Interaction of Ocean Currents and Seamounts: Role of Bottom Topography Around Atlantis II

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Abstract:

This work investigates through a modeling exercise the interaction of a strong, varying current with a seamount, focusing on the North Atlantic Current (NAC) and Atlantis II, one of the New England Seamounts. Two simulations are considered, one with realistic bathymetry (NESM run) and a second with a single Gaussian seamount of comparable width and height of the Atlantis II seamount (SEAMOUNT run). The idealized SEAMOUNT simulation captures a broad range of physical phenomena modeled by the NESM run near the Atlantis II seamount. By quantifying the energetics near the bottom, we show that a mixed barotropic-baroclinic mechanism is responsible for topographic eddy generation. On the anticyclonic side of the seamounts, the bottom boundary layer (BBL) separates from the bathymetry when the NAC flows atop the seamounts. The separation of the BBL arises from an overturning instability due to negative Ertel potential vorticity (PV) in the BBL, which is created when the currents flow in the direction of Kelvin wave propagation. The instability restratifies the BBL, moving the negative PV out of the BBL into the interior ocean. Finally, scattering of internal waves from the small-scale rough bottom topography results in increased vertical mixing, which is prominent in the NESM run and underestimated in the SEAMOUNT simulation.

Plain Language Summary:

Interaction of strong ocean currents with underwater mountains generates a variety of structures in the ocean. In this work we explore the role of bottom topography in generating these flow patterns in the case of a seamount with a complex bathymetry. In addition, we investigate if we can capture the key dynamics by replacing the complex bathymetry by an idealized one. We focus on the strong North Atlantic Current (NAC), that interacts with relatively isolated underwater mountains off the coast of New England, and in particular on the area near the Atlantis II seamount. We analyze the energetics near the bottom to identify the mechanism through which eddies are generated when the NAC flows atop the underwater mountains. We also uncover a mechanism responsible for the separation of the bottom boundary layer from the slopes of the seamount. Finally, by comparing realistic and idealized simulations, we quantify the contribution of rough small-scale topography to vertical mixing of waters in the vicinity of Atlantis II.