Cross-scale interactions and turbulent cascades in the North Atlantic Ocean

The dynamics of the ocean at a scale < 100km (fine-scales) is currently not well known. This is due to the lack of sufficient observational datasets at this scale-range in the ocean. There are suggestions from recent studies that classes of motions at this scale-range impacts the distribution and exchanges of kinetic energy in the ocean and that balanced submesoscale motions (<50km) and unbalanced internal gravity waves can play an active role in fluxing kinetic energy towards dissipative scales in the ocean. To better understand fine-scale motions, the Surface Water and Ocean Topography (SWOT) satellite with the task of providing an unprecedented view of the ocean down to a wavelength of 10-15km is been assembled and expected for launch in 2021. In anticipation of SWOT, numerical ocean models capable of resolving fine-scales has been designed and implemented. In this study, we use three of these simulations to investigate (i) the spatial and temporal variability of oceanic eddies at fine-scales down to 10km scale, (ii) cross-scale kinetic energy exchanges at fine-scales in a regime of active submesoscale motions and (iii) in a regime of externally forced internal tides. Our results show that the distribution of oceanic eddies at spatial scale < 100km undergo strong seasonality and that this seasonality is as a result of an increased population of submesoscales eddies (10 - 50km) in wintertime. We found that submesoscale turbulence (a class of oceanic turbulence at fine-scale) is responsible for the increase in the distribution of submesoscales eddies in winter. Further analysis showed that submesoscale turbulence also affects the kinetic energy cascade by providing a route towards dissipation both at the surface and in the interior of the ocean. Interestingly, this cascade in the presence of externally forced internal tides is increased by a factor of 3 in summertime due to enhanced wave activity by tidal motions. Our analysis also shows that not accounting for the ageostrophic flows in the calculation of cross-scale kinetic energy exchanges might underestimate the true magnitude of the forward cascade of energy.