On the dynamics of wind-driven shelf currents

Abstract

Measurements of currents, wind, and coastal sea level from off Oregon, northwest Africa and Peru are used to examine characteristics of the depth-integrated momentum balance at mid-shelf locations. Attention is focused on determining the nature of the balance, as a function of frequency, of time-dependent terms in the alongshore momentum equation. Decomposition of the estimated terms into empirical orthogonal functions, and regression of terms on the wind stress to obtain the wind-forced component, are methods used in an attempt to assess objectively the type of balance present. It is found, for Oregon, that there is a relatively large amount of variance in the depth-integrated cross-shelf velocity which is not balanced by other estimated terms. In addition, a substantial component of the flow is wind-forced, with bottom friction insignificant for periods less than 11 days. For northwest Africa, the motion is strongly wind-driven, with bottom friction playing an important role for periods greater than 6 days and with a quasi-steady response evident for periods longer than 10 days. For Peru, the motion is dominated by an inviscid, unforced balance which, for the 5- to 11-day frequency band, is primarily between the alongshore pressure gradient and the rate of change with time of the alongshore velocity.