Barotropic Zonal Jets Induced by Islands in the Southwest Pacific

Abstract

The oceanic circulation entering the tropical southwest Pacific (SWP) is dominated by the broad westward flow of the South Equatorial Current (SEC), which is forced by the trade winds. It has been argued that the numerous islands of the SWP are able to restructure the SEC into a series of deep and narrow zonal jets, which control important pathways connecting equatorial and extraequatorial signals. The primary objective of this paper is to improve the understanding of the structure and dynamics of SWP zonal jets, giving special attention to topographic effects. This study is based on the use of a high-resolution regional oceanic model, whose solution is compared with observations, as well as with solutions from global models and the Sverdrup relation. The model used here indicates that the regional topography drives a general equatorward shift of the SEC, which is beneficial to the North Fiji, North Vanuatu, and North Caledonian jets. A depth-integrated vorticity budget shows that this topographic effect is considerably attenuated by baroclinicity and advection processes, but not to the point of total compensation as often admitted for the interior ocean. The effect of nonlinear advection is to allow flow rectification of the jets fluctuations, taking the form of zonally elongated dipole circulations in the leeward side of the islands.