Vertical Interaction Between NBC Rings and Its Implications for South Atlantic Water Export

Abstract

AbstractNorthwestward‐propagating North Brazil Current (NBC) Rings are often destroyed upon reaching the Caribbean islands, carrying South Atlantic waters into the North Atlantic gyre and connecting the two branches of the Atlantic Meridional Overturning Circulation. Recent observations of NBC rings reported surface and subsurface cores separated by strong stratification. As independent structures, the subsurface eddies often appeared below—even if not aligned with—the surface rings. Motivated by these findings, this paper investigates the occurrence and consequences of vertical coupling between surface and subsurface NBC rings by applying a simplified theoretical model to a realistic numerical simulation. Eddy tracking in this complex simulation reveals around 1,600 instantaneous observations where eddies overlap. At each observation, we assess the eddies' vertical interaction with a 21/2‐layer quasi‐geostrophic framework. This interaction boils down to either coupling between the layers or a vertical splitting of the system, depending on eddy strength versus background shear. The effects of coupling include long‐lived eddies that can travel longer distances. These effects are particularly important for the lower layer, where differences between coupled and non‐coupled eddies are striking: about 40% of eddies with coupling travel 500 km, with ∼10% carrying South Atlantic Water up to the Caribbean sea. Without coupling, eddies propagate less than 150 km. During coupling, lower‐layer eddies could be driven by their surface counterparts to move offshore and bypass a topographic choke point, thus leaving the NBC retroflection region. Although still speculative, this idea explains the link between coupling and the increase in both lifetime and distance traveled for these eddies.