Interaction of the subtropical front with topography around southern New Zealand

Abstract

The subtropical front zone (STFZ) is a fundamental feature of the southwest Pacific Ocean. It separates waters in the subtropical gyres from subantarctic waters. In this study we examine a section of the global STFZ that passes around southern New Zealand, using hydrographic sections and remotely sensed sea surface temperature (SST) data collected during the austral autumns of 2007, 2008 and 2011. The STFZ was composed of a single southern subtropical front (S-STF) that marked the poleward edge of the STFZ, while the region north of the S-STF was characterised by a series of shallow fronts (<50m depth) and a freshwater-induced neritic front. The S-STF around southern New Zealand is also distinguished by moderate-to-weak density compensation, in contrast to the strong density compensation to the west and east of New Zealand. We find strong coupling between the surface and subsurface expressions of the S-STF and demonstrate the practicality of tracing the pathway of the front using its surface expression from remotely sensed images. The S-STF is steered by topography as it crosses the Macquarie Ridge close to 49.6°S and to the east of 166°E as it turns equatorward following the upper continental slope. This is contrary to some previous descriptions that placed the S-STF lying across the Snares Shelf close to 47.5°S. Topographic interactions between the S-STF and sharply defined bathymetry around southern New Zealand appear to strongly influence the position of the front in this region. Cross-frontal temperature and salinity gradients at the S-STF were also found to be intensified in regions of shallow bathymetry. Both results are attributed to topographic steering of the front by bathymetry at approximately 500m depth.