Spectrum analysis of inertial and subinertial motions based on analyzed winds and wind-driven currents from a primitive equation general ocean circulation model

Abstract

The relationship between the applied wind stress and currents predicted by a primitive equation ocean circulation model was analyzed and compared to theory and observations . Three one-year data sets were examined using Fourier and rotary spectrum analysis techniques. The Fourier analysis revealed three spectral peaks in the predicted currents with none in the wind stress. These peaks correspond to synoptic variability at low frequency, the inertial response at an intermediate frequency and a nonphysical response at high frequency, due to the finite time differencing procedure employed. This response at high frequency was two orders of magnitude smaller than the peaks at the synoptic and inertial periods. The inertial motion was the same order of magnitude as the synoptic motion near the surface, but much weaker below. It was identified by the rotary spectrum, and it was slightly shifted toward lower frequencies in direct proportion to the time step used by the model. The Ekma motion appeared to be restricted to the baroclinic response above 70 m. The time-varying "geostrophic" flow below 7 0 m was essentially barotropic.