Subinertial Frequency Response of Wind-Driven Currents in the Mixed layer Measured by Drifting Buoys in the Midlatitude North Pacific

Abstract

Previous analyses of satellite-tracked drifting buoy data (30 m drogue depth) and Fleet Numerical Ocean Center (FNOC) surface wind stress in the midlatitude North Pacific during autumn/winter have shown near-surface current vectors 25° to the right of the surface wind stress vector (i.e., approximately parallel to sea level air pressure isobars). In the present study, the complex coherence between time series of the two vectors, near-surface current and surface wind stress, is examined using the vector cross-spectral analysis technique developed by Mooers, yielding the frequency response of surface current to wind stress from the inertial frequency down to one cycle per 16 days. The analysis shows that during summer the near-surface a few days. In this season, the rotary spectrum of the near-surface current is dominated by anticyclonic motion, with periods of approximately 8 to 32 days, that is not locally wind-forced. In contrast, during autumn/winter, the two vectors are highly coherent over the subinertial frequency range corresponding to periods of 1 to 16 days. The phase estimates provided by vector cross-spectral analysis yield information on both the mean spatial angle and mean temporal phase difference between the major axes of the two ellipses described by the vector motions. Over the same subinertial frequency range where the coherence amplitude is significant, the average spatial angle between the major axis of the wind stress and the major axis of the near-surface current varies from 75° at the near-inertial frequencies to 15° at the low frequencies. The sign of the spatial angle is such that the near-surface current vector is always directed to the right of the surface wind stress vector. The temporal phase differences between the vectors show that the near-surface current vector lags the surface wind stress vector by 20° to 30° at near-inertial frequencies, diminishing to zero degrees with decreasing frequency. These phase lags correspond to temporal lags of up to four hours.