OCEANOGRAPHY: SHALLOW FRONTS
In the shallower ocean, the seas east of South Island are crossed by two major frontal systems that exhibit intensified meridional gradients in temperature, salinity and density (Fig. 1). At around 55°S, but then curling north around the southeastern corner of the Campbell Plateau to almost 50°S, the east-flowing ACC is bounded to the north by the Subantarctic Front (SAF) (Orsi et al., 1995). South of the SAF, the annual mean surface-water temperature is <10°C, and the nutrient-rich polar ocean is rich in both phosphate and silica. The SAF is also the site where low-salinity AAIW is subducted rapidly from the surface northward to depths around 1000 m. The observations of Bryden and Heath (1985), together with global circulation models (Carter and Wilkins, in press), indicate that another unnamed front may extend east into the Pacific from the south side of the Bounty Trough. This front, at about latitude 46°S, probably marks the northern limit of the ACC east of New Zealand, rather than the 56°S SAF that is usually taken as the ACC limit (e.g., Orsi et al., 1995).
About ten degrees of latitude north of the SAF, the Subtropical Convergence (STC) separates subantarctic water of salinity 34.5 and an annual mean surface-temperature range of 8°–15°C from subtropical water with salinity 35 and an annual mean temperature range of 15°C. East-flowing currents occur on both sides of the STC, the warm East Cape Current (a continuation of the East Australian current) on the north side, and the cold West Wind Drift on the south. However, these current systems are most active several degrees of latitude away from the STC, and sluggish flow is therefore characteristic of the frontal region itself. Eastward flow on the south side of the STC is augmented by the Southland Current (derived from the Tasman Current being deflected by wind-drift around the south end of South Island), which flows north along the eastern South Island continental shelf and then turns east along the southern side of the crest of Chatham Rise. Heat transfer from the equator to the pole takes place across the SAF and STC by a combination of wind drift and dynamic eddying, with a cold return flow at depth in the DWBC. In the open ocean, as demonstrated in the Indian Ocean sector of the Southern Ocean, these fronts may migrate backward and forward by up to 6° of latitude during a glacial/interglacial cycle (e.g., Howard and Prell, 1992). However, modern seasonal movements of the front of at least 2° of latitude occur also, as for example for the STC east of New Zealand (Chiswell, 1994). In contrast to the oceanic mobility of the STC, several authors have shown that east of South Island the STC probably remained fixed to the shallow Chatham Rise throughout at least the most recent climatic cycle (Fenner et al., 1992; Nelson et al., 1993; Weaver et al., 1998).
The pronounced bathymetry around New Zealand exercises a controlling influence on the disposition of both deep currents and oceanic frontal zones. Notably, the ACC and DWBC are steered around or through gaps in Macquarie Ridge, and are then guided northward along the eastern escarpment of the Campbell Plateau. Concomitantly, in near-surface waters, the SAF, and the ACC with it, is forced north around and along the southeastern corner of the Campbell Plateau, and the position of the STC is probably controlled by eastward currents both north and south of the Chatham Rise. Some sites on Leg 181 were chosen in order to track whether or not past changes in the position of these fronts has occurred.
To 181 Scientific Report: Sedimentary Record of the ACC-DWBC