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

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℃ isotherm was clearly revealed at a depth of 300–600 m for the δθ=26.8-26.9kgm-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℃ isotherm for 26.8-26.9kgm-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

Alternating zonal flows in a two-layer wind-driven ocean

Alternating zonal flows in an idealized wind-driven double-gyre ocean circulation have been investigated using a two-layer shallow-water eddy-permitting numerical model. While the alternating zonal flows are found almost everywhere in the time-mean zonal velocity field, their meridional scales differ from region to region. In the subpolar western boundary region, where the energetic eddy activity induces quasi two-dimensional turbulence, the alternating zonal flows are generated by the inverse energy cascade and its arrest by Rossby waves, and the meridional scale of the flows corresponds well to the Rhines scale. In the eastern part of the basin, where barotropic basin modes are dominant, the zonal structure is formed through the nonlinear effect of the basin modes and is wider than the Rhines scale. Both effects are likely to form zonal structure between the two regions. These results show that Rossby basin modes become an important factor in the formation of alternating zonal flows in a closed basin in addition to the arrest of the inverse energy cascade by Rossby waves. The wind-driven general circulation associated with eddy activities plays an essential role in determining which mechanism of the alternating zonal flows is possible in each region