Abstract
The Subtropical and Subantarctic Fronts, which separate Subtropical, Subantarctic, and Antarctic Intermediate Waters, are diverted to the south of New Zealand by the submerged continental landmass of Zealandia. In the upper ocean of this region, large volumes of dissolved or suspended material are intermittently transported across the Subtropical Front; however, the mechanisms of such transport processes are enigmatic. Understanding these oceanic boundaries in three dimensions generally depends on measurements collected from stationary vessels and moorings. The details of these data sets, which are critical for understanding how water masses interact and mix at the fine-scale (<10 m) to mesoscale (10-100 km), are inadequately constrained due to resolution considerations. Southeast of New Zealand, high-resolution seismic reflection images of oceanic water masses have been produced using petroleum industry data. These seismic sections clearly show three main water masses, the boundary zones (fronts) between them, and associated thermohaline fine structure that may be related to the mixing of water masses in this region. Interpretations of the data suggest that the Subtropical Front in this region is a landward-dipping zone, with a width that can vary between 20 and 40 km. The boundary zone between Subantarctic Waters and the underlying Antarctic Intermediate Waters is also observed to dip landward. Several isolated lenses have been identified on the three data sets, ranging in size from 9 to 30 km in diameter. These lenses are interpreted to be mesoscale eddies that form at relatively shallow depths along the south side of the Subtropical Front.
Plain Language Summary Seismic data originally collected by the petroleum industry are used in this paper to examine oceanic water masses southeast of New Zealand where Subtropical Waters meet Subantarctic Waters at the Subtropical Front. The method enables us to make cross-sectional pictures of the oceanmuch like an X-ray might work in medical applications. This region is of particular interest because it is a well-known region of oceanic mixing. In the data, water mass boundaries or fronts are observed as subhorizontal bands of high reflectivity. Also, distinct lens-shaped reflectivity patterns are interpreted to be caused by meso-scale eddies (up to 100 km in diameter)features that are important in understanding how oceanic water bodies with different salinities and temperatures mix.