J. Phys. Ocean., submitted, 2005.

Spatial variations of stirring in the surface ocean: A case study of the Tasman Sea.

Darryn W. Waugh
Department of Earth and Planetary Science, Johns Hopkins University, Baltimore, Maryland.

Edward R. Abraham and Melissa M. Bowen
National Institute of Water and Atmospheric Research, Wellington, New Zealand


Stirring in the Tasman Sea is examined using surface geostrophic currents derived from satellite altimeter measurements. Calculations of the distribution of finite-time Lyapunov exponents (FTLEs) indicate that the stirring in this region is not uniform and stretching rates over 15 days varying from less than 0.02 1/day to over 0.3 1/day. These variations occur at both small (~10 km) and large (~1000 km) scales, and in both cases are linked to dynamical features of the flow. The small scale variations are related to the characteristics of coherent vortex structures, and there are low FTLES inside vortices and filaments of high FTLEs in strain-dominated regions surrounding these vortices. Regional variations in the stirring are closely related to variations in mesoscale activity and eddy kinetic energy (EKE). High values of mean FTLE occur in regions of high EKE (highest mean values of around 0.2 1/day occur in the EAC separation region) whereas small values occur in regions with low EKE (mean values around 0.03 1/day in the east Tasman sea). There is a compact relationship between the mean FTLEs and EKE, raising the possibility of using the easily-calculated EKE to estimate the stirring. This possibility is even more intriguing as the FTLE distributions can be approximated, for the time scales considered here, by Weibull distributions with shape parameter equal to 1.6, which can be defined from the mean value alone.

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