Exchange Geometry Revealed by Float Trajectories in the Gulf Stream

Abstract

In an effort to understand the extent to which Lagrangian pathways in the Gulf Stream indicate fluid exchange between the stream and its surroundings, trajectories of RAFOS floats are viewed in a frame of reference moving with the dominant zonal phase speed associated with the periodic flow. In such a frame, geometrical structures emerge that more clearly delineate the position of the parcel in relation to the jet core and its surroundings. The basic premise of this work is that the pathways of fluid parcels in the vicinity of stagnation points, as defined in the moving frame of reference, are susceptible to changes in their pathways, thereby facilitating fluid exchange between different regions of the flow field. Four representative RAFOS float trajectories are shown to exhibit the expected behavior in the vicinity of stagnation points. To further examine the mechanism of exchange in the vicinity of these geometrical features, concepts from dynamical systems theory are applied to a numerically simulated flow field. The entrainment and detrainment of parcels from the jet core are explained in the context of stable and unstable manifolds and their associated lobes. It is shown that the Lagrangian pathways from the numerical flow and the observational trajectories exhibit a similarity based on the kinematics of a meandering flow field. Overall, this study provides the first look at RAFOS float trajectories in a moving frame and provides insight as to how the temporal variability of a jet creates chaotic exchange.