Combining IES and Hydrography to Observe The Time-Varying Baroclinic Structure of the Sub-Antarctic Front

D. Randolph Watts, Univ. of Rhode Island, Kingston, Rhode Island
Che Sun, Univ. of Rhode Island, Kingston, Rhode Island
Stephen R. Rintoul, CSIRO Marine Laboratories, Hobart, Australia


As a component of the SubAntarctic Flux and Dynamics Experiment (see Luther et al. poster), 18 Inverted Echo Sounders (IES) were moored along the WOCE SR3 transect south of Australia, 3/95--3/97. IESs measure the vertical acoustic travel time, tau, from the bottom to the surface. The thermocline and density fields slope steeply across the Subantarctic Front (SAF) with the associated baroclinicity almost extending to bottom. As geopotential height and tau both change systematically across the SAF, they exhibit an empirical functional relationship to each other and to the fundamental vertical structure of temperature and specific volume anomaly (svan). A technique is developed for interpretting IES data based on a "gravest empirical mode" (GEM) representation of the vertical structure. The GEM structure, determined from eight hydrographic sections across the SAF, also incorporates the average annual cycle in the near-surface layers. In the depth range 150--3000 dbar, more than 97% of the variance in the temperature field and >96% of the variance in the svan field is captured by the GEM representation. IES measurements of tau, combined with the GEM interpretation of the vertical structure, determine the geopotential thickness profile above each IES. Horizontally-separated IESs determine lateral gradients in geopotential height, from which geostrophic velocity profiles are calculated. Comparisons with moored temperature records confirm agreement within 0.19, 0.16, 0.08, and 0.03 C at depths 300, 600, 1000, and 2000 dbar. Velocity shear comparisons with moored current meters agree within 3--6 cm/s at 300 and 600 dbar relative to 3000 dbar; sampling differences from point measurements explain the residuals. Lastly, we indicate wider applicability of the GEM method to interpretting satellite altimetry observations.