stronger than downwelling exports.We find that the SBC and offshore-FC are coupled through onshore Ekman drift and the SBC and slope-FC are coupled through downwelling. The combined effect is a conversion of widespread northward Ekman drift into downwelling with little impact on the eastward SBC transport. In contrast, the onshore flows feeding into the FC are largely converted into increasing westward FC transport. Even though the Leeuwin Current System off western Australia is fed by geostrophic flows and the Southern Australia Current System is fed by Ekman drift, the annual-mean circulation of the two systems are remarkably similar.In summer, when the winds are upwelling favourable, the SBC are weaker and partly reversed (to westward). In autumn, the winds become downwelling favourable and the FC is weaker and partly reversed (to eastward). In contrast, the Leeuwin Current System persists annually. The balance mechanism maintaining the seasonal opposition between the SBC and FC warrants further research.