Talk Abstract:
Ocean turbulence transfers energy across scales, and is a key ingredient in shaping the ocean circulation. Conventional turbulence dynamics suggests that the kinetic energy in the ocean is transferred primarily to larger scales, via the inverse energy cascade that emerges under the influence of Earth's rotation. This raises the question of how dissipation is achieved in oceanic flows, which would necessarily require an energy transfer towards the small molecular scales. It has been argued that flows that are not formed by a balance between the Coriolis and pressure forces, such as ageostrophic fronts and internal waves, can produce a forward cascade of kinetic energy. This hypothesis, which is corroborated by evidence from direct numerical simulations, has been suggested as a potential route, among others, toward dissipation. However, so far no clear observational quantification of the energy transfer at the scales where this forward cascade might be potentially active has been made.Here we provide the first direct observational evidence of a dual energy cascade in the ocean, by analyzing in-situ velocity measurements from surface drifters, released in dense arrays in the Gulf of Mexico, with the help of a novel methodology, which can directly use non-gridded measurements, for quantifying energy transfer and energy injection scales in turbulent flow. Our results show that the kinetic energy cascades to both small and large scales, with the downscale flux dominating at scales smaller than approximately 1-10km. The results suggest the presence of two dominant range of scales at which energy is injected in the oceanic flow. We also show that there is a seasonality in the energy transfers, with a stronger release of kinetic energy into the surface flows and a more energetic cascade to smaller scales detected during the winter months.