Structure of Southern Ocean fronts at 140°E

The major fronts between Tasmania and Antarctica are described on the basis of repeat hydrographic and expendable bathythermograph (XBT) sections and satellite altimetry. The high spatial and temporal resolution allows the location, structure and variability of the fronts to be investigated in detail. A large number of criteria are examined in an effort to identify reliable indicators of the fronts (e.g. lateral gradients along isobars and isopycnals, transport maxima, and the latitude where a property isoline crosses a particular isobar). The location of the Subtropical Front (STF) varies by less than 1° from its mean latitude of 45.2°S between the Tasmanian continental slope and the South Tasman Rise. The high resolution sections resolve multiple branches or filaments of each of the main fronts of the Antarctic Circumpolar Current (ACC) south of Australia: the Subantarctic Front (SAF) has two cores at mean latitudes of 50.5° and 52°S, the Polar Front (PF) has two branches which are found between 53° and 54°S and between 59° and 60°S, and the southern ACC front crosses the section near 62°S and 64°S. The southern boundary of the ACC sometimes merges with the southern ACC front (SACCF). The Antarctic Slope Front is found over the upper continental slope on those sections, which extend sufficiently close to Antarctica. Each of the frontal filaments identified on the repeat sections corresponds to a narrow range of sea surface height (SSH) values. These SSH streamlines are also found to correspond to large lateral gradients of SSH (i.e. fronts) east and west of the repeat section. Maps of sea surface height are then used to determine the path and variability of the fronts. The maps confirm the multi-filament structure of the fronts and show that streamlines merge and split along the path of the fronts. Each of the ACC fronts extends throughout the water column; as a result, the path of the fronts and the width of their meander envelopes are strongly influenced by bathymetry. Meridional displacements of the fronts are correlated with variations in SST, suggesting shifts of the fronts contribute to SST variability observed on interannual time scales.