A. Simone et al.
52
hâve a radial accuracy of 3-4 cm rms [1]. For this reason T/P is the most accurate
altimeter satellite flown up to date. In this paper we analyzed the T/P dataset relative to the period October 1992-September 1994 in order to study the mesoscale
variability of the Southern Océan.
The Antarctic Circumpolar Current (ACC) is a strong eastward flow associated
with a région of transition between surface water masses with antarctic and subantarctic characteristics [2]. The position of the flow is seen to vary considerably
in latitude, covering a band between 40° and 60°S ail around Antarctica. The ACC
is the strongest current in the World Océan. The estimated transport in the Drake
Passage, where there are no barriers to the zonal flow, is about 130 Sv (1 Sv=106
m3/s). In situ measurements and numerical models show that the strong persistent winds of the polar cyclone provide the main surface wind stress which drives
the ACC. Nevertheless, the rôle of the coupling and the relative importance of the
wind and thermohaline forcing has not been understood yet.
An important characteristic of the ACC is the concentration of the transport in
narrow jets associated with frontal zones. This “zonation” of the ACC is clearly
visible along the cores of the current, with pronounced gradients of density and
other characteristics such as température (T),salinity (S) and nutrients [2]. Going
poleward, we can identify three fronts separating three water mass zones (Fig. 1);
the Subtropical Front (STF), the Subantarctic Zone (SAZ), the Subantarctic Front
(SAF), the Polar Front Zone (PFZ), the Polar Front (PF), and the Antarctic Zone
(AZ).
In the Drake Passage the fronts are rather narrow being about 50 km wide [3];
at the other longitudes, in open océan, they tend to be more widely separated and
about 100-150 km wide (i.e. 2-3 times the local deformation radius). The maximum geostrophic surface speeds in the ACC cores in Drake Passage are 25-45
cm/s.
Munk and Palmen [4] suggest that the main sink for momentum seems to be
the topographie form stress. The current is for most of its path in near Sverdrup
balance (southward drift); in some areas it is steered by topography (following
contours of f/H where f is the coriolis parameter and H the depth) and in most
cases (e.g. the South America shelf) the current flows northward, moreover balancing the deep océan Sverdrup southward drift.
Dissipation, mostly latéral, seems to play a rôle only in Drake Passage. Following
Hughes and Killworth [5], the bottom pressure torque, considered as a second
order process, is concentrated at the points where the current leaves the isobaths
toward the deep basins, mainly the eastern South America shelf, the Campbell
plateau, and the Pacific-Antarctic Ridge. So most of the global dynamics is
explained by localized processes. Almost the same conclusions hâve been reached
by Wells and de Cuevas [6] for the ACC vertically averaged vorticity budget.
In this dynamic scénario, strong eddy field becomes necessary because it could
represent the way in which the surface wind stress is transferred to the bottom,
via the action of the interfacial form stress [7]. The nature of the eddy-mean flow
interaction is still unclear. QG eddy-resolving experiments with zonal flows [7] or
zonal-averaged balances from the Fine Resolution Antarctic Model (FRAM) [8]
found that transient eddies accelerate the flow. However, again from FRAM, if the
balances are carried along the mean path of the current, the net effect is a drag-
52
hâve a radial accuracy of 3-4 cm rms [1]. For this reason T/P is the most accurate
altimeter satellite flown up to date. In this paper we analyzed the T/P dataset relative to the period October 1992-September 1994 in order to study the mesoscale
variability of the Southern Océan.
The Antarctic Circumpolar Current (ACC) is a strong eastward flow associated
with a région of transition between surface water masses with antarctic and subantarctic characteristics [2]. The position of the flow is seen to vary considerably
in latitude, covering a band between 40° and 60°S ail around Antarctica. The ACC
is the strongest current in the World Océan. The estimated transport in the Drake
Passage, where there are no barriers to the zonal flow, is about 130 Sv (1 Sv=106
m3/s). In situ measurements and numerical models show that the strong persistent winds of the polar cyclone provide the main surface wind stress which drives
the ACC. Nevertheless, the rôle of the coupling and the relative importance of the
wind and thermohaline forcing has not been understood yet.
An important characteristic of the ACC is the concentration of the transport in
narrow jets associated with frontal zones. This “zonation” of the ACC is clearly
visible along the cores of the current, with pronounced gradients of density and
other characteristics such as température (T),salinity (S) and nutrients [2]. Going
poleward, we can identify three fronts separating three water mass zones (Fig. 1);
the Subtropical Front (STF), the Subantarctic Zone (SAZ), the Subantarctic Front
(SAF), the Polar Front Zone (PFZ), the Polar Front (PF), and the Antarctic Zone
(AZ).
In the Drake Passage the fronts are rather narrow being about 50 km wide [3];
at the other longitudes, in open océan, they tend to be more widely separated and
about 100-150 km wide (i.e. 2-3 times the local deformation radius). The maximum geostrophic surface speeds in the ACC cores in Drake Passage are 25-45
cm/s.
Munk and Palmen [4] suggest that the main sink for momentum seems to be
the topographie form stress. The current is for most of its path in near Sverdrup
balance (southward drift); in some areas it is steered by topography (following
contours of f/H where f is the coriolis parameter and H the depth) and in most
cases (e.g. the South America shelf) the current flows northward, moreover balancing the deep océan Sverdrup southward drift.
Dissipation, mostly latéral, seems to play a rôle only in Drake Passage. Following
Hughes and Killworth [5], the bottom pressure torque, considered as a second
order process, is concentrated at the points where the current leaves the isobaths
toward the deep basins, mainly the eastern South America shelf, the Campbell
plateau, and the Pacific-Antarctic Ridge. So most of the global dynamics is
explained by localized processes. Almost the same conclusions hâve been reached
by Wells and de Cuevas [6] for the ACC vertically averaged vorticity budget.
In this dynamic scénario, strong eddy field becomes necessary because it could
represent the way in which the surface wind stress is transferred to the bottom,
via the action of the interfacial form stress [7]. The nature of the eddy-mean flow
interaction is still unclear. QG eddy-resolving experiments with zonal flows [7] or
zonal-averaged balances from the Fine Resolution Antarctic Model (FRAM) [8]
found that transient eddies accelerate the flow. However, again from FRAM, if the
balances are carried along the mean path of the current, the net effect is a drag-