Near inertial oscillations at the shelf off northern Patagonia

Abstract

Four current time series collected near 43/]S on the continental shelf off Patagonia from September 1991 to August 1992 have been band pass filtered with cutoff frequencies of 0.049 and 0.068 cycles per hour (cph) to extract the near-inertial component. The filtered time series have only small amplitude attenuation when compared to the original data and phase is not changed. At two near-shore moorings, where the water column is poorly stratified the energy contained in the inertial band is negligible. At mid-shelf the spectral peak of the inertial band is large and maximum speeds are > 25 cm s-1. The horizontal velocity vector is nearly circularly polarized (eccentricity of 1.04) and rotates counterclockwise with frequency a few percent greater than the local inertial frequency. The propagation direction is to 105oT, approximately perpendicular o the coast. The complex correlation between filtered currents at 17 and 67 m depth has a magnitude of 0.83 and a phase difference of 176o, indicating that the flow is dominated by the first baroclinic mode. The ratio between observed amplitudes above and below the pycnocline is controlled by picnocline depth, as predicted by a two-layer model. The simple model of Pollard and Millard (1970) forced with wind data from Puerto Madryn reproduced many of the observed features of the inertial currents in the surface layer. The lack of inertial oscillations in the mind-shelf during Austral winter and the reduced amplitude near the coast throughout the sampling period, are associated to the reduced stratification of the water column.