Impacts of Lee Waves on the Southern Ocean Circulation and its Sensitivity to Wind Stress

Luwei Yang, University of Tasmania, Institute for Marine and Antarctic Studies, Hobart, TAS, Australia, Maxim Nikurashin, Institute for Marine and Antarctic Studies, University of Tasmania, Hobart, TAS, Australia, Andrew M Hogg, Australian National University and ARC Centre of Excellence for Climate Extremes, Research School of Earth Sciences, Canberra, ACT, Australia and Bernadette Sloyan, Commonwealth Scientific and Industrial Research Organisation (CSIRO), Hobart, TAS, Australia
Abstract:
Observations and idealised numerical simulations suggest that enhanced turbulent energy dissipation and mixing in the Southern Ocean are modulated by the transient eddy field through the generation and breaking of lee waves at rough topography. Lee waves have been shown to extract a significant amount of energy from the transient eddy field through the work done by lee wave drag against bottom flow. In this study, we investigate how lee waves affect the Southern Ocean circulation and its sensitivity to wind stress using an idealized, eddy-resolving model of the Southern Ocean and an energetically-consistent lee wave drag and mixing parameterization.

Our results show that adding lee waves to the model increases the baroclinic volume transport of the Antarctic Circumpolar Current (ACC) by over 60 Sv (1 Sv = 106 m3s-1; 40%) and also have a significant impact on the overturning circulation and deep stratification. The changes induced by lee wave drag can be explained by the eddy kinetic energy (EKE) balance, in which the EKE dissipation by lee wave drag is compensated by the enhanced EKE generation through baroclinic instability of the ACC. The lee-wave-driven mixing plays a minor role in modulating the ACC transport but makes a significant impact on the overturning circulation and deep stratification. We also find that lee waves significantly alter the sensitivity of the baroclinic ACC transport and lower overturning circulation to wind stress. Our results show that the effects of lee wave drag and lee-wave-driven mixing are coupled through bottom stratification and bottom kinetic energy. The coupling leads to a nonlinear combination of the individual effects of drag and mixing. Our results demonstrate that both drag and mixing effects need to be parameterized in global models to properly represent the impact of lee waves on the Southern Ocean circulation.

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