Quasi-geostrophic 3D circulation and mass transport in the western Bransfield Strait during Austral summer 1995/96

Abstract

In the framework of the FRUELA carbon-flux study, two oceanographic surveys (December 1995 and January 1996) were conducted by B.I.O. Hesperides in the western basin of the Bransfield Strait and southern Drake Passage. The horizontal (geostrophic) velocity field is characterized by two jet-like features linked to frontal hydrographic structures: a meandering current off the continental shelf edge of the southern Drake Passage, corresponding to the Southern front of the Antarctic Circumpolar Current, and the Bransfield Current, related to the Bransfield front. The first jet shows some changes in the meandering pattern from December 1995 to January 1996, whereas the second has a very stationary location and structure. The estimated horizontal circulation and mass transports are compared with available current meter data and with previous geostrophic estimates in the context of a sensitivity analysis that takes into account the choice of the reference level and the station distribution. The vertical velocity field has been computed by integrating the quasi-geostrophic Omega equation. Maximum estimated values are of the order of 1 m/day, which suggests that a phytoplankton bloom observed in the northern part of the study area in December (and vanished one month later) should not be attributed to frontal dynamic instabilities inducing upward motion and nutricline-related lifting, but rather to other hydrographic factors