74
A. Russo et al.
this signal occurs 60-80 m from the surface, but a weaker signature can be
observed also in the surface layer. Just north of the PF, other water bodies are separating from it, more evidently in February. Comparing these two sections with
the two corresponding 1995 ones, we observe that the Tmin layer in January and
February 1996 is thicker than in January and February 1995, probably due to a
minor surface warming during summer 1996.
5 Discussion and Conclusions
Main fronts between New Zealand and Antarctica hâve been individuated on the
basis of thermal structure, with the exception of STF (too close to the New
Zealand coast in this area) and Bdy (difficult to locate using température data
only). According to Orsi et al. [3], the Bdy marks the séparation between the ACC
and the subpolar régime and can be individuated as the Southern limit of the
Upper Circumpolar Deep Water (UCDW); in this area, the maps of Orsi et al.
report the Bdy very close to the Southern front. We detected the Southern front
on the northern flanks of the Southeast Indian and Pacific-Antarctic Ridges, a
position in good agreement with Orsi et al., so we can consider these two ridges
as the northern limit of the subpolar area.
We observed that the SAF is not uniquely individuated in our sections, so, following a définition of a similar situation observed sporadically in the Tasmanian
sector [4], we hypothesize in the New Zealand area a splitting of the SAF into a
“northern” and a “southern” part. These two fronts being persistent, at least for
what concerning our six sections, we propose the names Northern SubAntarctic
Front (NSAF) and Southern SubAntarctic Front (SSAF) respectively. The NSAF is
defined by a température variation between 6.5 and 4.5 °C at 400 m and the SSAF
by a température variation between 4.5 and 3.5 °C at the same depth.
We observed NSAF just south of the Campbell Plateau, while SSAF is located
around 58° S, arriving at 56° 50’ S in November 1994 and at 59° 50’ S in December
1994-January 1995, where the PF was in November. The PF appears to retreat its
position southwards, going from austral spring to summer, presumably related to
the surface layer warming caused by the increasing solar heating. The Southern
front shows the most stable location, being always on the northern flanks of the
Pacific-Antarctic and Southeast Indian Ridges.
We observe many cold features between the NSAF and PF, and most of them
could be detached from the PF at around 165° S, where the Southeast Indian
Ridge and the Campbell Plateau form a relatively narrow passage in which,
according to the Belkin and Gordon fronts location map, PF and SAF are very
close. Then these cold eddies and/or meanders would continue to propagate
toward north-east, some of them along the Campbell Plateau continental slope.
The hypothesis that these features were detached from the PF is supported by the
évident subsurface température minima observed between 200 and 300 m in
most of them (fourth and sixth sections in particular, where they show up to 1000
km north of the PF). These mesoscale phenomena transfer cold water in the
upper layer toward lower latitudes and could play a relevant rôle in the global
heat budget.
A. Russo et al.
this signal occurs 60-80 m from the surface, but a weaker signature can be
observed also in the surface layer. Just north of the PF, other water bodies are separating from it, more evidently in February. Comparing these two sections with
the two corresponding 1995 ones, we observe that the Tmin layer in January and
February 1996 is thicker than in January and February 1995, probably due to a
minor surface warming during summer 1996.
5 Discussion and Conclusions
Main fronts between New Zealand and Antarctica hâve been individuated on the
basis of thermal structure, with the exception of STF (too close to the New
Zealand coast in this area) and Bdy (difficult to locate using température data
only). According to Orsi et al. [3], the Bdy marks the séparation between the ACC
and the subpolar régime and can be individuated as the Southern limit of the
Upper Circumpolar Deep Water (UCDW); in this area, the maps of Orsi et al.
report the Bdy very close to the Southern front. We detected the Southern front
on the northern flanks of the Southeast Indian and Pacific-Antarctic Ridges, a
position in good agreement with Orsi et al., so we can consider these two ridges
as the northern limit of the subpolar area.
We observed that the SAF is not uniquely individuated in our sections, so, following a définition of a similar situation observed sporadically in the Tasmanian
sector [4], we hypothesize in the New Zealand area a splitting of the SAF into a
“northern” and a “southern” part. These two fronts being persistent, at least for
what concerning our six sections, we propose the names Northern SubAntarctic
Front (NSAF) and Southern SubAntarctic Front (SSAF) respectively. The NSAF is
defined by a température variation between 6.5 and 4.5 °C at 400 m and the SSAF
by a température variation between 4.5 and 3.5 °C at the same depth.
We observed NSAF just south of the Campbell Plateau, while SSAF is located
around 58° S, arriving at 56° 50’ S in November 1994 and at 59° 50’ S in December
1994-January 1995, where the PF was in November. The PF appears to retreat its
position southwards, going from austral spring to summer, presumably related to
the surface layer warming caused by the increasing solar heating. The Southern
front shows the most stable location, being always on the northern flanks of the
Pacific-Antarctic and Southeast Indian Ridges.
We observe many cold features between the NSAF and PF, and most of them
could be detached from the PF at around 165° S, where the Southeast Indian
Ridge and the Campbell Plateau form a relatively narrow passage in which,
according to the Belkin and Gordon fronts location map, PF and SAF are very
close. Then these cold eddies and/or meanders would continue to propagate
toward north-east, some of them along the Campbell Plateau continental slope.
The hypothesis that these features were detached from the PF is supported by the
évident subsurface température minima observed between 200 and 300 m in
most of them (fourth and sixth sections in particular, where they show up to 1000
km north of the PF). These mesoscale phenomena transfer cold water in the
upper layer toward lower latitudes and could play a relevant rôle in the global
heat budget.