Modelling the early evolution of a Loop Current ring

The Colorado University Princeton Ocean Model (CUPOM) is used here to study the early stages in the life of Millennium, a mesoscale anticyclonic ring that detached from the Loop Current on April 2001 and lasted for more than 100 days. The numerical near-surface velocity field for the Gulf of Mexico is validated with the altimetry geostrophic velocities. The first 30 days of numerical data, before Millennium interacts with other mesoscalar features, are closely examined both from Eulerian and Lagrangian perspectives. During this time Millennium had a near-constant rotation period of 6.5 days, and particles do not leave the ring. Nevertheless, the distributions of temperature, salinity, and angular velocity confirm the existence of significant (possibly numerical) radial diffusion. Polar-coordinate phase plots for temperature–salinity anomalies and tangential–radial velocities, at several depths, illustrate the presence of an evolving oscillating pattern. Radial and tangential velocities change in phase, associated with vertical displacements of the isothermal and isohaline surfaces. A simple diffusion model with an effective diffusion coefficient of 200 m2 s− 1 is appropriate to grossly simulate the temporal evolution of angular velocity within Millennium.