The analysis of some high-resolution hydrological data sets acquired during the 1997, 1998 and
2001 austral summers across the Ross Sea continental shelf break are here presented. The main
focus of these cruises carried out in the framework of the Italian National Antarctic Program was
the investigation of the overflow of the dense waters originated inside the Ross Sea. Such dense
waters, already detected in the same area during previous cruises, flow near the bottom and,
reaching the continental shelf break, ventilate the deep ocean. Two Antarctic continental shelf
mechanisms can originate dense and deep waters. The former mechanism involves the formation,
along the Victoria Land coasts, of a dense and saline water mass, the High Salinity Shelf Water
(HSSW). The HSSW formation is linked to the rejection of salt into the water column as sea ice
freezes, especially during winter, in the polynya areas, where the ice is continuously pushed
offshore by the strong katabatic winds. The latter one is responsible of the formation of a supercold
water mass, the Ice Shelf Water (ISW). The salt supplied by the HSSW recirculated below the Ross
Ice Shelf, the latent heat of melting and the heat sink provided by the Ross Ice Shelf give rise to
plumes of ISW, characterized by temperatures below the sea-surface freezing point. The dense shelf
waters migrate to the continental shelf-break, spill over the shelf edge and descend the continental
slope as a shelf-break gravity current, subject to friction and possibly enhanced by topographic
channelling. Friction, in particular, breaks the constraint of potential vorticity conservation,
counteracting the geostrophic tendency for along slope flow. The density-driven downslope motion
or cascading entrains ambient water, namely the lower layer of the CDW, reaches a depth where
density is the same and spreads off-slope. In fact, the cascading event is inhibited by friction
without entrainment. The downslope processes are important for the ocean and climate system
because they play a crucial role in the formation of oceanic deep water responsible for
ocean/continental shelf exchange of organic carbon, suspended material and dissolved gases around
Antarctica. In this context, this work presents the analysis of the 1997 and 2001 high-resolution
surveys carried out in the western Ross Sea near Cape Adare, where the HSSW flows down the
continental slope. The third study area was investigated during the 1998 survey of the Italian
National Programme for Antarctic Research of the CLIMA Project, in order to follow the ISW
overflow path at the shelf break in the central Ross Sea. A 3D primitive equation model was also
implemented as a first step in the construction of a high-resolution process study model to explore
the dynamical constraints involved in the downslope motion. |
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