Dynamics of Deep Eastern Boundary Currents (DEBCs)

Tracers form tongue-like structures along the eastern boundaries in the ocean basins in the Southern Hemisphere which implies presence of southward flow of northern water masses. Realistic regional simulations of southeast Atlantic/Indian/Pacific Oceans forced by monthly climatologies from SOSE successfully simulate DEBCs that carries volume transport comparable to western boundary currents. They are mainly governed by interior vorticity budget with a boundary "hydrostatic" balance.

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Dissolved oxygen at  2.5 km depth
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Westward cumulative transport (top) and bathymetry (bottom) in southeast Atlantic Ocean (a,b), Indian Ocean (c,d) and Pacific Ocean (e,f).
Southward flow (colors) in the southeast Atlantic/Indian/Pacific Oceans.
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Vorticity budget terms (top) and bathymetric profile (bottom) in the southeast (a,d) Atlantic; (b,e) Indian; (c,f) Pacific Oceans.

Idealized non-eddying regional simulations of DEBCs  show that the currents is sensitive to bathymetry, which is later shown by a semi-analytical model to be an important source of vortex stretching along with parameterized eddy temperature diffusion. The right structure of voriticity budget is recovered by the simple model only when both topography and eddy diffusion are included. In these non-eddying simulations, the vertical structure of the current is bottom unrealistically intensified. The right vertical structure can be simulated by including the forcing of Agulhas rings that carries a mid-depth core of eddy vortex stretching signal.

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Idealized simulations of DEBCs using slopes of increasing widths. The magnitude and latitudinal scale of this current increase with the bathymetric width.

Idealize global simulation with an re-entrant channel in the south that is partially blocked below 2.5 km depth. The trench that has been shown important to the dynamics of the DEBC is added to near the eastern boundary. 

Idealize eddy-permitting global simulation with an re-entrant channel in the south that is partially blocked below 2.5 km depth. The trench that has been shown important to the dynamics of the DEBC is added near the eastern boundary. Three-dimensional floats are added to the steady state flow field and their trajectories show how the eastern boundary contributes to the global meridional overturning circulation. 

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A northward passage through the bottom of the trench (colors: time in year)
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A southward passage through the DEBC (colors: time in year)
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A linkage to the western boundary through the DEBC (colors: time in year)