Abstract
The subtropical front (STF) is a fundamental feature of the Southern Ocean. It separates waters in the subtropical gyres from Subantarctic waters. At present however only a rudimentary understanding exists about its temporal variability. Variations in the physical state of the STF (i.e. its location, intensity and hydrography) have implications for atmospheric circulation, marine productivity and climate. This thesis focuses on improving temporal understanding of this major Southern Ocean front. Observations of the temporal variability of the STF in the Tasman Sea are presented. These are obtained through a synthesis of repeat hydrographic sections and remotely sensed sea surface temperature (SST) and sea surface salinity (SSS) data. This work aims to contribute to an improved understanding of how the physical state of the STF varies in time, and of role that topography and surface winds play in this variability.
Using three hydrographic surveys and remotely sensed SST data collected during the austral autumns of 2007, 2008 and 2011, the interaction of the south subtropical front (S-STF) with topography around southern New Zealand is investigated. The poleward limit of the S-STF is found to be remarkably consistent, crossing the Macquarie Ridge north of a seamount at 49.6°S, before turning equatorward east of 166°E to follow the upper continental shelf. This is contrary to some previous descriptions that placed the S-STF lying across the Snares Shelf close to 47.5°S. Cross-frontal temperature and salinity gradients at the S-STF are also found to be intensified in regions of shallow bathymetry. Both results are attributed to topographic steering of the S-STF by bathymetry at approximately 500 m depth.
Thermal fronts detected using thirty years (1982-2011) of remotely sensed SST data are combined with 27 months (2013-2015) of remotely sensed SSS data, to provide new insights on the typical position and seasonal migration of the S-STF in the Tasman Sea. By examining thirty year annual-mean and seasonal-mean distributions of hundreds of thermal fronts that are identified as likely to be surface expressions of the S-STF, a long-term, statistical view of the S-STF’s position in the western and eastern Tasman Sea is developed. It is apparent that seasonal migrations of the S-STF in these locations do not exceed 1°, contrary to a recent suggestion that the subtropical frontal zone undergoes a global, seasonal shift of 5-7°. The distributions of thermal fronts also contain an intriguing suggestion that the S-STF might behave bimodally at the Macquarie Ridge.
SSTs across the S-STF south of New Zealand exhibit marked interannual variability. SST variability in this region may have significant impacts on regional climate. Recent studies have shown that variability in the basin-wide wind stress curl significantly influence SSTs in other parts of the South Pacific subtropical gyre. Using thirty years of remotely sensed SST and atmospheric reanalysis data, variability of subtropical and Subantarctic SSTs at the S-STF south of New Zealand are found to be significantly lag-correlated to both basin-wide and local-scale winds. Potential mechanisms linking SST with variations in the South Pacific wind field are also discussed.