The turbulent dynamics of anticyclonic submesoscale headland wakes

Abstract

Flow interacting with bathymetry has been posited to be important for dissipation and mixing in the global ocean. Despite this, there are large uncertainties regarding mixing in these environments, particularly as it pertains to the role of submesoscale structures in the dynamics and energetics. In this work we study such flows with a series of Large-Eddy simulations of a submesoscale flow past a headland where the turbulence is resolved, allowing us to probe into the small-scale processes responsible for the energy cascade. One key finding is that the kinetic energy (KE) dissipation rate, buoyancy mixing rate, and eddy diffusivity of the flow organize as linear functions of the bulk Rossby and Froude numbers across all simulations, despite very different dynamical regimes. The slope Burger number (Rossby over Froude number) was found to be particularly useful as it can organize aspects of both the dynamics and energetics. Moreover, comparison of KE dissipation rates with previous works suggests an underestimation of dissipation rates by regional models of up to an order of magnitude, with potential implications for global energy budgets. Consistent with hypotheses from previous studies, but resolved here for the first time up to small scales, we find evidence of submesoscale centrifugal-symmetric instabilities (CSIs) in the wake leading to a forward energy cascade. However, given that dissipation and mixing rates seem to follow the same scaling across regimes with and without CSIs, their effect on flow energetics here differs from what has been observed in the upper ocean, where CSI turbulence seems to follow a different scaling from their non-CSI counterparts.

Overturning instability in forced ageostrophic oceanic flows

“…Ageostrophic theory for identifying AOI is also needed to improve our understanding of its phenomenology. A specific application of this updated theory is the growing interest in instabilities along boundaries (Naveira Garabato & Coauthors, 2019;Wenegrat & Thomas, 2020;Yankovsky et al, 2021;Chor & Wenegrat, 2024). Flows strongly influenced by topography (e.g., topographic shearing or drag) are not in geostrophic balance, and we must consider ageostrophic flow properties when investigating their effects and developing appropriate parameterizations.…”

Overturning instability in forced ageostrophic oceanic flows

“…Ageostrophic theory for identifying AOI is also needed to improve our understanding of its phenomenology. A specific application of this updated theory is the growing interest in instabilities along boundaries (Naveira Garabato & Coauthors, 2019;Wenegrat & Thomas, 2020;Yankovsky et al, 2021;Chor & Wenegrat, 2024). Flows strongly influenced by topography (e.g., topographic shearing or drag) are not in geostrophic balance, and we must consider ageostrophic flow properties when investigating their effects and developing appropriate parameterizations.…”

Overturning instability in forced ageostrophic oceanic flows

“…Ageostrophic theory for identifying AOI is also needed to improve our understanding of its phenomenology. A specific application of this updated theory is the growing interest in instabilities along boundaries (Naveira Garabato & Coauthors, 2019;Wenegrat & Thomas, 2020;Yankovsky et al, 2021;Chor & Wenegrat, 2024). Flows strongly influenced by topography (e.g., topographic shearing or drag) are not in geostrophic balance, and we must consider ageostrophic flow properties when investigating their effects and developing appropriate parameterizations.…”

Overturning instability in forced ageostrophic oceanic flows

“…Ageostrophic theory for identifying AOI is also needed to improve our understanding of its phenomenology. A specific application of this updated theory is the growing interest in instabilities along boundaries (Naveira Garabato & Coauthors, 2019;Wenegrat & Thomas, 2020;Yankovsky et al, 2021;Chor & Wenegrat, 2024). Flows strongly influenced by topography (e.g., topographic shearing or drag) are not in geostrophic balance, and we must consider ageostrophic flow properties when investigating their effects and developing appropriate parameterizations.…”