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S. Gremes Cordero, E. Salusti
Particularly good candidates for steering dense bottom water flows are submarine canyons and deep troughs. Quadfasel et al. [10] used a sequence of
hydrographie sections to follow a bottom water plume down the Storfjord
Trough into Fram Strait, where the plume entered the deep basin. Another good
example is represented by Barrow Canyon, which runs parallel to the northern
coast of Alaska [11].
Surprisingly few studies hâve attempted to model the formation and offshore
transport of dense shelf water in polar seas. Moreover, in general, theoretical analyses of density currents over a regular slope are essential for a better understanding
of the behaviour of various natural phenomena [12-14]. For such currents bottom
érosion is a particular complication: the bottom stress entrains bottom sédiment
and also suspended sédiment is constantly settling out of the current.
For océans, such flows hâve been studied by many authors [1,2,15-18] to find
the équations of motion spécifie to these density-turbidity currents [16, 20, 21].
Of particular interest is the analysis by Parker et al. [16] referring to a simple
model of a continuous steady flow where the exchange of sédiment from the bed
is allowed. Stacey and Bowen [17, 18] make a particularly careful analysis of the
vertical structure of these currents, also in comparison with the few known
marine cases [19]. Salusti [3] considers the hydrodynamic instabilities for a twofluid problem as discussed in Caserta et al. [2], and has been able to take into
account both space and time variability of such density-turbidity currents.
On theoretical grounds our analysis consists of two parts: first, the study of
the quasi-steady main flow [22] and, second, the instabilities of these currents,
which is the main purpose of the présent analysis. We find that these nonlinear
oscillations can affect the bottom current in a catastrophic way, giving rise to an
“ignition” like those of classical turbidity currents. A novel criterion discriminating between regular flows and catastrophic events is discussed below. In particular, in Section 5, we apply these considérations to a cold dense water
observed in the shelf break near Cape Adare and to dense salty shelf water on
the oceanic bottom off Victoria Land.
2 Deep Océan Currents in the Ross Sea and Victoria Land Shelf
The study zone in the Ross Sea, as shown in Fig. 1, is located off Cape Adare,
around 72° S, 173° E. The bottom topography is rather irregular; the shelf of about
500-m depth is isolated from the shore by a deep canyon; the océan bottom can
reach 2000 m in depth. It is of interest that the glaceologic and météorologie conditions of Terra Nova Bay and the adjacent basins differ from most of Antarctica.
During the warm season, indeed, it is not difficult to find the océan free of ice,
even for long periods. Consequently, in the Ross Sea catabatic winds dramatically affect the overall marine situation, giving rise to vortices, meanders and other
two-dimensional turbulent phenomena.
In particular, Jacobs and Comiso [23] discuss the formation of polynyas associated with violent air-sea exchanges, such as those due to catabatic winds.
Interestingly, they explain the maintenance of the Pennell and Ross Sea Passage
polynyas (Fig. 1) near the continental shelf break (our zone of interest) as being
S. Gremes Cordero, E. Salusti
Particularly good candidates for steering dense bottom water flows are submarine canyons and deep troughs. Quadfasel et al. [10] used a sequence of
hydrographie sections to follow a bottom water plume down the Storfjord
Trough into Fram Strait, where the plume entered the deep basin. Another good
example is represented by Barrow Canyon, which runs parallel to the northern
coast of Alaska [11].
Surprisingly few studies hâve attempted to model the formation and offshore
transport of dense shelf water in polar seas. Moreover, in general, theoretical analyses of density currents over a regular slope are essential for a better understanding
of the behaviour of various natural phenomena [12-14]. For such currents bottom
érosion is a particular complication: the bottom stress entrains bottom sédiment
and also suspended sédiment is constantly settling out of the current.
For océans, such flows hâve been studied by many authors [1,2,15-18] to find
the équations of motion spécifie to these density-turbidity currents [16, 20, 21].
Of particular interest is the analysis by Parker et al. [16] referring to a simple
model of a continuous steady flow where the exchange of sédiment from the bed
is allowed. Stacey and Bowen [17, 18] make a particularly careful analysis of the
vertical structure of these currents, also in comparison with the few known
marine cases [19]. Salusti [3] considers the hydrodynamic instabilities for a twofluid problem as discussed in Caserta et al. [2], and has been able to take into
account both space and time variability of such density-turbidity currents.
On theoretical grounds our analysis consists of two parts: first, the study of
the quasi-steady main flow [22] and, second, the instabilities of these currents,
which is the main purpose of the présent analysis. We find that these nonlinear
oscillations can affect the bottom current in a catastrophic way, giving rise to an
“ignition” like those of classical turbidity currents. A novel criterion discriminating between regular flows and catastrophic events is discussed below. In particular, in Section 5, we apply these considérations to a cold dense water
observed in the shelf break near Cape Adare and to dense salty shelf water on
the oceanic bottom off Victoria Land.
2 Deep Océan Currents in the Ross Sea and Victoria Land Shelf
The study zone in the Ross Sea, as shown in Fig. 1, is located off Cape Adare,
around 72° S, 173° E. The bottom topography is rather irregular; the shelf of about
500-m depth is isolated from the shore by a deep canyon; the océan bottom can
reach 2000 m in depth. It is of interest that the glaceologic and météorologie conditions of Terra Nova Bay and the adjacent basins differ from most of Antarctica.
During the warm season, indeed, it is not difficult to find the océan free of ice,
even for long periods. Consequently, in the Ross Sea catabatic winds dramatically affect the overall marine situation, giving rise to vortices, meanders and other
two-dimensional turbulent phenomena.
In particular, Jacobs and Comiso [23] discuss the formation of polynyas associated with violent air-sea exchanges, such as those due to catabatic winds.
Interestingly, they explain the maintenance of the Pennell and Ross Sea Passage
polynyas (Fig. 1) near the continental shelf break (our zone of interest) as being