Eddy trains and eddy jets tracked by constellated altimetry

Abstract

Systematic eddy propagation with a westward dominance is a well-documented feature in the global ocean, thanks to the availability of constellated altimeter measurement of sea level anomaly with a centimeter-level accuracy. According to their zonal speed relative to the phase speed of the first baroclinic Rossby wave, we propose that eddies can be divided into two modes: “eddy trains” and “eddy jets”. The former refers to isolated eddies intrinsically guided by the long Rossby wave theory in zonal propagation, meanwhile significantly advected by energetic extratropical currents (via Doppler-like effect) and equatorial planetary waves (basically Kelvin waves). The latter refers mostly to “geared” eddy dipoles (also known as “modons” governed by the equivalent barotropic vorticity equation) that propagate with a “super” speed escaping from the Rossby wave guide. In this study, departure and reversal of eddy trains from the Rossby theory are analyzed and compensated, meanwhile the signature and mechanism of eddy jets are identified and revealed. As a result, three distinct regimes are observed in oceanic eddy propagation: A primary Rossby regime (∼66%, westward), a secondary anti-Rossby regime (∼33%, eastward, modulated by Doppler-like and Kelvin wave effects), and a third minor non-Rossby regime (∼1%, zonally neutral, of dipolar nature with gear effect). The first two regimes combine to form the eddy trains, while the third regime belongs to the eddy jets. A clear understanding of the mechanism and pattern of the complex eddy propagations is critical to marine science in general, and to eddy science in particular.