V. Gouretski
80
3 Geographical Position of the Ross Gyre
To define dynamic boundaries for the Ross Gyre we consider geostrophic circulations (Fig. 2) referenced to a number of assumed zero-velocity levels. Choice of
the levels is subjective, with the intention to illustrate the circulation of the nearsurface, intermediate and deep waters respectively. Ail maps portray the Ross
Gyre as a dynamic height dépréssion south of the mid-ocean ridge. The steric
anomaly map 100/500 m reveals the center of the gyre at about 164°W, 68°S. The
gyre's center seems to shift with depth to the north-east, whereas the 1500/3000m map (Fig. 2c) depicts the center of the gyre near 150°W, 63°S. A northward shift
of the gyre axis is reported by Orsi et al. [17] for the Weddell Gyre. Our averaged
maps do not show much différence in the horizontal shear between the ACC and
the northern limb of the gyre. The steric height anomaly isoline 0.155 m for the
layer 100/500 m lies within the zone of high latéral geostrophic shear of the
Antarctic Circumpolar Current and may be chosen to represent the location of
the gyre's northern boundary. In contrast, the Southern limb of the gyre is characterized by a much weaker shear. The steric height différence across the gyre is
about 5 cm for the layer 100/500 m, whereas below 500 m it is only about 3 cm,
indicating a decrease of the vertical shear with depth.
At ail levels isolines of steric height are aligned quasi-meridionally between
150 and 135°W, indicating the eastern boundary of the gyre at about 140°W.
Compared with the Weddell Gyre no pronounced western boundary is revealed
by the dynamic topography at ail levels. This may be explained by a quasi-zonal
orientation of the continental slope in the area. In contrast, the Antarctic
Peninsula in the Weddell Sea provides a méridional barrier along which a northward boundary current with a total transport of about 28 Sv is developed [18].
Intégral volume transports within the study area based on the gridded density fields are shown in Fig. 3. The océan bottom was used as a zéro velocity
level. Accumulated westward transports were calculated for the area between
the Antarctic continent and 65°S (north of this latitude the direction of the
water transport is to the east everywhere). The maximum westward transport of
8.5 Sv is found near 150°W, with transport magnitudes exceeding 5 Sv between
165 and 145°W. These estimâtes include both transports within the narrow
Antarctic Slope Front described by Jacobs [19] and within the Southern limb of
the Ross Gyre. The non-zero westward transports are found only between 179
and 135°W, thus implying the virtual abscence of the Antarctic Slope Current
west of 190°W and east of 135°W.
Geostrophic transports of 24-26 Sv relative to the bottom are reported by
Fahrbach et al. [20] for the Weddell Gyre at 6°W. Thus, in terms of geostrophic
transports the Ross Gyre is about a factor of 3-4 weaker than its Weddell Sea
counterpart. At ail levels depicted in Fig. 2 the ACC is characterized by a strong
horizontal shear north of the mid-ocean ridge crest. Being a deep-reaching current, the ACC is strongly controlled by the bottom topography. Two deep gaps
provide the conduits for two main high-speed cores of the ACC. The Antarctic
Polar Front core flows through the Udintsev Fracture Zone near 143°W. It is well
pronounced on the 100/500-m steric height anomaly map (Fig. 2a) as an intensive
jet between the mid-ocean ridge and 120°W. The current core, associated with the
80
3 Geographical Position of the Ross Gyre
To define dynamic boundaries for the Ross Gyre we consider geostrophic circulations (Fig. 2) referenced to a number of assumed zero-velocity levels. Choice of
the levels is subjective, with the intention to illustrate the circulation of the nearsurface, intermediate and deep waters respectively. Ail maps portray the Ross
Gyre as a dynamic height dépréssion south of the mid-ocean ridge. The steric
anomaly map 100/500 m reveals the center of the gyre at about 164°W, 68°S. The
gyre's center seems to shift with depth to the north-east, whereas the 1500/3000m map (Fig. 2c) depicts the center of the gyre near 150°W, 63°S. A northward shift
of the gyre axis is reported by Orsi et al. [17] for the Weddell Gyre. Our averaged
maps do not show much différence in the horizontal shear between the ACC and
the northern limb of the gyre. The steric height anomaly isoline 0.155 m for the
layer 100/500 m lies within the zone of high latéral geostrophic shear of the
Antarctic Circumpolar Current and may be chosen to represent the location of
the gyre's northern boundary. In contrast, the Southern limb of the gyre is characterized by a much weaker shear. The steric height différence across the gyre is
about 5 cm for the layer 100/500 m, whereas below 500 m it is only about 3 cm,
indicating a decrease of the vertical shear with depth.
At ail levels isolines of steric height are aligned quasi-meridionally between
150 and 135°W, indicating the eastern boundary of the gyre at about 140°W.
Compared with the Weddell Gyre no pronounced western boundary is revealed
by the dynamic topography at ail levels. This may be explained by a quasi-zonal
orientation of the continental slope in the area. In contrast, the Antarctic
Peninsula in the Weddell Sea provides a méridional barrier along which a northward boundary current with a total transport of about 28 Sv is developed [18].
Intégral volume transports within the study area based on the gridded density fields are shown in Fig. 3. The océan bottom was used as a zéro velocity
level. Accumulated westward transports were calculated for the area between
the Antarctic continent and 65°S (north of this latitude the direction of the
water transport is to the east everywhere). The maximum westward transport of
8.5 Sv is found near 150°W, with transport magnitudes exceeding 5 Sv between
165 and 145°W. These estimâtes include both transports within the narrow
Antarctic Slope Front described by Jacobs [19] and within the Southern limb of
the Ross Gyre. The non-zero westward transports are found only between 179
and 135°W, thus implying the virtual abscence of the Antarctic Slope Current
west of 190°W and east of 135°W.
Geostrophic transports of 24-26 Sv relative to the bottom are reported by
Fahrbach et al. [20] for the Weddell Gyre at 6°W. Thus, in terms of geostrophic
transports the Ross Gyre is about a factor of 3-4 weaker than its Weddell Sea
counterpart. At ail levels depicted in Fig. 2 the ACC is characterized by a strong
horizontal shear north of the mid-ocean ridge crest. Being a deep-reaching current, the ACC is strongly controlled by the bottom topography. Two deep gaps
provide the conduits for two main high-speed cores of the ACC. The Antarctic
Polar Front core flows through the Udintsev Fracture Zone near 143°W. It is well
pronounced on the 100/500-m steric height anomaly map (Fig. 2a) as an intensive
jet between the mid-ocean ridge and 120°W. The current core, associated with the