Fractionally modified flow in a deep ocean channel: Application to the Vema Channel

Abstract

The modification of the exchange flow in a deep southern hemisphere passage, resembling the Vema Channel, by frictionally induced secondary circulation is investigated numerically. The hydrostatic primitive equation model is a two‐dimensional version of the sigma‐coordinate Princeton Ocean Model. The time dependent response of a stratified along‐channel flow, forced by barotropic or baroclinic pressure gradients, is examined. Near the bottom, where the along‐channel flow is retarded, there is cross‐channel Ekman flux that is associated with downwelling on the eastern side and upwelling on the western side of the channel. In the presence of stratification the cross‐channel flow rearranges the density structure, which in turn acts on the along‐channel velocity via the thermal wind relation. Eventually the cross‐isobath Ekman flux is shut down. In the case of baroclinically driven flow of Antarctic Bottom Water through the Vema Channel the model reproduces the observed shape of the deep temperature profiles and their cross‐channel asymmetry. The model offers an explanation that is alternative or supplementary to inviscid multilayer hydraulic theory that was proposed in earlier studies. It explains the extremely thick bottom boundary layers in the center and on the western slope of the channel. The deep thermocline is spread out in the west and sharpened in the east, and the coldest water is found on the eastern side of the deep trough. The modified density field reduces the along‐channel flow near the bottom and focuses it into a narrow jet on the eastern side of the channel.