Variability of the Subantarctic and Polar fronts in the Drake Passage as deduced from altimetry
Résumé
The magnitude and time variability of the total volume transport of the Antarctic Circumpolar Current through Drake Passage is a key climatic index, yet still uncertain. We processed ten years of TOPEX/Poseïdon data along two adjacent descending tracks in Drake Passage assuming (following Gille, 1994) that the surface current velocity has a profile structure of double Gaussian jets corresponding respectively to the Subantarctic Front (SAF) and the Polar Front (PF). We seek the location, width and intensity of each jet as a function of time. Results from the two tracks are coherent and show that the time evolution of the flows is well constrained, although the absolute magnitude of the flow is not. Robust conclusions can be drawn using correlations between parameters and spectral analysis of their time evolution. The two front locations are positively correlated (0.68). When the SAF is farther south, it is also wider and has a larger surface transport. When the PF is farther south, it narrows with a smaller surface transport. The two widths are significantly negatively correlated (-0.58) and the maximum velocities are independent (0.08). The surface transports associated with the SAF and PF are strongly anti-correlated (-0.98) so that the variance of total surface transport is much smaller than the variance of the surface transport of each individual front. Spectral peaks for SAF and PF surface transports are, in order of decreasing amplitude, less than 4 months, semi-annual, 2.6 years and annual. The total surface transport variations are distributed into two broad bands (less than 4 months-the most energetic-and from 0.5 to 3 years).