Anatomy of energy pathways in the subtropical North Atlantic

The North Atlantic is a central region in the global ocean circulation and for regulating our climate. Its circulation is constrained by energy transfers between time-mean currents, flowing at the basin scale, and time-varying (eddy) flows ranging from hundreds to order one kilometer. Eddy flows include low-frequency mesoscale and submesoscale currents and high-frequency internal waves. Here, we evaluate the eddy kinetic energy budget in the subtropical North Atlantic (ST) using two state-of-the-art numerical simulations, one without and one with a tidal forcing. We evaluate sources and sinks of energy, non-local effects spatial energy redistribution and eddy-mean energy conversions. We show that the energy budget varies within the ST, from the leading-order energy balances to the contributions of eddy flows. In the STs Interior, where submesoscale currents and internal waves dominate the eddy reservoir, the primary energy sources correspond to wind, tide, and baroclinic instability. At western and eastern boundaries, energy balances are the most complex. Mean-to-eddy energy conversion accounts for a substantial energy source, and non-local effects largely recirculate energy. At the western boundary, both energy contributions primarily involve mesoscale and submesoscale currents and internal tides (net contributions increased by a factor of 2 in the tidal scenario). At the eastern boundary, both energy contributions primarily involve internal tides (net contribution increased by a factor of 3-4 in the tidal scenario). Our results emphasise the substantial impact of internal tides on the time-mean circulation of the subtropical North Atlantic. This advocates for representing their contributions in ocean models, along with the ones of mesoscale and submesoscale currents.