Western boundaries have been suggested as mesoscale eddy graveyards, using a diagnostic of the eddy kinetic energy (EKE) flux divergence based on sea surface height (η). The graveyard's paradigm relies on the approximation of geostrophy—required by the use of η—and other approximations that support long baroclinic Rossby waves as the dominant contribution to the EKE flux divergence. However, a recent study showed an opposite paradigm in the Agulhas Current region using an unapproximated EKE flux divergence. Here, we assess the validity of the approximations used to derive the η-based EKE flux divergence using a regional numerical simulation of the Agulhas Current. The EKE flux divergence consists of the eddy pressure work (EPW) and the EKE advection (AEKE). We show that geostrophy is valid for inferring AEKE, but that all approximations are invalid for inferring EPW. A scale analysis shows that at mesoscale (L > O(30) km), EPW is dominated by coupled geostrophic-ageostrophic EKE flux and that Rossby waves effect is weak. There is also a hitherto neglected topographic contribution, which can be locally dominant. AEKE is dominated by the geostrophic EKE flux, which makes a substantial contribution (54%) to the net regional mesoscale EKE source represented by the EKE flux divergence. Other contributions, including topographic and ageostrophic effects, are also significant. Our results support the use of η to infer a qualitative estimate of the EKE flux divergence in the Agulhas Current region. However, they invalidate the approximations on mesoscale eddy dynamics that underlie the graveyard's paradigm.

Key Points

We assess whether the mesoscale eddy energy flux divergence can be calculated from sea surface height in the Agulhas Current region

Geostrophy allows a qualitative estimate of eddy energy advection, but not of eddy pressure work

This favors the use of sea surface height, but challenges the founding approximations of an earlier paradigm

Plain Language Summary

In the ocean, the most energetic motions are large-scale eddies with horizontal scales ranging from tens to hundreds of kilometers. These are major components of the ocean energy budget, and unraveling their lifecycles is crucial to improving our understanding of ocean dynamics. Although the generation of large-scale eddies is well documented, how their energy is dissipated remains uncertain. Based on satellite observations of the sea surface and approximations to the dynamics of large-scale eddies, it has been suggested that they decay at western boundaries of oceanic basins, thereby closing their lifecycle. However, based on different data and approximations, a recent study has suggested that large-scale eddies are predominantly generated in a specific western boundary region, such as the Agulhas Current. Our study explains which of the data (sea surface observations) or the assumed leading order dynamics (approximations) explains the opposite eddy energy sources and sinks shown by the two studies in the Agulhas Current region. Our results show that the use of sea surface observations is valid for qualitatively inferring the regional eddy energy source, but not the assumed leading order dynamics. This has implications for (a) our understanding and (b) study strategies of the energetics of large-scale eddies.