A Laboratory Study of the Zonal Structure of Western Boundary Currents

Abstract

The zonal structure of strongly nonlinear inertial western boundary currents (WBCs) is studied experimentally along a straight ?meridional? coast in a 5-m-diameter rotating basin by analyzing the ?zonal? profile of the meridional velocity field as a function of transport intensity and other dynamical parameters. The return flow that is generated by the surface wind stress curl in the oceanic interior is forced in the rotating basin by the motion of a piston, in the absence of any surface stress. The laboratory setup consists of two parallel rectangular channels separated by an island and linked by two curved connections: in the first channel, a piston is forced at a constant speed up ranging from 0.5 to 3 cm s?1 over a distance of 2.5 m, producing a virtually unsheared current at the entrance of the second channel. In the latter, a linear reduction of the water depth provides the topographic beta effect that is necessary for the development of the westward intensification. Nearly steady currents are obtained and measured photogrammetrically over a region of about 1 m2. In all of the experiments performed, an appropriate horizontal Reynolds number (Re = ε/E, where ε and E are dimensionless numbers measuring the importance of nonlinearity and lateral friction, respectively) is Re ? 1. The zonal profile of the meridional velocity is always found to have (away from the viscous boundary layer) a nearly exponential structure typical of inertial WBCs, whose width agrees well with the classical inertial boundary layer length scale δI. A control experiment (with up = 1 cm s?1) is analyzed in detail: it has the same ε as the Gulf Stream (GS) but a much smaller E. This implies that the laboratory flow is expected to be geometrically similar to the GS outside the viscous boundary layer, but to differ within it. To assess the effect of such a departure from dynamic similarity, a mathematical model is used that numerically simulates a flow that is fully dynamically similar to the GS. The comparison between the profile thus obtained numerically and the one obtained experimentally shows that they are, indeed, virtually coincident outside the viscous boundary layer, except for a small offset that tends to vanish as Re ? ∞. Moreover, additional sensitivity experiments in which the piston speed, the rotation rate of the basin, the topographic beta effect, and the width of the main channel are varied provide further information on the zonal structure of WBCs.