Seasonal differences in low‐frequency current fluctuations over the Oregon continental shelf

Abstract

Current observations over three isobaths (50, 100, and 200 m) on the Oregon continental shelf are available for winter and spring 1975 and summer 1973. In each season there are low‐frequency current fluctuations with periods of several days, predominantly alongshore, that are correlated with the along‐shore wind stress and sea level at Newport. The amplitude of the current fluctuations is greatest in winter and least in summer, but the ratio between current and wind stress fluctuations does not change appreciably. In each season the alongshore fluctuations have the same sign throughout the water column, and they are nearly in phase across the shelf. The vertical and offshore structure of the fluctuations was examined by means of empirical orthogonal functions. In winter the amplitude decreases with depth at each location; in summer it is nearly constant at all locations. The near‐bottom shear is consistent with Ekman veering. Away from the bottom the shear is in approximate geostrophic balance. The fluctuations have a significant baroclinic component in winter but not in summer. In each season the amplitude of the fluctuations decreases with distance from shore, but the offshore length scale is about twice as great in winter as it is in spring.