‘Explosive’ meridional migration of cyclones and anticyclones

We examine the very rapid meridional migration of barotropic eddies and show that the main balance of forces is between the meridional β-induced force and the opposing form-drag generated by the flow surrounding the rapidly moving eddy. The solution is constructed by assuming that, as the flow around a solid cylinder, the flow surrounding the eddy detaches from the eddy downstream. We find the migration speed to be (3πβVm/5)1/2rm, where Vm is the maximum orbital eddy speed and rm is the radius where the maximum speed occurs. It is typically an order of magnitude faster than the classical westward speed induced by the baroclinic component (βrm 2). The above formula gives values that are within 30–50% of the speeds measured in both our own numerical experiments, numerical experiments conducted by others, as well laboratory experiments (conducted by others). We suggest that it is the barotropic component that very rapidly (10 km d−1) pulls young Agulhas rings towards the equator. Poleward propagating cyclones are not very common but, during POLYMODE, an extremely rapidly propagating cyclone was observed in the North Atlantic. It is speculated here that its rapid propagation was also due to a barotropic component.