48
E. Zambianchi et al.
around 800 XBTs hâve been launched along the transect connecting Cape Adare
and New Zealand, covered eight times. The most complété température section,
gathered in February 1995 along the section, can be found in [1].
In order to get a picture as consistent as possible with our Lagrangian data, we
examine here an earlier and, unfortunately, less complété section which was performed between December 28,1994, and January 1,1995, i.e. in coincidence with
the January 1995 surface drifter deployment. The data collected by our Sippican
T7 and T5 XBTs hâve been First of ail edited in order to get rid of spikes and obviously spurious values, and then low-pass filtered. The resulting température section is shown in Fig. 6a, and présents very satisfactory evidence of the three fronts
we expect to see across the ACC, which can be reconstructed on the basis of the
définitions summarized in Table 3 of [7]: in particular, the most obvious feature
in our section is the Polar Front, which can be traced in the subsurface minimum
température tongue, characterized by a value of 2.0 °C around 60°30'S; also very
évident is the deep-reaching frontal structure corresponding with the South
Antarctic Front, located around 52°S. As to the Southern ACC Front, its two température indicators (0 < 0 °C along the température minimum at the subsurface
and 0 > 1.8 °C along the température maximum; see also Fig. 10 in [7]) do not
perfectly coincide in our section, but the front can be clearly traced around
64°30'S.
In order to reconstruct the geostrophic flow field perpendicular to the transect, we needed to associate a salinity section to the température one, so as to
infer a picture of the mass field. The only possibility we had, in absence of direct
salinity measurements, was to resort to climatological data. A similar hydrological section, and precisely data collected by different vessels in February 1971 and
in December 1971 along the 180° meridian, was taken from the Southern Océan
Atlas [9], checked so as to make sure that the correspondent thermal field was
similar to that of 1994-1995, and digitized at 50-db depth intervals. The
geostrophic field resulting from the mass field obtained by this procedure using
as the reference level the maximum XBT température measurement depth (750
db) is shown in Fig. 6b. The complexity of the zonal flow discussed in the préviens sections is shown quite evidently: we even hâve reversais of the velocity. This
is an obvious artifact due to the fact that our reference level is too shallow.
Hydrological investigations of the ACC by geostrophic analysis hâve shown that,
even if using 1000 db as a reference level provides meaningful flow patterns [8],
still a realistic reference level is more likely to be, when bottom topography allows
for this, below 2500 m (see, again, [7,8]).
In order to avoid this effect, and to get a doser quantitative agreement with the
Lagrangian surface velocities, a straightforward addition of a barotropic field of
10 cm/s was tried. As to the surface flow, the results are quite satisfactory: the outcoming flow is much doser to the Lagrangian one, and the transport is similar to
that relative to section 23 in [7],presenting two maxima in correspondence of the
Polar (stronger) and ACC (weaker) Front and quite weaker flow in between - no
salinity data were available for the latitudes of the South Antarctic Front.
However, this is obviously a very crude approximation, since it implies a constant
10 cm/s velocity below 750 m; as was seen in Section 3, at 1000 m we still hâve
eastward zonal speeds, but of the order of 4 cm/s.
E. Zambianchi et al.
around 800 XBTs hâve been launched along the transect connecting Cape Adare
and New Zealand, covered eight times. The most complété température section,
gathered in February 1995 along the section, can be found in [1].
In order to get a picture as consistent as possible with our Lagrangian data, we
examine here an earlier and, unfortunately, less complété section which was performed between December 28,1994, and January 1,1995, i.e. in coincidence with
the January 1995 surface drifter deployment. The data collected by our Sippican
T7 and T5 XBTs hâve been First of ail edited in order to get rid of spikes and obviously spurious values, and then low-pass filtered. The resulting température section is shown in Fig. 6a, and présents very satisfactory evidence of the three fronts
we expect to see across the ACC, which can be reconstructed on the basis of the
définitions summarized in Table 3 of [7]: in particular, the most obvious feature
in our section is the Polar Front, which can be traced in the subsurface minimum
température tongue, characterized by a value of 2.0 °C around 60°30'S; also very
évident is the deep-reaching frontal structure corresponding with the South
Antarctic Front, located around 52°S. As to the Southern ACC Front, its two température indicators (0 < 0 °C along the température minimum at the subsurface
and 0 > 1.8 °C along the température maximum; see also Fig. 10 in [7]) do not
perfectly coincide in our section, but the front can be clearly traced around
64°30'S.
In order to reconstruct the geostrophic flow field perpendicular to the transect, we needed to associate a salinity section to the température one, so as to
infer a picture of the mass field. The only possibility we had, in absence of direct
salinity measurements, was to resort to climatological data. A similar hydrological section, and precisely data collected by different vessels in February 1971 and
in December 1971 along the 180° meridian, was taken from the Southern Océan
Atlas [9], checked so as to make sure that the correspondent thermal field was
similar to that of 1994-1995, and digitized at 50-db depth intervals. The
geostrophic field resulting from the mass field obtained by this procedure using
as the reference level the maximum XBT température measurement depth (750
db) is shown in Fig. 6b. The complexity of the zonal flow discussed in the préviens sections is shown quite evidently: we even hâve reversais of the velocity. This
is an obvious artifact due to the fact that our reference level is too shallow.
Hydrological investigations of the ACC by geostrophic analysis hâve shown that,
even if using 1000 db as a reference level provides meaningful flow patterns [8],
still a realistic reference level is more likely to be, when bottom topography allows
for this, below 2500 m (see, again, [7,8]).
In order to avoid this effect, and to get a doser quantitative agreement with the
Lagrangian surface velocities, a straightforward addition of a barotropic field of
10 cm/s was tried. As to the surface flow, the results are quite satisfactory: the outcoming flow is much doser to the Lagrangian one, and the transport is similar to
that relative to section 23 in [7],presenting two maxima in correspondence of the
Polar (stronger) and ACC (weaker) Front and quite weaker flow in between - no
salinity data were available for the latitudes of the South Antarctic Front.
However, this is obviously a very crude approximation, since it implies a constant
10 cm/s velocity below 750 m; as was seen in Section 3, at 1000 m we still hâve
eastward zonal speeds, but of the order of 4 cm/s.