Spectral resolution of oceanic baroclinic production: Exploring a novel eddy energy sink

The oceanic Lorenz energy cycle and its baroclinic eddy production B is analysed in realistic global and idealized models. B shows a dipolar structure in spectral space, acting as source of eddy kinetic energy at smaller, and as sink (B<0) at larger scales, partly balancing the inverse kinetic energy cascade. Together with a forward potential energy cascade, this opens a potential pathway for small scale eddy dissipation. To understand this structure of B, its geographical relation to different dynamical regimes is examined.
In the realistic model, B<0 is found predominantly poleward of 30° latitude, where the zonal mean current U tends to be eastward. Simulations using idealized models also show a connection between B<0 and the sign of U<0 consistently appears for U>0 but diminishes for U<0. When the meridional gradient of planetary vorticity β is set to zero, B<0 disappears, suggesting that planetary Rossby waves are also essential for its existence.
Linear stability analysis can explain the findings: it shows no B<0 for β=0, but B<0 at large-scales when β ≠ 0. It also shows that the vertical structure of B<0 changes with the sign of U: for eastward currents, B<0 is located in the upper half of the water column, whereas for U<0, it shifts to the bottom. Since eddy energy and B is surface intensified, the near-bottom energy sink at large-scales by B for U<0 is damped and becomes unimportant, while the near-surface B<0 for U>0 is amplified, which we see in turn in the models.