The continuous eastward flow of the ACC causes the horizontal distribution of water properties in the Southern Ocean to be relatively uniform in the zonal direction while varying meridionally. In most longitudes, the meridional gradients in water properties are not uniform, but occur in steps. That is, relatively broad zonal bands (zones) characterized by weak horizontal gradients are separated by narrow fronts with sharp gradients (e.g., Gordon and Goldberg, 1970; Gordon and Molinelli, 1982). A schematic representation of the zonation is given in El-Sayed (this volume, Fig. 9.1). The gross features of this zonation in water properties have long been recognized (e.g., Deacon, 1937; Mackintosh, 1946) but only in recent years with the advent of instruments capable of recording continuous vertical profiles of temperature and salinity has this phenomenon been described in some detail south of Australia and New Zealand (Gordon, 1973, 1975; Gordon et al., 1977b), in Drake Passage (Nowlin et al., 1977; Whitworth, 1980; Nowlin and Clifford, 1982), and in the western Scotia Sea (Gordon et al., 1977a). The studies have revealed that the various zones can be characterized on the basis of distinctive vertical stratification or temperature-salinity relationships of the surface and intermediate waters. Characteristics of the deep and bottom waters undergo more subtle transitions across the ACC. The zonation within these deep layers is most evident as an abrupt change in depth of common water properties from one zone to the next.

In each of the regions studied, the zonation is remarkably similar. The zones are, from north to south, the Subantarctic Zone, the Polar Frontal Zone, the Antarctic Zone, and the Continental Zone. As noted earlier, the narrow boundaries or fronts separating the zones in Drake Passage and the western Scotia Sea coincide with jets of eastward flow. The front separating the Subantarctic Zone from the Polar Frontal Zone is called the Subantarctic Front, that separating the Polar Frontal Zone from the Antarctic Zone is called the Polar Front, and that separating the Antarctic Zone from the Continental Zone is called the Continental Water Boundary. A similar configuration is observed south of Australia and New Zealand (cf. Callahan, 1971; Gordon et al., 1977b). One difference observed in this latter region is the presence of an additional zone, the Subtropical Zone, to the north of the Subantarctic Zone. The front separating the Subtropical waters from the Subantarctic waters is called the Subtropical Front or Subtropical Convergence.

The similarity of zonation and velocity structure among the regions which have been studied so far has led to the conjecture that the ACC may everywhere be characterized by two separate and distinct fronts, the Subantarctic Front and the Polar Front, and their associated eastward jets. As will be discussed later, the circumpolar continuity of this structure has not yet been convincingly demonstrated. However, this conceptual model of the ACC provides a useful framework for the following discussion of zonation in the South Pacific.

Figs 3.6a-e present distributions of potential temperature, salinity, oxygen, silicate, and potential density on a meridional section through the central part of the Pacific Sector. The locations of the stations used to construct the section are shown in Fig. 3.7. Since the section combines data from different seasons and different cruises, it is not synoptic. Nonetheless, the major features revealed in the distributions are representative of conditions in the area. Although it would have been desirable to present distributions of phosphate and nitrate along this section, high quality nutrient data were not available.

Subtropical Front

The Subtropical Front separates the relatively warm, salty subtropical waters from the cooler, fresher waters of the Subantarctic Zone. Although this front passes south of New Zealand (Heath, 1981), it lies to the north of 40°S throughout most of the South Pacific east of 150°W (Deacon, 1982). Since this front is not observed in Drake Passage, it is not circumpolar in extent.

Subantarctic Zone

The northernmost zone in the ACC is the Subantarctic Zone. This zone has two distinguishing characteristics. First, vertical sections across the ACC show that

Fig. 3.6a. Distribution of potential temperature (°C) on a meridional section approximately along 135°W. Dotted line indicates the core of the subsurface temperature minimum. Station positions are shown in Fig. 3.7.

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