Lagrangian motion and fluid exchange in a barotropic meandering jet

Kinematic models predict that a coherent structure, such as a jet or an edd y, in an unsteady flow can exchange fluid with its surroundings. The author s consider the significance of this effect for a fully nonlinear, dynamical ly consistent, barotropic model of a meandering jet. The calculated volume transport associated with this fluid exchange is comparable to that of flui d crossing the Gulf Stream through the detachment of rings. Although the mo del is barotropic and idealized in other ways, the transport calculations s uggest that this exchange mechanism may be important in lateral transport o r potential vorticity budget analyses for the Gulf Stream and other oceanic jets. The numerically simulated meandering jet is obtained by allowing a s mall-amplitude unstable meander to grow until a saturated state occurs. The resulting flow is characterized by finite-amplitude meanders propagating w ith nearly constant speed, and the results clearly illustrate the stretchin g and stirring of fluid particles along the edges of the recirculation regi ons south of the meander crests and north of the troughs. The fluid exchang e and resulting transport across boundaries separating regions of predomina ntly prograde, retrograde, and recirculating motion is quantified using a d ynamical systems analysis. The geometrical structures that result from the analysis are shown to be closely correlated with regions of the how that ar e susceptible to high potential vorticity dissipation. Moreover, in a relat ed study this analysis has been used to effectively predict the entrainment and detrainment of particles to and from the jet.