58
A. Simone et al.
the ACC eddy variability is not affected by large seasonal and interannual variation, even if the main forcing of ACC is the wind stress. This is in agreement with
the hypothesis that ACC is a partial topographical controlled. The géographie distribution of SLV, in which most of the high variability values are downstream of
topographie features, confirms this hypothesis. The increased rôle of bottom
topography would reduce also the rôle played by baroclinie instabilities in the formation of eddies released by the ACC [15].
Consequently, we expect to see the major eddy System in the proximity of
these areas of high variability in the SLA maps. Nevertheless, a considération
must be added before looking at the SLA maps that the internai Rossby radius,
which is supposed to be the preferred scale for eddies, decreases going southward.
At the latitudes of Antarctica it is about 10 km, reducing the possibilities of a reliable SLA map reconstruction using actual altimeter data.
Interesting known features described above are easily recognizable in the SLA
maps computed every 10 days using 74 cycles of T/P altimeter data. Incidentally,
we found it useful to show in this paper only some of these maps, corresponding
to cycles 12 (austral summer 1993, Fig. 5a) and 33 (austral winter 1993, Fig. 5b).
Also, it is worthwhile underlining that from altimétrie data, since the geoid surface is not adequately known yet, we can get only information about déviation
from the mean sea surface level.
Traces of large eddy-like structure are évident in ail the high energetic areas.
The presence of warm and cold rings is shown on the maps white and black spots
respectively, especially in the sector 10-50°E and 30-60°W, with a typical diameter
of 150-200 km and sea level amplitude relative to the background <-30 cm (cold
rings) and >30 cm (warm rings). These results are in agreement with the resuit
obtained by Gouretsky and Danilov [19] using Geosat altimeter data.
T/P altimetry data indicate that in the South Indian Océan, most of the warmcore rings remain detached from the ACC moving southward, confined to a rather
narrow corridor of about 400 km wide. This fact implies poleward transport of
heat and momentum by the rings just in these areas.
In the Southwest Atlantic Océan eddies can be noted in the area of the Malvinas
Current and Brazil Current confluence. Here annual and semiannual variability
are mostly due to the méridional variation of the Malvinas Current pénétration,
while, on scales of about 150 days and 300 km, the Brazil Current oscillation produces rings, mainly with warm core. Indeed, these are scales larger than those of
other Southern areas.
However, analysis of the SLA maps reveals a quite clear seasonal variation of
the warm-rings activities; indeed, it improves during the summer and it might be
a matter for future investigations to understand the coupling with the régional
climatological régime.
4 Crossover Analysis
I he distribution of anisotropic eddy kinetic energy and horizontal eddy momentum flux has been diffïcult to détermine from surface altimeter data. This is due
to the problem of resolving north/east directional components of the surface
A. Simone et al.
the ACC eddy variability is not affected by large seasonal and interannual variation, even if the main forcing of ACC is the wind stress. This is in agreement with
the hypothesis that ACC is a partial topographical controlled. The géographie distribution of SLV, in which most of the high variability values are downstream of
topographie features, confirms this hypothesis. The increased rôle of bottom
topography would reduce also the rôle played by baroclinie instabilities in the formation of eddies released by the ACC [15].
Consequently, we expect to see the major eddy System in the proximity of
these areas of high variability in the SLA maps. Nevertheless, a considération
must be added before looking at the SLA maps that the internai Rossby radius,
which is supposed to be the preferred scale for eddies, decreases going southward.
At the latitudes of Antarctica it is about 10 km, reducing the possibilities of a reliable SLA map reconstruction using actual altimeter data.
Interesting known features described above are easily recognizable in the SLA
maps computed every 10 days using 74 cycles of T/P altimeter data. Incidentally,
we found it useful to show in this paper only some of these maps, corresponding
to cycles 12 (austral summer 1993, Fig. 5a) and 33 (austral winter 1993, Fig. 5b).
Also, it is worthwhile underlining that from altimétrie data, since the geoid surface is not adequately known yet, we can get only information about déviation
from the mean sea surface level.
Traces of large eddy-like structure are évident in ail the high energetic areas.
The presence of warm and cold rings is shown on the maps white and black spots
respectively, especially in the sector 10-50°E and 30-60°W, with a typical diameter
of 150-200 km and sea level amplitude relative to the background <-30 cm (cold
rings) and >30 cm (warm rings). These results are in agreement with the resuit
obtained by Gouretsky and Danilov [19] using Geosat altimeter data.
T/P altimetry data indicate that in the South Indian Océan, most of the warmcore rings remain detached from the ACC moving southward, confined to a rather
narrow corridor of about 400 km wide. This fact implies poleward transport of
heat and momentum by the rings just in these areas.
In the Southwest Atlantic Océan eddies can be noted in the area of the Malvinas
Current and Brazil Current confluence. Here annual and semiannual variability
are mostly due to the méridional variation of the Malvinas Current pénétration,
while, on scales of about 150 days and 300 km, the Brazil Current oscillation produces rings, mainly with warm core. Indeed, these are scales larger than those of
other Southern areas.
However, analysis of the SLA maps reveals a quite clear seasonal variation of
the warm-rings activities; indeed, it improves during the summer and it might be
a matter for future investigations to understand the coupling with the régional
climatological régime.
4 Crossover Analysis
I he distribution of anisotropic eddy kinetic energy and horizontal eddy momentum flux has been diffïcult to détermine from surface altimeter data. This is due
to the problem of resolving north/east directional components of the surface