On the generation of North Brazil Current rings

A high resolution general circulationmodel (OGCM) is used to investigate the life cycle of North Brazil Current (NBC) rings. The focus of the study is on explainingthe generationmechanismsof the rings and their vertical structure.The OGCM is used in a con guration simpli ed as much as possible but capable of reproducinga realisticmean circulationof the tropical Atlantic. Three numerical experiments are analyzed: no wind; steadywind; seasonally varyingwinds. With wind forcing, a consistent scenario emerges for the generationof NBC rings. It is shown rst that the NECC is barotropicallyunstable, radiatingRossbywaves of the rst baroclinicmode with a period of 63 days (steady wind) or 50 days (variable wind). These waves re ect 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. 1