Diagnosing Cross‐Scale Kinetic Energy Exchanges From Two Submesoscale Permitting Ocean Models.

Fine‐scale motions (<100 km) contribute significantly to the exchanges and dissipation of kinetic energy in the upper ocean. However, knowledge of ocean kinetic energy at fine‐scales (in terms of density and transfers) is currently limited due to the lack of sufficient observational data sets at these scales. The sea‐surface height measurements of the upcoming Surface Water and Ocean Topography (SWOT) altimeter mission should provide information on kinetic energy exchanges in the upper ocean down to 10–15 km. Numerical ocean models, able to describe ocean dynamics down to ∼10 km, have been developed in anticipation of the SWOT mission. In this study, we use two state‐of‐the‐art, realistic, North Atlantic simulations, with horizontal resolutions ∼1.5 km, to investigate the distribution and exchanges of kinetic energy at fine‐scales in the open ocean. Our results show that the distribution of kinetic energy at fine‐scales approximately follows the predictions of quasigeostrophic dynamics in summertime but is somewhat consistent with submesoscale fronts‐dominated regimes in wintertime. The kinetic energy spectral fluxes are found to exhibit both inverse and forward cascade over the top 1,000 m, with a maximum inverse cascade close to the average energy‐containing scale. The forward cascade is confined to the ocean surface and shows a strong seasonality, both in magnitude and range of scales affected. Our analysis further indicates that high‐frequency motions (<1 day) play a key role in the forward cascade and that the estimates of the spectral fluxes based on geostrophic velocities fail to capture some quantitative aspects of kinetic energy exchanges across scales. Plain Language Summary: The dynamics of oceanic motions with scales <100 km (fine‐scales) are currently not well known. This is due to the lack of sufficient observational datasets at these scales in the ocean. There are suggestions from recent studies that this class of motions impacts the distribution and exchanges of kinetic energy in the ocean. To better understand fine‐scale motions, the Surface Water and Ocean Topography (SWOT) satellite has been assembled. SWOT is expected for lunch in 2022 and will provide an unprecedented view of the ocean down to a wavelength of 10–15 km. In anticipation of the SWOT mission, numerical ocean models capable of resolving fine‐scale oceanic motions have been designed and implemented. In this study, we use two of these simulations to investigate how kinetic energy is exchanged between oceanic motions at fine‐scales. Our results show that submesoscale turbulence (a class of oceanic turbulence at fine‐scale) and high‐frequency motions affect the kinetic energy exchanges by providing a route to kinetic energy toward dissipation. Also, we found that kinetic energy exchanges based on the future SWOT data set might underestimate the true magnitude of the transfer of kinetic energy toward finer scales. Key Points: We used two submesoscale permitting ocean models of the North Atlantic Ocean to investigate kinetic energy exchanges at fine‐scalesKE fluxes at fine‐scales are strongly impacted by submesoscale turbulence with a stronger forward cascade in winter within the mixed‐layerNot accounting for ageostrophic motions yields a significant underestimation of the forward cascade [ABSTRACT FROM AUTHOR]