A two-dimensional gravest empirical mode determined from hydrographic observations in the subantarctic front

South of Australia, where the baroclinicity in the Subantarctic Front exten ds almost to the seafloor, the geopotential height of the sea surface (phi) and the vertical acoustic travel time (tau) exhibit a tight empirical rela tionship to each other and to the entire vertical structure of temperature T(p), salinity S(p), and specific volume anomaly delta (p). Measurements of tau provide proxy estimates of the profiles T-G (tau ,p), S-G(tau ,p), and delta (G)(tau ,p), based on a two-dimensional "gravest empirical mode'' (G EM) representation of the vertical structure fitted to 212 hydrographic sta tions. A seasonal model was fitted in the near-surface layers using additio nal historical data. At each depth in the range 150-3000 dbar, more than 96 % of the variance in both the T and delta fields is captured by the GEM rep resentation. During the Subantarctic Flux and Dynamics Experiment (Mar 1995-Mar 1997), i nverted echo sounders (IES) and current meters were moored in a 2D array al ong the WOCE SR3 transect south of Australia. At each IES site, proxy-estim ated T( p) time series were compared with independent moored temperature re cords, confirming agreement within 0.29 degrees, 0.26 degrees, 0.10 degrees , and 0.04 degreesC rms differences at depths 300, 600, 1000, and 2000 dbar . Between laterally separated IES sites, geostrophic velocity profiles esti mated from horizontal gradients in f( p) agree with moored current records within 0.07, 0.05, and 0.03 m s(-1) at 300, 600, and 1000 dbar relative to 2000 dbar. A simple argument based on conservation of potential vorticity i s suggested to account for GEM dominance of the spatiotemporal variability.