Anatomy of energy pathways in the subtropical gyre of the North Atlantic

The North Atlantic is a key region in the global ocean circulation and plays a central role in regulating our climate. Its circulation is constrained by energy transfers between time-mean currents at the basin-scale and time-varying ("eddy") flows from hundreds to order one kilometer, including low-frequency mesoscale and submesoscale currents and high-frequency internal waves. Here, we evaluate the eddy kinetic energy budget in the subtropical gyre of the North Atlantic (STG), with a focus on the contribution of the different flows. Sources and sinks of energy, non-local effects — spatial energy redistribution — and eddy-mean energy conversions are evaluated using two state-of-the-art numerical simulations, one without and with a tidal forcing. We show that the energy budget varies within the STG, from the leading-order energy balances to the contributions of time-varying flows. In the STG's Interior, where submesoscale currents and internal waves dominate the eddy reservoir, the main energy sources correspond to wind, tide, and baroclinic instability. At western and eastern boundaries, energy balances are the most complex. (i) Another major energy source is due to mean to eddy energy conversion and (ii) energy is significantly redistributed by non-local effects. At the western boundary, eddy-mean interactions and non-local effects primarily involve mesoscale and submesoscale currents and also internal tides (net contributions increased by a factor >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 North Atlantic's STG. This advocates for representing their contributions, along with the ones of mesoscale and submesoscale currents, in ocean models.