Hydrography and circulation of the west Antarctic Peninsula continental shelf

The water mass structure and circulation of the continental shelf waters we st of the Antarctic Peninsula are described from hydrographic observations made in March-May 1993. The observations cover an area that extends 900 km alongshore and 200 km offshore and represent the most extensive hydrographi c data set currently available for this region. Waters above 100-150 m are composed of Antarctic Surface Water and its end member Winter Water. Below the permanent pycnocline is a modified version of Circumpolar Deep Water, w hich is a cooled and freshened version of Upper Circumpolar Deep Water. The distinctive signature of cold and salty water from the Bransfield Strait i s found at some inshore locations: but there is little indication of signif icant exchange between Bransfield Strait and the west Antarctic Peninsula s helf. Dynamic topography at 200 m relative to 400 m indicates that the baro clinic circulation on the shelf is composed of a large, weak, cyclonic gyre , with sub-gyres at the northeastern and southwestern ends of the shelf. Th e total transport of the shelf gyre is 0.15 Sv, with geostrophic currents o f order 0.01 m s(-1). A simple model that balances across-shelf diffusion o f heat and salt from offshore Upper Circumpolar Deep Water with vertical di ffusion of heat and salt across the permanent pycnocline into Winter Water is used to explain the formation of the modified Circumpolar Deep Water tha t is found on the shelf. Model results show that the observed thermohaline distributions across the shelf can be maintained with a coefficient of vert ical diffusion of 10(-4) m(2) s(-1) and horizontal diffusion coefficients f or heat and salt of 200 and 1200 m(2) s(-1), respectively. When the effects of double diffusion are included in the model, the required horizontal dif fusion coefficients for heat and salt are 200 and 400 m(2) s(-1), respectiv ely. (C) 1999 Elsevier Science Ltd. All rights reserved.