Alongshore coherence at low frequencies in currents observed over the continental shelf off Oregon and Washington

Abstract

Current observations over the continental shelf at locations off central Oregon and southern Washington had the period from July 18 to September 18, 1972, in common. Low-frequency fluctuations (less than one cycle per day) in the currents are compared by means of visual display, linear regression, and spectral analysis. The currents are found to be highly coherent over an alongshore separation of 200 km. Coherent signals occur at 0.16, 0.3, and 0.44 cpd. The signal at 0.16 cpd occurs with high mutual coherence in wind, current, and sea level and may be a forced shelf wave driven by the wind. The signal at 0.3 cpd has high coherence between current and sea level and may be a free shelf wave generated by the wind. Correlation between observations is higher for separations in the alongshore direction than in the offshore direction in spite of greater separations.