Coupled Ocean–Sea Ice Dynamics of the Antarctic Slope Current Driven by Topographic Eddy Suppression and Sea Ice Momentum Redistribution

Abstract

The Antarctic Slope Current (ASC) plays a central role in redistributing water masses, sea ice, and tracer properties around the Antarctic margins, and in mediating cross-slope exchanges. While the ASC has historically been understood as a wind-driven circulation, recent studies have highlighted important momentum transfers due to mesoscale eddies and tidal flows. Furthermore, momentum input due to wind stress is transferred through sea ice to the ASC during most of the year, yet previous studies have typically considered the circulations of the ocean and sea ice independently. Thus it remains unclear how the momentum input from the winds is mediated by sea ice, tidal forcing, and transient eddies in the ocean, and how the resulting momentum transfers serve to structure the ASC. In this study the dynamics of the coupled ocean/sea ice ASC circulation are investigated using high-resolution process-oriented simulations, and interpreted with the aid of a reduced-order model. In almost all simulations considered here, sea ice redistributes almost 100% of the wind stress away from the continental slope, resulting in approximately identical sea ice and ocean surface flows in the core of the ASC in a fully spun-up equilibrium state. This ice-ocean coupling results from suppression of vertical momentum transfer by mesoscale eddies over the continental slope, which allows the sea ice to accelerate the ocean surface flow until the speeds coincide. Tidal acceleration of the along-slope flow exaggerates this effect, and may even result in ocean-to-ice momentum transfer. The implications of these findings for along- and across-slope transport of water masses and sea ice around Antarctica are discussed.

Scalings for Eddy Buoyancy Fluxes Across Prograde Shelf/Slope Fronts

“…In line with the findings of Si et al. (2022), transient eddy fluxes are larger than standing eddy fluxes by a factor of at least eight (not shown), which indicates that all our results supported by “Smooth” simulations should readily apply to “Corrugated” runs.…”

“…Notwithstanding these issues, Si et al. (2022) showed that the vertical eddy fluxes of prograde momentum, determined by the cross‐slope buoyancy fluxes (Greatbatch & Lamb, 1990), are not dominated by standing eddies in the prograde Antarctic Slope Front. To assess this, we calculate the standing eddy fluxes in our “Corrugated” simulations defined by $$\left\langle \int \nolimits_{-\vert H\vert }^{0}{\overline{v}}^{{\dagger}}{\overline{\theta }}^{{\dagger}}\hspace*{.5em}\mathrm{d}z\right\rangle $$, where $${\bullet }^{{\dagger}}=\bullet -\left\langle \bullet \right\rangle $$ represents the deviation from the zonal‐mean (e.g., Bischoff & Thompson, 2014).…”

“…In the presence of zonal topographic corrugations, standing eddies can arise and contribute to the total meridional eddy buoyancy fluxes (Abernathey & Cessi, 2014; Bai et al., 2021; Si et al., 2022; Stewart & Hogg, 2017; Thompson & Naveira Garabato, 2014), yet the eddy buoyancy diffusivity K θ defined by Equation only accounts for transient eddy fluxes. Notwithstanding these issues, Si et al.…”

“…In the presence of zonal topographic corrugations, standing eddies can arise and contribute to the total meridional eddy buoyancy fluxes (Abernathey & Cessi, 2014;Bai et al, 2021;Si et al, 2022;Stewart & Hogg, 2017;Thompson & Naveira Garabato, 2014), yet the eddy buoyancy diffusivity K θ defined by Equation 5 only accounts for transient eddy fluxes. Notwithstanding these issues, Si et al (2022) showed that the vertical eddy fluxes of prograde momentum, determined by the cross-slope buoyancy fluxes (Greatbatch & Lamb, 1990), are not dominated by standing eddies in the prograde Antarctic Slope Front. To assess this, we calculate the standing eddy fluxes in our "Corrugated" simulations defined by 𝐴𝐴…”

“…For instance, Stewart and Thompson (2013) employed a simple analytical fit for the cross-slope eddy buoyancy diffusivity as a function of δ, based on an idealized simulation of the Antarctic Slope Front. This scaling was later adopted by Si et al (2022) to construct a reduced-order model for the momentum balance of the Antarctic Slope Current. Brink (2012) and Brink and Cherian (2013) performed idealized eddy-resolving simulations of WEI ET AL.…”