A laboratory model of exchange and mixing between western boundary layers and subbasin recirculation gyres

Abstract

Chaotic advection is suggested as a possible mechanism for fluid exchange and mixing among a western boundary current and subbasin recirculation gyres. Applications include the North Atlantic Deep Western Boundary Current and its adjacent mesoscale recirculation gyres. Visualization and quantification of certain aspects of chaotic advection in a laboratory analog are described. Depending on the strength of the forcing. recirculating fluid offshore of the western boundary layer may be contained in a single gyre (not favorable for chaotic advection) or twin gyre with a "figure-eight" geometry (favorable for chaotic advection). When time dependence is imposed on these steady flows by varying the forcing periodically, the resulting fluid exchange, stirring, and mixing is most dramatic in the case of the twin gyre. A template for these processes can be formed by highlighting certain material contours (invariant manifolds) using dye and other techniques. These objects can be used to identify blobs of fluid (turnstile lobes) that are carried into and out of the gyres. The associated transports and flushing times can be estimated. The preferential stirring and mixing in the twin-gyre case is quantified by calculating the effective diffusivity of the flow field based on snapshots of the dye fields at longer times. The experiment suggests how tracers in a western boundary current might be transported into and out of neighboring recirculations and where regions of strong zonal or meridional transport might occur.