Surface currents in the Bransfield and Gerlache Straits, Antarctica

We used 39 tracks of mixed layer drifters deployed during the period from November 1988 to January 1990 to study the surface flow characteristics in the Bransfield and Gerlache Straits, Antarctica. The results revealed both the Gerlache Strait Current and the Bransfield Strait Current, which flows along the deep channel of the Gerlache Strait, northeastward to the southern continental margin of the South Shetland Islands following the 750 m isobath. The observed strongest sustained daily mean current reached approximately 40 cm s−1 in the Bransfield Strait and was confined to the shelf break south of the South Shetland Islands. The computed acceleration of drifters in the Bransfield Strait Current indicates the southward transversal component limits drifters from approaching isobaths shallower than 750 m. The southern side of the Current is rich in cyclonic eddies. Drifters spun off and circulated in cyclonic eddies over deep basins. The residence time of a water parcel in the current is approximately 10–20 days. Anticyclonic circulations were observed around Tower, Hoseason and Liege Islands, and long residence times were found for drifters in shallows and bays of up to 70 days. Results also indicate the Gerlache Strait water can extend along the shelf of the Antarctic peninsula to Tower Island, where it meets the southewestward Weddell Sea water. Most of the Gerlache Strait water exits northward and enters the Bransfield Strait Current. It Spins off and mixes with other waters in the Bransfield Strait. Several long tracks indicated the existence of a cyclonic large circulation gyre in the Bransfield Strait during the ice-free condition. The circulation patterns in both Bransfield and Gerlache Straits change seasonally. The analysis of force balance indicates that currents and eddies are geostrophic though the ageostrophic components are important to maintain currents and form eddies. This composition of eddies and currents provides ideal physical settings for zooplankton growth in eddies and bays and zooplankton dispersion in currents.