The impact of lee waves on the Southern Ocean circulation and its sensitivity to wind stress

The westerly wind over the Southern Ocean has been strengthening and shifting poleward for the last few decades. It remains an outstanding problem to understand the response of the Antarctic Circumpolar Current (ACC) and Southern Ocean Meridional Overturning Circulation (MOC) to the changing wind conditions, noting that the local changes in the Southern Ocean will have remote impacts on global ocean and climate through the interbasin connection and the exchange of heat and carbon with the atmosphere. The response of the Southern Ocean circulation to the changing wind is regulated by the energetic eddy field, which acts to transfer momentum, heat, and tracers horizontally and to transfer momentum vertically. These eddy effects are particularly important for the Southern Ocean in the latitudinal band of Drake Passage. Therefore, the sensitivity of the Southern Ocean circulation to wind stress is usually explored using eddy-permitting and eddy-resolving global ocean models, where eddy generation is well resolved. However, eddy-resolving models do not fully represent the dynamics of eddy energy dissipation. A potential route of eddy energy dissipation occurs in the deep ocean where deep-reaching eddies interact with small-scale topography, generating internal lee waves. It is suggested that lee waves sustain mixing in the deep Southern Ocean and apply a drag on the time-mean flow of the ACC. In this study, we evaluate the role of lee waves for the dissipation of transient eddies in the Southern Ocean and investigate the impacts of lee waves on the Southern Ocean circulation and its response to changes in wind stress. Using linear lee wave theory, abyssal hill topographic datasets, and bottom velocity from a global eddy-resolving model, we estimate the contribution of lee waves to the dissipation of transient eddies. The results show that the energy dissipation of the eddy field due to lee wave generation (0.12 TW) exceeds its dissipation due to bottom friction in the turbulent bottom boundary (0.05 TW), and that lee waves make a stronger contribution to the dissipation of transient eddies (0.12 TW) than to the dissipation of the time-mean ACC (0.04 TW). We then develop an energetically consistent lee wave drag and mixing parameterisation and use it in an idealised, eddy-resolving model of the Southern Ocean to investigate the impact of lee waves on the ACC and MOC and their response to changes in wind stress. The results show that adding lee waves to the model increases the baroclinic transport of the ACC by over 60 Sv (by 40%). The results are explained by the eddy kinetic energy (EKE) balance in which the EKE dissipation by the lee wave drag is compensated by the enhanced EKE generation through baroclinic instability of the ACC, and hence isopycnal slope, leading to an increase in the ACC transport. We find that the lee-wave-driven mixing plays a minor role in the baroclinic ACC transport increase but has a significant impact on the overturning circulation and deep stratification. The parameterisation of lee waves significantly alters the sensitivity of the baroclinic ACC transport and lower overturning circulation to wind stress. The baroclinic ACC transport increases with wind stress in the presence of lee waves, in contrast to the reference case where the eddy saturation is reproduced without lee waves. The lower overturning circulation increases considerably with wind stress, contrary to the decrease found in the reference case without lee waves. We also find a coupling between lee wave drag and lee-wave-driven mixing, through bottom stratification and bottom kinetic energy. The coupling effect leads to a nonlinear combination of the effect from each component and demonstrates that both drag and mixing effects need to be parameterised to properly represent the impact of lee waves on the Southern Ocean circulation. Our results show that lee waves are an important player in the eddy energy balance in the Southern Ocean and regulate the strength of the ACC, MOC, and how they respond to the changing climate. These findings suggest that the presence of lee waves in global eddy-resolving ocean models can improve the representation of the eddy field, the Southern Ocean circulation and its sensitivity to changing climate. Hence, lee waves need to be parameterised in the global ocean models.