... exploit the existence of a pair of Lagrangian invariants in the model,it is natural to introduce the Lagrangian coordinates X(x, t) of the fluid “parcels” (in fact, fluid lines along the y-axis). They ... describe the first stage of the beta-drift in which the in uence of the far-field of the Rossby wave wake is not important. In the ocean,his effect becomes dominant after a few weeks. This wake drains ... vortices in the ocean, Rev. Geophys., 23, 165(1985). 65 100 . Mc Williams, J.C.: Geostrophic vortices. In: Nonlinear Topics in Ocean Physics. Proceed-ings of the International School of Physics...