Three-Dimensional Flow in a Shallow Coastal Upwelling Zone: Alongshore Convergence and Divergence on the New Jersey Shelf

Abstract

Alongshore flow field divergence is investigated on the inner shelf off the coast of New Jersey during the summer upwelling season of 1996. Velocities obtained from an array of moorings are used to examine the flow field. Empirical orthogonal function (EOF) analyses are employed to extract those features of the flow field that are resolved by the moorings and to construct a measure of alongshore divergence. Multi-input single-output linear systems analysis is used to examine the role of buoyancy intrusions and wind stress in determining alongshore divergence or convergence of the flow field and to construct a proxy for divergence derived from a time series of stratification gradients and wind stress. Alongshore divergence on this shallow shelf (<25-m depth) is shown to be a function of both wind stress and buoyancy intrusions. The latter originate in the Hudson River outflow and produce downshelf flow (i.e., in the direction of Kelvin wave propagation). Examination of the first EOF mode (containing 67% of the variance) reveals that the largest resolved fluctuations are oriented alongshore and are characterized by strong counterclockwise veering with depth. Momentum balances demonstrate that the depth-averaged flow fields are governed by wind stress, bottom stress, and the across-shore pressure gradient. The second EOF mode describes significantly less variance (8%); however, it represents the across- and alongshore variations of the flow field, composing the bulk of the alongshore divergence. The production of alongshore divergence by a combination of wind stress and buoyancy intrusions is a likely outcome in upwelling regimes downshelf of a major buoyancy source.