Deep Eastern Boundary Currents: idealized models and dynamics

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  • 1 Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA, USA.
  • 2 Department of Earth and Planetary Sciences and School of Engineering and Applied Sciences, Harvard University, Cambridge, MA, USA.
  • 3 Geophysical Fluid Dynamics Institute and the Department of Scientific Computing, Florida State University, Tallahassee, Florida, USA.
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Abstract

Concentrated poleward flows along eastern boundaries between two and four kilometers depth in the southeast Pacific, Atlantic, and Indian Oceans have been observed, and appear in data-assimilation products and regional model simulations at sufficiently high horizontal resolution, but their dynamics are still not well understood. We study the local dynamics of these Deep Eastern Boundary Currents (DEBCs) using idealized GCM simulations and use a conceptual vorticity model for the DEBCs, to gain additional insights into the dynamics. Over most of the zonal width of the DEBCs, the vorticity balance is between meridional advection of planetary vorticity and vortex stretching, which is an interior-like vorticity balance. Over a thinner layer very close to the eastern boundary, a balance between vorticity tendencies due to friction and stretching that rapidly decay away from the boundary is found. Over the part of the DEBC that is governed by an interior-like vorticity balance, vertical stretching is driven by both the topography and temperature diffusion, while in the thinner boundary layer, it is driven instead by parameterized horizontal temperature mixing. The topographic driving acts via a cross-isobath flow that leads to stretching and thus to vorticity forcing for the concentrated DEBCs.

Corresponding author address: 24 Oxford Street, Cambridge, MA, USA. E-mail: xiaoting_yang@g.harvard.edu

Abstract

Concentrated poleward flows along eastern boundaries between two and four kilometers depth in the southeast Pacific, Atlantic, and Indian Oceans have been observed, and appear in data-assimilation products and regional model simulations at sufficiently high horizontal resolution, but their dynamics are still not well understood. We study the local dynamics of these Deep Eastern Boundary Currents (DEBCs) using idealized GCM simulations and use a conceptual vorticity model for the DEBCs, to gain additional insights into the dynamics. Over most of the zonal width of the DEBCs, the vorticity balance is between meridional advection of planetary vorticity and vortex stretching, which is an interior-like vorticity balance. Over a thinner layer very close to the eastern boundary, a balance between vorticity tendencies due to friction and stretching that rapidly decay away from the boundary is found. Over the part of the DEBC that is governed by an interior-like vorticity balance, vertical stretching is driven by both the topography and temperature diffusion, while in the thinner boundary layer, it is driven instead by parameterized horizontal temperature mixing. The topographic driving acts via a cross-isobath flow that leads to stretching and thus to vorticity forcing for the concentrated DEBCs.

Corresponding author address: 24 Oxford Street, Cambridge, MA, USA. E-mail: xiaoting_yang@g.harvard.edu
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