Processes influencing wind-induced current profiles in near coastal stratified regions

Abstract

Cross sectional and single point models of wind-induced currents in a stratified sea coastal region are used to determine the role of internal waves due to various wind periods upon mixing and current profiles. The limitations of a point model in reproducing current profiles are also examined. Calculations are performed with both fixed vertical diffusion coefficients and those derived from a turbulence closure model. Wind forcing at sub-inertial, inertial and super-inertial frequencies together with a wind impulse are considered. Results show that the offshore extent of the coastal boundary layer where mixing occurs depends upon the frequency of the wind forcing. The extent of this region and offshore propagation of internal waves influences the offshore variability of current profiles and the extent to which they can be reproduced by a single point model. Calculations with periodic wind forcing show that within the coastal boundary layer a point model cannot reproduce the current profile due to the internal pressure gradients associated with the internal wave field. However outside this region, current profiles from a point model are in good agreement with those computed with the cross section model when a barotropic pressure gradient forcing proportional to the local wind forcing is included. In the case of transient wind forcing, the inertial period dominates and determines the offshore extent of the coastal boundary layer. The transient nature of the response is such that although the single point model, outside the coastal region can reproduce the surface current due to direct wind forcing, the current at depth is not reproduced. The offshore extent of the coastal boundary layer computed with the turbulence energy model is comparable to that found with fixed diffusion coefficients although in some cases there are important differences which influence current profiles.