Observations of small-scale disturbances of the Subantarctic Front south of Australia

Abstract

High-resolution XCTD and XBT observations were conducted to study eddy variability in the Subantarctic Front (SAF) and its possible impact on the properties of the adjacent Subantarctic Mode Water (SAMW) in the region south of Australia. The stations were occupied along cross-frontal transects in February 2004 and in February and March 2002. Coarse-resolution data from previous WOCE observations were also examined to reveal the water mass properties, including dissolved oxygen (DO). Small-scale (10–20 km) features were detected in the SAF for all high-resolution sections. Fluctuation of the 7 ∘ C isotherm was clearly revealed at a depth of 300–600 m for the σ θ = 26.8 – 26.9 kg m - 3 density range. Temperature and salinity inversions of a similar spatial scale were also found on the same density surface. The spatial scale and depth range of the small-scale anomalies could be consistent with those of the baroclinic instability in the top several hundred-meter layer. In the potential temperature–salinity ( θ – S) diagram, the small-scale fluctuations constitute cold/fresh intrusions from the warm/saline SAMW curve. The intrusions were found on the θ – S line connecting the deeper SAMW and the shallower Antarctic Surface Water (AASW) of the higher latitudes. From the WOCE observations, the presence of SAMW with high DO was indicated north of the SAF, as previously reported. Within the SAF, intrusions in the θ – S diagram with relatively high DO were found at around the 7 ∘ C isotherm for 26.8 – 26.9 kg m - 3 , the same densities for the small-scale anomalies and the high-DO SAMW. Analysis of heat and salt deficit suggests that the small-scale disturbances have a comparable or larger role than those of meso-scale anomalies. These imply that the small-scale anomalies contribute to the cross-frontal water exchange and to the formation of high-DO SAMW.