The Ross Sea is a large continental sea in the Pacific sector of the Southern Ocean. It is
characterized by a rather irregular topography with several relieves and depressions, some of
which deeper than the platform edge (about 700 m deep). The dense Ross Sea shelf waters
are generally formed during the austral winter, when the upper layers cool and freeze,
releasing part of their saline content and so increasing the salinity (and the density) of the
subsurface waters. Such dense waters flow near the bottom and, reaching the continental shelf break, ventilate the deep ocean.
Recently, transient tracers such as chlorofluorocarbons (CFCs) have been used in addition to
hydrological parameters to study the formation and the circulation of deep and bottom waters.
In this paper, CFC-11, CFC-12 and CFC-113 data,collected during the Italian cruise carried
on in the 2000-01 austral summer, are presented and discussed in the context of water
masses distribution and mixing processes. In the Ross Sea shelf area, an increasing CFCs
concentration has been observed in the bottom layer, proving the presence of recently
ventilated water masses. Low CFCs content at intermediate depths shows, instead, the
evidence of intrusions of older waters from offshore.
Based on actual knowledge of the phenomenology, following Trumbore’s approach and
taking into account the CFC fluxes from the atmosphere, a multi-box model was set up. The
main goal is the estimate of the time scales involved in the major processes such as the High
Salinity Shelf Water (HSSW) formation and spreading, the Ross Sea shelf waters/Ross Ice
Shelf cavity interactions and the ventilation of deep waters.
We use temperature, salinity and CFC concentrations to asses the initial conditions and to
evaluate the dilution of shelf waters, as they move from the origin areas to the shelf break.
The estimation of the age and mass transports are presented and analysed. The results show
the importance of the mixed layer entrainment at the beginning of the winter and the
ventilation in the polynya area during winter. Moreover, the observed freshening of the
HSSW core can be related to an increasing of the residence time found with our
calculations. A comparison with previous data acquired in the same region and their
previous modelling is also done. Our data and estimations confirm previous analysis as well
as the mechanism involved in the dynamics. |
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