http://journals.cambridge.org Downloaded: 16 Dec 2013 IP address: 177.72.156.29 Introduction The deep circulation in the world ocean is driven by dense water formation at high latitudes (Gordon 1986). The primary source of deep waters is located in the Southern Ocean, in the Weddell and Ross seas. The Weddell Sea is thought to be the most important source (Jacobs et al. 1970, Foster et al. 1987), but the Ross Sea seems to have a crucial importance in the circulation of the Pacific Ocean (Jacobs et al. 1970, Locarnini 1994). In particular, the Ross Sea is the formation site of two shelf waters (SWs), which constitute an important part of the Antarctic Bottom Waters (AABWs): the High Salinity Shelf Water (HSSW), characterized by salinity values ranging from 34.75 to 34.85 psu (Jacobs et al. 1985), and the Ice Shelf Water (ISW), defined by temperatures below the surface freezing point. The Circumpolar Deep Water (CDW), the most voluminous water mass carried by the Antarctic Circumpolar Current, flows in the Ross Gyre, reaches the continental slope and, at the shelf-break, mixes with the SWs forming the AABWs (Locarnini 1994). The AABWs overflow influences the mass exchange budget at basin scale, and it affects to some extent the global ocean circulation and the climate of the planet. The dense waters, accumulated on the shallow continental shelves, migrate to the shelf-break, spill over the shelf edge and descend the continental slope as a shelf-break gravity current (Whitehead 1987), subject to friction and possibly enhanced by topographic channelling. For mesoscale processes like this density-driven downslope motion or cascading friction is important, because it breaks the constraint of potential vorticity conservation and counteracts the geostrophic tendency for alongslope flow (Huthnance 1995, Shapiro & Hill 1997). The downslope motion entrains ambient water, namely the CDW, reaches a depth where density is the same and spreads off-slope (Huthnance 1995). The role of entrainment is fundamental because without entrainment the cascading event is inhibited by friction (Shapiro & Hill 1997). The downslope mechanisms are important for ocean–continental shelf exchange (Huthnance 1995), in particular for the export of organic carbon, suspended material and dissolved gases such as oxygen, responsible for the ventilation of the deep ocean. Dense overflows were observed in different marginal seas (Huthnance 1995) and around Antarctica in the Weddell Sea (Gill 1973, Foster & Carmack 1976, Foster et al. 1987) and in the Ross Sea (Baines & Condie 1998). This paper presents the first analysis of field observations focussed on describing a possible cascading-like overflow of ISW on the continental slope in the central Ross Sea, where a five day mesoscale experiment was carried out. A description of the water masses present in the investigation area and of the sampling design is given. A classical θ/S analysis was performed in order to investigate the water masses interactions on the shelf-slope edge. Our observations show evidence of the presence of ISW down to 1200 m depth on the slope, suggesting an overflow occurring in very localized areas. Antarctic Science 14 (3): 271–277 (2002) © Antarctic Science Ltd Printed in the UK DOI: 10.1017/S0954102002000068 271 Evidence of dense water overflow on the Ross Sea shelf-break A. BERGAMASCO 1 , V. DEFENDI 1 , E. ZAMBIANCHI 2 and G. SPEZIE 2 1 National Research Council, Institute for the Study of the Dynamics of Large Masses, 1364 S. Polo, I-30125 Venice, Italy 2 Università degli Studi di Napoli “Parthenope”, Institute of Meteorology and Oceanography, via F. Acton 38, I-80133 Naples, Italy Abstract: This paper presents the results of the analysis of hydrological data of a 5-day mesoscale experiment (53 CTD casts) conducted during the XIIIth Italian Expedition to Antarctica (1997–98 cruise) in the framework of the CLIMA (Climatic Longterm Interaction for the Mass balance in Antarctica) Project of the Italian National Programme for Antarctic Research (PNRA). The experiment site was chosen for studying the dense water overflow in relation to the shelf-break in the central Ross Sea, after a large-scale synoptic survey, aimed to detect the general hydrological characteristics of the basin. A classical θ/S analysis was carried out for better understanding of the shelf-slope connection and the interactions between the water masses of this zone: the Circumpolar Deep Water (CDW) coming from the oceanic domain and the Ice Shelf Water (ISW) spreading from the Ross Ice Shelf (RIS) edge. Our results show the evidence of an overflow of dense water, originated on the continental shelf, on the shelf-break. This supercold water signal is found on the continental slope down to 1200 m depth. The shape of this tongue of modified ISW, whose thickness reaches up to 100 m, is very narrow, suggesting that the overflow occurs in very localized areas. Received 15 May 2001, accepted 1 May 2002 Key words: Ice Shelf Water, Ross Sea, shelf-slope interactions, water masses, θ/S analysis