The impact of lee waves on the Southern Ocean circulation

Abstract

Lee waves play an important role in transferring energy from the geostrophic eddy field to turbulent mixing in the Southern Ocean. As such, lee waves can impact the Southern Ocean circulation and modulate its response to changing climate through their regulation on the eddy field and turbulent mixing. The drag effect of lee waves on the eddy field and the mixing effect of lee waves on the tracer field have been studied separately to show their importance. However, it remains unclear how the drag and mixing effects act together to modify the Southern Ocean circulation. In this study, a lee wave parameterization that includes both lee wave drag and its associated lee-wave-driven mixing is developed and implemented in an eddy-resolving idealized model of the Southern Ocean to simulate and quantify the impacts of lee waves on the Southern Ocean circulation. The results show that lee waves enhance the baroclinic transport of the Antarctic Circumpolar Current (ACC) and strengthen the lower overturning circulation. The impact of lee waves on the large-scale circulation are explained by the control of lee wave drag on isopycnal slopes through their effect on eddies, and by the control of lee-wave-driven mixing on deep stratification and water mass transformation. The results also show that the drag and mixing effects are coupled such that they act to weaken one another. The implication is that the future parameterization of lee waves in global ocean and climate models should take both drag and mixing effects into consideration for a more accurate representation of their impact on the ocean circulation.

The Impact of Representations of Realistic Topography on Parameterized Oceanic Lee Wave Energy Flux

“…Implementations of lee wave mixing parameterizations in global models have generally employed a static map of lee wave energy flux (Melet et al., 2014; Nikurashin & Ferrari, 2013), rather than using online bottom velocities and stratification to update the energy flux and inferred dissipation and mixing as the model runs. However, there is evidence from idealized models that feedbacks between bottom velocities, stratification, and lee waves can significantly alter the ocean state (Broadbridge et al., 2016; Yang et al., 2021), motivating the need for online parameterizations. Both the spectral and peaks methods considered here would be computationally expensive if implemented directly at run‐time in a coarse global ocean model.…”

Lee Waves Break Eddy Saturation of the Antarctic Circumpolar Current

“…The only difference worth noting in the model setup between Yang et al. (2021) and the current study is that the bottom frictional drag is changed from quadratic in Yang et al. (2021) to linear in this study.…”

“…The model configuration and numerical experiments are described in Section 2. The details of parameters used in the configuration are provided in Yang et al (2021). The processed data that support the figures are archived at https://doi.org/10.5281/zenodo.7794086.…”

“…Mixing sustained by lee waves has been shown to drive the Southern Ocean overturning circulation (Melet et al, 2014;Nikurashin & Ferrari, 2013) and regulate its sensitivity to changing winds (Broadbridge et al, 2016;Stanley & Saenko, 2014). Lee waves have also been shown to increase the ACC transport through their drag effect (Yang et al, 2021): lee waves apply drag to eddies and thus, as in Marshall et al (2017), lead to the increase in ACC shear, and therefore the ACC baroclinic transport. In this study, using simulations with and without lee wave parameterization in an idealized eddy-resolving model of the Southern Ocean, we investigate the impact of lee waves on the sensitivity of the ACC transport to increasing wind stress.…”

Internal Lee Waves Generated by Shear Flow Over Small‐Scale Topography

“…Although the magnitude of global energy input into lee waves is smaller than that into internal tides, lee wave‐driven mixing significantly affects the ocean state. It can reduce ocean stratification associated with warming of the abyssal ocean, accelerate global meridional overturning circulation because of its promotion of deep‐water upwelling, and hence facilitating the renewal of deep water (e.g., MacKinnon et al., 2017; Mashayek et al., 2017; Melet et al., 2014; Trossman et al., 2016; Yang et al., 2021). Internal lee waves have received extensive attention in recent years (e.g., Baker & Mashayek, 2021; De Marez et al., 2020; Johnston et al., 2019; Kunze & Lien, 2019; Legg, 2020; Mayer & Fringer, 2020; Shakespeare, 2020; Shakespeare et al., 2021; Shakespeare & Hogg, 2017; Voet et al., 2020; Xie et al., 2017; Xie & Li, 2019; Xie & Vanneste, 2017).…”