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

Abstract:

This work investigates through a modeling exercise the interaction of a strong, varying current with a seamount, focusing on the Gulf Stream and Atlantis II, one of the New England Seamounts. Five simulations are considered, with realistic bathymetry at different horizontal resolution (NESM runs) or with Atlantis II and the surrounding seamounts approximated by Gaussian-shaped obstacles (SEAMOUNT runs). The quasi-idealized SEAMOUNT simulations capture a broad range of the physical phenomena modeled by the NESM runs near the Atlantis II seamount. By quantifying the energetics near the bottom, we show that in general, a barotropic mechanism is responsible for topographic eddy generation, although baroclinic mechanism can also contribute. On the anticyclonic side of the seamounts, an overturning centrifugal instability due to negative Ertel potential vorticity (PV) in the bottom boundary layer (BBL) is identified. The negative/low PV is created when the currents flow in the direction of Kelvin wave propagation. The instability moves the negative PV out of the BBL into the interior ocean, increasing diapycnal mixing near Atlantis II. A combination of internal wave activity and centrifugal instability contribute to vertical mixing near Atlantis II, which depends on both resolution and topographic complexity. The NESM run with 5km horizontal resolution as well as the quasi-ideal SEAMOUNT runs underestimate vertical mixing as compared to the NESM run with 1 km horizontal resolution.