Cross-shelf variations of near-inertial current oscillations

Observations and numerical simulations show that cross-shelf variations of current oscillations at near-inertial frequencies increase exponentially in amplitude from nearshore to waters with depths of 100-200 m. An assessment of a semi-analytical theory of the cross-shelf variation of near-inertial current oscillations (Chen and Xie, 1997, Journal of Geophysical Research 1 02(C7), 15,583-15,593) indicates that, although instructive, the theory doe s not capture all the elements seen in numerical simulations. Here a purely analytical approach is taken. It assumes only that the cross-shelf current s are of the form AB cos ft, where B is an amplitude modifier (a function o f the distance offshore) and f is the Coriolis parameter. The solution to t he governing equations gives B = 1 - e(-x/L), where x is the distance offsh ore and L is a radius of deformation length scale. The solutions show that exponentially increasing current oscillations over continental shelves are actually a form of inertial-gravity waves. In non-equatorial regions, the e ffect of the sea surface pressure gradient is found to be in phase with tha t of the Coriolis effect but with a much larger magnitude over the inner co ntinental shelves. The kinematic boundary condition results in the oscillat ing sea level setup and setdown, the resulting pressure gradient drives the oscillatory cross-shelf currents over the inner continental shelf, and the Coriolis effect results in a corresponding oscillatory flow in the longsho re direction. One of the more notable findings is that the length scale L, referred to here as the inertial-gravity barotropic radius of deformation, is found to be significantly larger than the classical barotropic radius of deformation. (C) 2001 Elsevier Science Ltd. All rights reserved.