'Explosive' meridional migration of cyclones and anticyclones

• Published in: Tellus. Series A, Dynamic meteorology and oceanography Vol. 60; no. 2; pp. 372 - 383
• Main Author: Nof, Doron
• Format: Journal Article
• Language: English
• Published: Oxford, UK Taylor & Francis 01.03.2008; Blackwell Publishing Ltd; Blackwell
• Subjects: Dynamics of the ocean (upper and deep oceans); Earth, ocean, space; Exact sciences and technology; External geophysics; Marine; Physics of the oceans; Atlantic Ocean; Agulhas current; AN, North Atlantic; ISW, West Indian Ocean, Agulhas Current; Barotropic instability; Cyclonic eddy; Meridional flow; Anticyclonic eddy; Vortex motion; Rapid process; Ocean dynamics; kinetics; Western boundary current
• ISSN: 0280-6495; 1600-0870; 1600-0870
• DOI: 10.1111/j.1600-0870.2007.00297.x

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 (3πβV m /5) 1/2 r m , where V m 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 (βr m 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.