S.S. Jacobs, C.F. Giulivi
8
World Océan Atlas [22]. Model results can vary markedly and hâve global impacts
[23, 24] depending upon whether salinities are restored to values as low as 34.0,
characteristic of zonal average summer surface waters, or as high as 35.0, which
exceed anything reported on the shelf for the past three décades.
Substantial différences are also évident between our summer observations and
NODC Atlas objectively averaged 1-degree grid data over the same months in
each méridional band (Fig. 3). The Atlas data are consistently saltier, perhaps
because more recent, lower-salinity measurements [15] were not in the NODC
archive. Atlas températures also tend to be higher and more variable and, like
salinities, do not extend below 500 m. These anomalies might resuit from mislocated stations, from the incorporation of bathythermograph data into the Atlas,
and from irregular shelf depths over which isolated deep values are removed [25],
Atlas data on the shelf may also be biased by different water types from north of
the shelf break, near which the strong Antarctic Slope Front [26] could also influence data averaged on pressure surfaces, as in the North Atlantic [27]. Gridded
and averaged data are valuable for many purposes, but caution must be exercised
in their use for modelling or other work on the Antarctic continental shelf.
Annual averages are compromised by the scarcity of non-summer measurements, and spatial averages can show a high variance due to the strong east-west
gradient resulting from the variable sait fluxes and wind fields [2, 28]. The zonal
gradients in Fig. 3 are largest below the upper 100 m, with the highest salinities
and coldest températures in the western sector. The salinity gradient may hâve
implications for classical bottom water formation arguments, which frequently
hinge on much lower changes in density (salinity). Indeed, the entire région below
350 m is saltier than the Pacific continental shelf average of 34.46 [29]. If bottom
water formation simply required a shelf component with a salinity above -34.51
[30], then most water on the Ross Sea continental shelf would qualify. However,
the Ross Sea is widely believed to produce much less bottom water than the
Weddell Sea (e.g., [29]), in spite of similar environments. This may in part be an
artifact of water mass définitions, or be a conséquence of the greater warmth of
the Ross Gyre, éléments of which intrude onto the continental shelf. An apparent
drift toward lower shelf water salinities in the Ross Sea over recent décades might
also hâve altered the régional bottom water production rate or properties [15].
4 Water masses
Circumpolar Deep Water evolves into several new water masses on and near the
Antarctic continental shelves. As these waters mix and interact with the atmosphère, sea ice and shelf ice, some ventilating the deep océan, their properties and
variability are of more than taxonomie interest in this région. Names and définitions hâve evolved over time,but common use has tended to follow Carmack [29].
The north-south and east-west température and salinity sections across the shelf
in Figs. 4 and 5 are plotted at the same vertical scale, with the following water
mass abbreviations and characteristics:
HSSW (High Salinity Shelf Water), identified by salinities of > 34.6 and températures at the sea surface freezing point, dominâtes the subsurface western sector in
summer. Previously referred to as Ross Sea Shelf Water (and Western Shelf Water
in the Weddell Sea), HSSW appears on ail sections but the one in Fig. 5d.
8
World Océan Atlas [22]. Model results can vary markedly and hâve global impacts
[23, 24] depending upon whether salinities are restored to values as low as 34.0,
characteristic of zonal average summer surface waters, or as high as 35.0, which
exceed anything reported on the shelf for the past three décades.
Substantial différences are also évident between our summer observations and
NODC Atlas objectively averaged 1-degree grid data over the same months in
each méridional band (Fig. 3). The Atlas data are consistently saltier, perhaps
because more recent, lower-salinity measurements [15] were not in the NODC
archive. Atlas températures also tend to be higher and more variable and, like
salinities, do not extend below 500 m. These anomalies might resuit from mislocated stations, from the incorporation of bathythermograph data into the Atlas,
and from irregular shelf depths over which isolated deep values are removed [25],
Atlas data on the shelf may also be biased by different water types from north of
the shelf break, near which the strong Antarctic Slope Front [26] could also influence data averaged on pressure surfaces, as in the North Atlantic [27]. Gridded
and averaged data are valuable for many purposes, but caution must be exercised
in their use for modelling or other work on the Antarctic continental shelf.
Annual averages are compromised by the scarcity of non-summer measurements, and spatial averages can show a high variance due to the strong east-west
gradient resulting from the variable sait fluxes and wind fields [2, 28]. The zonal
gradients in Fig. 3 are largest below the upper 100 m, with the highest salinities
and coldest températures in the western sector. The salinity gradient may hâve
implications for classical bottom water formation arguments, which frequently
hinge on much lower changes in density (salinity). Indeed, the entire région below
350 m is saltier than the Pacific continental shelf average of 34.46 [29]. If bottom
water formation simply required a shelf component with a salinity above -34.51
[30], then most water on the Ross Sea continental shelf would qualify. However,
the Ross Sea is widely believed to produce much less bottom water than the
Weddell Sea (e.g., [29]), in spite of similar environments. This may in part be an
artifact of water mass définitions, or be a conséquence of the greater warmth of
the Ross Gyre, éléments of which intrude onto the continental shelf. An apparent
drift toward lower shelf water salinities in the Ross Sea over recent décades might
also hâve altered the régional bottom water production rate or properties [15].
4 Water masses
Circumpolar Deep Water evolves into several new water masses on and near the
Antarctic continental shelves. As these waters mix and interact with the atmosphère, sea ice and shelf ice, some ventilating the deep océan, their properties and
variability are of more than taxonomie interest in this région. Names and définitions hâve evolved over time,but common use has tended to follow Carmack [29].
The north-south and east-west température and salinity sections across the shelf
in Figs. 4 and 5 are plotted at the same vertical scale, with the following water
mass abbreviations and characteristics:
HSSW (High Salinity Shelf Water), identified by salinities of > 34.6 and températures at the sea surface freezing point, dominâtes the subsurface western sector in
summer. Previously referred to as Ross Sea Shelf Water (and Western Shelf Water
in the Weddell Sea), HSSW appears on ail sections but the one in Fig. 5d.