A Simple Kinematic Mechanism for Mixing Fluid Parcels across a Meandering Jet

Abstract

Recent observations of fluid parcel pathways in the Gulf Stream using isopycnal RAFOS floats revealed a striking pattern of cross-stream and vertical motion associated with meanders (Bower and Rossby 1989). In an attempt to explain the observed pattern, a two-dimensional kinematic model of a meandering jet has been developed which enables examination of the relationship between streamfunction patterns and fluid parcel trajectories. The streamfunction fields are displayed in a reference frame moving with the wave pattern so motions of fluid parcels relative to the jet can be seen more easily. The results suggest that the observed pattern of cross-stream motion results primarily from the downstream phase propagation of meanders. The model successfully reproduces several of the most distinctive features of the float observations: 1 ) entrainment of fluid into the Gulf Stream occurs at the leading edges of meander extrema while detrainment takes place at the trailing edges; 2) exchange between the Gulf Stream and its surroundings increases with a) increasing depth, b) increasing meander amplitude, and c) increasing wave phase speed. Transport calculations from the model streamfunction fields indicate that for typical phase speeds (10 km d⁻¹) and amplitudes (50 km), roughly 90% of the fluid in the surface layers of the Gulf Stream flows downstream in the jet while 10% continuously recirculates into the surroundings. In the deep main thermocline, where downstream speeds are less, only about 40% of the fluid is retained in the jet and 60% is trapped in the recirculating cells. It is concluded that this simple kinematic mechanism could lead to cross-stream mixing of fluid parcels, especially in the deeper layers of the Gulf Stream.