Water masses and 200 m relative geostrophic circulation in the western Bransfield Strait region

Four hydrographic surveys to 200 db of 16–20 nm horizontal resolution carried out in the vicinity of western Bransfield Strait in the period November 1986 to March 1987 are used to provide a description of water masses and relative geostrophic circulation in the survey area. The water mass structure is characterized by two frontal structures with a complex vertical and horizontal interleaving of water from the Southern Drake Passage, Orkney Island shelf, Bellingshausen Sea and the shelf of Antarctic Peninsula. In the northwestern segment of the survey area, intrusions of Drake Passage water into the Bransfield are found beneath a frontal zone at a 100-m depth between Snow and Smith Islands. In the southeastern segment fresh, warm water from the Gerlache Straits flows over the cold salty water from the Antarctic Peninsula. A geostrophically balanced relative flow, termed the Bransfield Current, transports water along a second front between Bellingshausen Sea and Antarctic Peninsula from the Bellingshausen Sea and Gerlache Straits to the northeast. A cyclonic gyre exists around Low Island. In this current, the 200 db relative geostrophic mean current maximum is 8 cm s−1 and the geostrophic eddy kinetic energy is 18–55 cm2 s−2, being strongest near the flow axis. Based on estimates of other mid-latitude ocean current areas with similar mean flows and eddy energies, the single particle diffusion coefficient is 1–3 × 107 cm2 s−1. This circulation and diffusion could transport particles from the biologically rich Gerlache Strait to Livingston Island and the Drake Passage in 15–30 days, while particles can spread an average ± 50 km to either side of a track traced out by the mean flow.