Entrainment and Diffusion in a Gulf Stream Cyclonic Ring

Abstract

The mixing and entrainment processes present in a cyclonic ring are investigated by means of a parametric model which is fitted to serial temperature data for a North Atlantic ring. The physical model assumes an axially symmetric ring in which the temperature is presumed to be governed bywhere KA and KZ are the horizontal and vertical eddy diffusivity coefficients and J(ψT) is the (r,z) Jacobian of the transverse streamfunction ψ and the temperature T. Data from two cruises during the 1967 observation of a ring by the Woods Hole Oceanographic Institution provided estimates of the derivatives of the temperature. Regression analysis was used to determine the coefficients for polynomial representations of ψ(r,z) for selected combinations of Kh and Kz. The study indicates upper bounds on the order of magnitude for the diffusivities (Kh,Kz)=(105,10) cm2 s−1 based upon near-minimum least-squares error estimates from the regression analysis. An important result is that little differentiation exists between a purely advective entrainment regime and those regimes including both entrainment and diffusion; i.e., the entrainment circulation appears to be the dominant mechanism in the temporal changes of the ring for a time scale of at least two months. The results provide streamline patterns for the transverse flow from the surface to 1000 m depth consistent with isotherm movement and changes in the ring water masses.