Abstract
Boyd's previous work on equatorial Rossby solitary waves which derived the Korteweg-deVries equation using the method of multiple scales is here extended in several ways. First, the perturbation theory is carried, to the next highest order to (i) assess the accuracy and limitations of the zeroth-order theory and (ii) analytically explore solitons of moderate amplitude. Second, using the refined theory, it is shown that Rossby solitary waves will carry a region of closed recirculating fluid along with the wave as it propagates provided that the amplitude of the wave is greater than some (moderate) threshold. The presence of such closed “streaklines”, i.e., closed streamlines in a coordinate system moving with the wave, is an important property of modons in the theory of Flieri, McWilliams and others. The “closed-streakline” Rossby waves have many other properties in common with modons including (i) phase speed outside the linear range, (ii) two vortex centers of equal magnitude and opposite sign, (iii) vortex centers aligned due north-south, (iv) propagation east-west only and (v) a roughly circular shape for the outermost closed streakline, which bounds the region of recirculating fluid. Because of these similarities, it seems reasonable to use “equatorial modon” as a shorthand for “closed-streakline, moderate amplitude equatorial Rossby soliton,” but it should not be inferred that the relationship between midlatitude modons and equatorial solitary waves is fully understood or that all aspects of their behavior are qualitatively the same. Kindle's numerical experiments which showed that small amplitude Rossby solitons readily appear in El Niño simulations, suggest—but do not prove—that the very large El Niño of 1982 could have generated equatorial modons.
