1 Introduction Mesoscale eddies account for 80% of the surface kinetic energy and are a key component of the global ocean energy budget (Ferrari & Wunsch, 2009; Müller et al., 2005; Wunsch, 2007). They have horizontal scales of the order of the first Rossby deformation radius (Rd) or larger (Chelton et al., 2011). At these scales, the velocity field can be decomposed into a leading order geostrophic and a weaker ageostrophic component, following the quasi-geostrophic theory (Gill, 1982).
