Introduction The Ross Sea is a particular oceanographic environment where important water mass formation and transformation processes occur at several temporal and spatial scales. Interest in the processes occurring in the Ross Sea has increased in recent years largely due to an appreciation of the climatic relevance of this area, which plays a major role in the production of dense water at high southern latitudes (Jacobs et al. 1985, Jacobs & Comiso 1989, Locarnini 1994, Jacobs & Giulivi 1998, 1999, Hofmann & Klinck 1998). In fact, most of the dense deep water production in the Southern Ocean originates from the continental shelves of Antarctica. Flowing from oceanic regions next to the continental shelf they mix with the ambient water (typically less dense) as they flow down to the continental slope before reaching their equilibrium depth. If they reach the bottom of the slope they may become Antarctic Bottom Water (AABW) (Jacobs et al. 1970, Jacobs 1991, Hoffman & Klinck 1998, Bindoff et al. 2000). Many details about dense water formation on the Antarctic continental slope are still obscure, due to the difficulty in obtaining an adequate temporal and spatial resolution for atmospheric and oceanic phenomena related to the oceanic convection in the Southern Oceans from in situ data (a useful discussion on possible downslope mechanisms can be found in Baines & Condie 1998). The Ross Sea, located in the Pacific sector of the Antarctic continental shelf, is characterized by a rather irregular topography (Fig. 1) with several reliefs and depressions, some of which are deeper than the shelf break depth (about 700 m deep). The Ross Ice Shelf (RIS), a broad ice cover which extends over nearly half of the continental shelf, marks the southern limit of the Ross Sea only for the near surface water layers, while deeper waters can flow freely between the bottom and the floating ice shelf (Jacobs et al. 1979). Reaching the continental shelf, the Circumpolar Deep Water (CDW) moves upward and mixes with the shelf waters forming the modified CDW (MCDW). The MCDW is the only water mass flowing over the continental shelf and therefore plays a fundamental role in the renewal of shelf waters and in the total heat budget of the Ross Sea (Budillon et al. 2000). The dense shelf waters in the Ross Sea are generally produced during winter, when the upper layers cool and freeze, releasing part of their saline content and, therefore, increasing the salinity of the subsurface waters. This process is frequently enhanced by the occurrence of strong katabatic winds in the Terra Nova Bay (TNB) Polynya, where a large amount of High Salinity Shelf Waters (HSSW) is therefore formed (Jacobs et al. 1985). Part of the HSSW is known to flow northward, towards the shelf break along the western sector of the Ross Sea (Budillon et al. 1999), and to take part in the formation of the AABW. Another branch moves southward under the Antarctic Science 15 (1): 25–30 (2003) © Antarctic Science Ltd Printed in the UK DOI: 10.1017/S0954102003001020 25 A model for the spreading and sinking of the Deep Ice Shelf Water in the Ross Sea ANGELO RUBINO 1,2 , GIORGIO BUDILLON 3 , STEFANO PIERINI 3 and GIANCARLO SPEZIE 3 1 Institut für Meereskunde, Universität Hamburg, Troplowitzstrasse 7, D-22529 Hamburg, Germany rubino@ifm.uni-hamburg.de 2 Dipartimento di Scienze Ambientali, Università Ca’Foscari, Dorsoduro 2137, 30123 Venezia, Italy 3 Istituto di Meteorologia e Oceanografia, Università di Napoli “Parthenope”, Via De Gasperi 5, 80127 Napoli, Italy Abstract: Spreading and sinking of the Deep Ice Shelf Water (DISW) in the Ross Sea are analysed using in situ observations and the results of a nonlinear, reduced gravity, layered numerical model, which is able to simulate the motion of a bottom trapped current over realistic topography. The model is forced by prescribing thickness and density of the DISW layer at the southern model boundary as well as ambient density stratification above the DISW layer. This density structure is imposed using hydrographic data acquired by the Italian PNRA-CLIMA project. In the model water of the quiescent ambient ocean is allowed to entrain in the active deep layer due to a simple entrainment parameterization. The importance of forcing the model with a realistic ambient density is demonstrated by carrying out a numerical simulation using an idealized ambient density. In a more realistic simulation the path and the density structure of the DISW vein flowing over the Challenger Basin are obtained and are found to be in good agreement with data. It is found that entrainment, which is particularly active in regions of strong topographic variation, significantly influences the pattern followed by the DISW layer. The evolution of the DISW layer beyond the continental shelf, i.e., in a region where the paucity of experimental data does not allow for a detailed description of the deep ocean dynamics, is also investigated. Received 13 February 2002, accepted 24 September 2002 Key words: Antarctica, dense water formation, oceanography, water mass transformations