Mechanisms of baroclinic eddy damping on large scales

The oceanic Lorenz energy cycle and its baroclinic eddy conversion B are analysed in realistic global and idealized models. We find a dipolar structure of B 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. In the realistic model, B < 0 is found predominantly poleward of 30° latitude, where the zonal mean current ū tends to be eastward. Simulations using idealized models also show a connection between B < 0 and the sign of ū : B < 0 consistently appears for ū > 0, but diminishes for ū < 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 ū : for eastward currents, B < 0 is located in the upper half of the water column, whereas for westward currents, it shifts to the bottom. Since eddy energy and B are surface intensified, the near-bottom energy sink at large-scales by B for ū < 0 is damped and becomes unimportant, while the near-surface B < 0 for ū > 0 is amplified, which we see in turn in the models.