Global Ocean Cascades and a Direct Gyrescale-Mesoscale Energy Pathway Induced by the Atmosphere*

Our understanding of the ocean's spatial scales and their coupling has been derived mostly from Fourier analysis in small "representative" regions, typically a few hundred kilometers in size, that cannot capture the vast dynamic range at planetary scales. Using coarse-graining on a 1/12-degree reanalysis dataset, we probe a range of spatial scales spanning more than three orders of magnitude, including mesoscales and planetary scales, and quantify both the kinetic energy (KE) spectral density and the across-scale KE transfer. Since coarse-graining preserves the temporal signal, we are able to observe in the oceanic mesoscale, a steady propagation of energy to larger scales across the seasonal cycle, indicative of a scale-local energy cascade. This 'spectral advection' signal presents a characteristic lag time of ~27 days per octave, and is present in both the KE spectra and the KE transfer. Combined with a phase-lag between the KE spectra and KE transfer signals, this suggests that energy transferred across scale L is primarily deposited at scales 4×L. We also find that the highly energetic mesoscales have a direct transfer with gyrescales (> 1000km), which is induced by the atmospheric circulation cells (Hadley, Ferrel, Polar). This gyrescale-mesoscale exchange is new and absent from standard theories of gyre circulation.