Abstract

A nonlinear mechanism for the generation of anticyclonic lens-like eddies from boundary currents is proposed. In contrast to the familiar generation processes that rely on unstable long waves that grow and close upon themselves or vortex shedding due to the geometry of the boundary, the present mechanism is related to intermittency in the current's mass transport. The essence of the new mechanism is that intermittencies in the transport (such as those in the Denmark Strait or the Mediterranean outflow) lead to unbalanced patches of fluid which break up into a discrete sets of eddies that interact with the boundary. The process is highly nonlinear because both the amplitude and the Rossby number are of order unity. It is modeled as follows: we begin with a rectangular box containing the motionless (light) fluid near the boundary. At, say, t = 0, the conceptual box is removed the unbalanced fluid undergoes two main processes. The first involves the establishment of a set of eddies via breakup and geostrophic adjustment, whereas the second is associated with the interaction of the set with the wall. These two processes are examined independently even though in reality the processes are, obviously, taking place at the same time. To examine the first processes we consider the nonlinear collapse of a (light) rectangular box in the open ocean away from the boundary. The breakup processes involves, of course, some sort of instability (because the patch does not remain intact) but this is not necessarily related to the long wave instability that is usually associated with long gravity currents. The general structures of the resulting final chain of eddies can be computed analytically by using the usual connecting principles, the conservation of potential vorticity and mass. It turns out, however, that the number of eddies and their detailed structure cannot be computed unless one invokes an additional constraint. To resolve this closure difficulty, the integrated angular momentum constraint, which is rarely used in oceanographic modelling, has been applied. The details of the second process (i.e. chain-wall interaction) are examined with the aid of the (above) results for the chained offshore eddies and the single eddy-wall interaction analysis of Nof (1988, Journal of Marine Research, 46, 527dash555). A combination of these two studies shows that a chain of lens-like eddies forced against a wall would leak fluid until all the eddies in the chain are merely ‘kissing’ the wall. Simple qualitative laboratory experiments on a rotating table support the conclusion that intermittencies in the current's transport lead to a group of eddies that leak along the wall. Such a group is formed even if the equivalent steady current is stable to long wave perturbation, i.e. the laboratory current would not have broken up had it not been terminating shoprlty after its formation. It is suggested that the observed mid-Atlantic eddies resulting fromt he Mediterranean outflow (Meddies) might be formed by a mechanism similar to our newly suggested process.

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