Subantarctic Front Polar Front

Fig. 3.6d. Distribution of silicate (|o.g-atoms/l) on the section.

Fig.3. 6e. Distribution of the density parameter ü [a = (p -1) x 1,000) where p is potential density] on the section. Units are mg/cm3.

the temperature-salinity relationships of stations in the Subantarctic Zone fall within a very tight envelope (e.g., Nowlin et al., 1977). Second, the southern part of the zone, just to the north of the Subantarctic Front, contains a thick, nearly homogeneous layer extending from near the sea surface to depths of 400 m or more (e.g., stations 964 and 1189 in Fig. 3.6). The degree of homogeneity varies with geographical location and season. The deepest and most homogeneous layers seem to be those in the southeast Indian Ocean, while the coldest, freshest and densest layers are observed in the southeastern Pacific (McCartney, 1977). The homogeneity is apparently the result of deep vertical convection induced within the Subantarctic Zone by air-sea interaction during the austral winter (McCartney, 1977). However, some stratification within this layer is noted. In Drake Passage and the western Scotia Sea, the Subantarctic Zone exhibits a slight decrease in salinity with depth (a "nearly isohaline layer") associated with a weak thermocline (Gordon et al., 1977a). Molinelli (1978) concluded that the cold, fresh characteristics observed at the base of this "isohaline thermocline" could not be explained by vertical convection but instead require northward spreading of Antarctic waters along isopycnal surfaces. Below the homogeneous layer, temperature decreases monotonically to the bottom while salinity increases to the deep salinity maximum associated with Circumpolar Deep Water.

Within the Subantarctic Zone, the salinity of the near-surface waters (100 m) decreases from west to east across the South Pacific (Fig. 3.8). South of Australia

Fig. 3.7. The locations of the Subantarctic and Polar Fronts as determined by the diving of the salinity and temperature minima, respectively. The dashed lines show the 400 m and 600 m isobaths of the salinity minimum layer. The heavy solid lines show the 200 m and 400 m isobaths of the temperature minimum layer. The contours are from Gordon and Molinelli (1982). The light solid line is the 3,000 m isobath. Dots indicate the positions of stations used to construct the vertical sections shown in Fig. 3.6.

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