Instabilities and Interior Transport Pathways of the Deep Western Boundary Current

Recent observations and modeling of the Deep Western Boundary Current (DWBC) in the North Atlantic have revealed that its role as a southward conduit for the Atlantic Meridional Overturning Circulation (AMOC) is less coherent than implied by classical theories. Instead, the southward AMOC transport is accomplished not only by the coherent, relatively steady component of the DWBC, but also via interior pathways. These pathways tend to have chaotic trajectories, leaving the continental slope to the basin interior before returning southwestward, and hence have longer transit times in the North Atlantic. Floats released within the DWBC almost invariably escape into the ocean interior, sometimes returning to the DWBC much further downstream.

 We investigate the development of interior pathways from the DWBC by conducting high-resolution (2.5 km grid spacing) numerical simulations of the northwest Atlantic. We focus on the formation and development of instabilities near the Grand Banks of Newfoundland (GB), a site known to exhibit pronounced eddy activity. We find that production of eddy kinetic energy (EKE) is dominated by conversion from available potential energy (APE), indicating that baroclinic instabilities dominate eddy generation over the continental slope and throughout the GB region. We further investigate the intermittent occurrence of DWBC restratification events that contribute to the time-averaged APE->EKE transformation, and their statistical and dynamical relation to external disturbances such as Gulf Stream Rings and proximity to the North Atlantic Current. Finally, the finite-time evolution of these disturbances into coherent eddies, and their propagation and relation to interior pathways are investigated via the use of particle tracking.