Deep Cyclones in the western North Atlantic: Insight from a Regional Numerical Model with High Vertical Resolution

Deep cyclones (DCs) were observed in the western North Atlantic under meander troughs of the Gulf Stream (GS) west of the New England seamounts during the Synoptic Ocean Prediction (SYNOP) field campaign. Although subsequent dedicated observations have been sparse, DCs appear common underneath major surface-intensified western boundary current extensions. Recent model studies with idealised domains suggest that DCs are important sources of eddy kinetic energy in the deep ocean, key sites of energy dissipation, and potential contributors to episodes of strong near-bottom currents and sediment resuspension known as “benthic storms”. In the western North Atlantic, DCs that form within GS meander troughs could play a role in the Atlantic meridional overturning circulation by providing a path for recirculation of water between the Deep Western Boundary Current (DWBC) and the adjacent oceanic basins. However, most numerical ocean models lack the vertical resolution that is needed to simulate in detail both the vertical structure of DCs and the near-bottom flows.

In this study, we configure the MIT general circulation model to produce eddy-rich simulations of western North Atlantic circulation at a horizontal resolution of 1/20^o and with high vertical resolution (550 levels with uniform Dz = 10 m). Emphasis is placed on the role of DCs in the time-mean abyssal circulation and on their contribution to Lagrangian transport, particularly to the exchange of water between the DWBC and the adjacent basins and between the bottom mixed layer and the stratified interior. In the simulations, deep cyclones are found to form west of the New England seamounts, consistent with field observations from SYNOP. They also form in the Sohm abyssal plain – east of the seamounts – although observations are lacking to confirm or refute this result. In our simulations, the DCs typically persist for 30-90 days and move eastward at a speed of ~1.5 cm s^-1 in tandem with the GS meander troughs near the surface. The time-averaged horizontal velocity field in the Sohm abyssal plain depicts a large-scale cyclonic circulation cell that is reminiscent of the Northern Recirculation Gyre proposed from sparse observations of deep current meters. Preliminary results of Lagrangian transport suggest that water parcels in the DWBC may enter the interior through entrainment of DCs near 68ºW (west of the New England seamounts). Further Lagrangian analyses of the simulated deep circulation are ongoing to elucidate the temporal and spatial scales of particle transport associated with the DCs and benthic storms.