• Energy Research

We examine the ocean energy cycle where the eddies are defined about the ensemble mean of a partially air-sea coupled, eddy-rich ensemble simulation of the North Atlantic. The decomposition about the ensemble mean leads to a parameter-free definition of eddies, which is interpreted as the expression of oceanic chaos. Using the ensemble framework, we define the reservoirs of mean and eddy kinetic energy (MKE and EKE respectively) and mean total dynamic enthalpy (MTDE). We opt for the usage of dynamic enthalpy (DE) as a proxy for potential energy due to its dynamically consistent relation to hydrostatic pressure in Boussinesq fluids and non-reliance on any reference stratification. The curious result that emerges is that the potential energy reservoir cannot be decomposed into its mean and eddy components, and the eddy flux of DE can be absorbed into the EKE budget as pressure work.We find from the energy cycle that while baroclinic instability, associated with a positive vertical eddy buoyancy flux, tends to peak around February, EKE takes its maximum around September in the wind-driven gyre. Interestingly, the energy input from MKE to EKE, a process sometimes associated with barotropic processes, becomes larger than the vertical eddy buoyancy flux towards the summer and autumn. Our results question the common notion that the inverse energy cascade of winter-time EKE energized by baroclinic instability within the mixed layer is solely responsible for the summer-to-autumn peak in EKE, and suggest that the non-local eddy transport of DE and local transfer of energy from MKE to EKE could also contribute to the seasonal EKE maxima.

AbstractWestern 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.