Non-local Eddy-Mean Kinetic Energy Transfers in Submesoscale-Permitting Ensemble Simulations

Abstract

Understanding processes associated with eddy-mean flow interactions helps our interpretation of ocean energetics, and guides the development of parameterizations. Here, we focus on the non-local nature of Kinetic Energy (KE) transfers between mean and turbulent reservoirs. Transfers are interpreted as nonlocal when the energy extracted from the mean flow does not locally sustain an growth of energy in the turbulent flow, or vice versa. The novelty of our approach is to use ensemble statistics to define the mean and the turbulent flow. Based on KE budget considerations, we first rationalize the eddy-mean separation in the ensemble framework, and discuss the interpretation of a mean flow driven by the prescribed (surface and boundary) forcing and a turbulent flow u′ driven by non-linear dynamics sensitive to initial conditions. We then analyze 120-day long, 20-member ensemble simulations of the Western Mediterranean basin run at resolution. Our main contribution is to recognize the prominent contribution of the cross energy term to explain non-local energy transfers, which provides a strong constraint on the horizontal organization of eddy-mean flow KE transfers since the cross energy term vanishes identically for perturbations orthogonal to the mean flow . We also highlight the prominent contribution of vertical turbulent fluxes for energy transfers within the surface mixed layer. Analyzing the scale dependence of non-local energy transfers supports the local approximation usually made in the development of mesoscale, energy-aware parameterizations for non-eddying models, but points out to the necessity of accounting for non-local dynamics in the meso-to-submeso scale range. Key Points► Ensemble-based eddy-mean decomposition of kinetic energy budget supports the view of an ocean turbulence driven by internal dynamics ► Turbulent fluxes of the cross energy term provide a potentially strong horizontal constraint on eddy-mean flow interactions ► Non-localities are leading order at small scales and should be accounted for in submesoscale parameterizations Please note that this is an author-produced PDF of an article accepted for publication following peer review. The definitive publisher-authenticated version is available on the publisher Web site.