Chaotic stirring by a mesoscale surface-ocean flow

Abstract

The horizontal stirring properties of the flow in a region of the East-Australian Current are calculated. A surface velocity field derived from remotely sensed data, using the Maximum Cross Correlation method, is integrated to derive the distribution of the finite-time Lyapunov exponents. For the region studied (between latitudes 36 S and 41 S and longitudes 150 E and 156 E) the mean Lyapunov exponent during 1997 is estimated to be 4 x 10^{-7} s^{-1}. This is in close agreement with the few other measurements of stirring rates in the surface ocean which are available. Recent theoretical results on the multifractal spectra of advected reactive tracers are applied to an analysis of a sea-surface temperature image of the study region. The spatial pattern seen in the image compares well with the pattern seen in an advected tracer with a first-order response to changes in surface forcing. The response timescale is estimated to be 20 days.