On the generation of North brazil current rings

A high resolution general circulation model (OGCM) is used to investigate the life cycle of North Brazil Current (NBC) rings. The focus of the study is on explaining the generation mechanisms of the rings and their vertical structure. The OGCM is used in a configuration simplified as much as possible but capable of reproducing a realistic mean circulation of the tropical Atlantic. Three numerical experiments are analyzed: no wind; steady wind; seasonally varying winds. With wind forcing, a consistent scenario emerges for the generation of NBC rings. It is shown first that the NECC is barotropically unstable, radiating Rossby waves of the first baroclinic mode with a period of 63 days (steady wind) or 50 days (variable wind). These waves reflect at the Brazilian coast and create about 6-7 anticyclones per year. These anticyclones intensify as they propagate north-westward along the Brazilian coast because of potential vorticity conservation and become NBC rings. Among them, about one per year reaches a depth below 1000 m. These deep rings are created by the merger of a surface NBC ring with an intermediate eddy. The intermediate eddies are produced by the Intermediate Western Boundary Current (IWBC) which becomes unstable upon crossing the equator.