Trajectory and propagation speed (cm/s) of a dipole eddy pair (modon) in the Tasman Sea that formed off Tasmania, Australia (left) and together propagated eastward at speeds of 10-20 cm/s before separating off New Zealand (right). In contrast, Rossby wave speeds in the region are typically ~1.2 cm/s. The start date for eddy tracking was 2 September 2010 (black dot), unfilled circles indicate every 30th day, with circle size indicating time from formation (large) to split up (smaller). Credit: Hughes and Miller, 2017, Figure 2e
Source: Geophysical Research Letters

Typical ocean eddies travel predominantly westwards at speeds close to the local baroclinic Rossby wave speed that may be accelerated or decelerated by the speed and direction of the (often) stronger mean background flow. However, an exception to this is theoretically expected to occur for dipole eddy pairs, known as modons, that couple together to have a zero net mass anomaly so there is no longer any constraint on the propagation speed or direction. Hughes and Miller [2017] observed nine such modons in altimetry data in the southern midlatitude ocean. The rapidly moving modons were tracked over large distances, both with and against the mean flow, and were coherent for more than six months. The modons thus offer an efficient means to carry vast amounts of anomalous heat, nutrients and carbon that might have important ecological consequences.

Citation: Hughes, C. W., & Miller, P. I. [2017]. Rapid water transport by long-lasting modon eddy pairs in the southern midlatitude oceans. Geophysical Research Letters, 44.

—Janet Sprintall, Editor, Geophysical Research Letters


Sprintall, J. (2018), Two paired eddies travel faster and further than one, Eos, 99, . Published on 02 February 2018.

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