On the physics of the Agulhas Current: Steady retroflection regimes

From previous model studies, it has become clear that several physical mechanisms may be at work in the retroflection of the Agulhas Current. Here, a systematic study of steady barotropic flows connecting the Indian Ocean and South Atlantic Ocean in several idealized setups is performed. By solving directly for the steady circulation with continuation methods, the connection between different retroflection regimes can be monitored as external conditions, such as the wind forcing or bottom topography, as well as parameters, such as the lateral friction and layer depth, are changed. To distinguish the different steady retroflecting flows, an objective measure of the degree of retroflection, a retroflection index R, is introduced. By monitoring R along a branch of steady solutions, using the horizontal friction as control parameter, several steady retroflecting regimes are found. At large friction there exist stable steady states with viscously dominated retroflection. When friction is decreased, inertial effects become more dominant, and eventually unstable steady states with strong retroflection characteristics exist. Within this framework, different results from earlier studies can be reconciled.