Mean flow, eddy variability and energetics of the Subantarctic Front south of Australia

Abstract

This thesis describes the variability and mean flow of the Subantarctic Front (SAF) south of Australia using time series measurements of velocity and temperature from 1993 to 1995, and six hydrographic transects along WOCE line SR3 from Tasmania to Antarctica over the period 1991 to 1996. The SAF is the strongest jet of the Antarctic Circumpolar Current (ACC) south of Australia. The time series of velocity and temperature are only the third such dataset collected in the ACC and provide insight into the dynamics of this massive current and into the heat and momentum balances of the Southern Ocean. The SAF was found to be an energetic, meandering jet with vertically coherent fluctuations. These fluctuations varied on a timescale of 20 days, and had a typical amplitude of 30 cm/s at 1150 dbar. The analysis used a coordinate frame that rotated daily to be in alignment with the direction of flow. This allowed the mesoscale variability of the SAF to be isolated from variability due to meandering of the front and proved very successful for examining eddy fluxes. Vertically averaged cross-stream eddy heat flux was 11.3 kW/m^2 poleward and was significantly different from zero at the 95% confidence level for fluctuations with periods between 2 and 90 days. Zonally integrated, this eddy heat flux (=0.9x10^15 W) is more than large enough to balance the heat lost south of the Polar Front and is as large as cross-SAF fluxes found in Drake Passage. Cross-stream eddy momentum fluxes were small and not significantly different from zero but were tending to decelerate the mean flow. A relationship between vertical motion and meander phase identified in the Gulf Stream was found to hold for the SAF. Eddy kinetic energy levels were similar to those in Drake Passage and southeast of New Zealand. Eddy potential energy was up to an order of magnitude larger than at the other ACC sites, most likely because meandering of the front is more common south of Australia. Baroclinic conversion was found to be the dominant mechanism by which eddies grow south of Australia. The typical time for the growth of an eddy is estimated to be 30 days, approximately half that in Drake Passage. This is consistent with observations from satellite altimetry which indicate that eddy energy is growing rapidly downstream of the Australian measurement site, while the eddy field in Drake Passage is mature. Mean cross-stream profiles of absolute and baroclinic velocity in the SAF at five current meter levels have been obtained from two streamwise profiling techniques using specific volume anomaly at 780 dbar as the cross-stream coordinate. One of the techniques, using hydrographic data to estimate the baroclinic velocity profile, is presented for the first time. The mean SAF velocity profile is composed of one central peak, reaching 52 and 34 cm/s at 420 dbar, absolute and baroclinic respectively, and several smaller peaks. The SAF flow is coherent at all levels, reaches the sea floor, and is at least 220 km wide. The cross-stream structure of baroclinic and absolute transport of the SAF has been characterized for the first time. The integrated mean transport is at least 116+/-10 x 10^6 m^3/s, of which approximately 14% is barotropic. The linear conditions for baroclinic and barotropic instability are satisfied at the array, consistent with the eddy growth rates calculated.