2.6 P′ 2.9 1.1 0.1 0.4 2.5 2.3 13 9.4 1.4 0.7 +1.9 1.5 +1.0 0.8 -0.9 P K K′ (x 3.4) (x 3) (x 4) (x 2.4) (x 3.3) (x 4) (x 3.3) (x 4) 7.7 P′ 3.7 3.5 4.2 0.5 3.3 6.1 30.0 24.0 1.4 0.3 +1.8 4.0 +0.3 3.65 -1.5 P K K′ (x 1.3) (x 1.8) (x 1.4) (x 9.5) (x 7.7) (x 3.7) (x 2) Anatomy of energy pathways in the subtropical gyre of the North Atlantic Pauline Tedesco1, Jonathan Gula2,3, Ali Mashayek1 1University of Cambridge, Earth Sciences, United Kingdom, 2University of Western Brittany, Laboratory of Ocean Physics and Satellite remote sensing (LOPS), France; 3Institut Universitaire de France, France pfmt2@cam.ac.uk Our energy analysis uses the concept of energy separation in a time-mean ( ) and time-varying ( ) component: with the horizontal velocity components, the reference density, a 4-year average, and deviation from this average. The steady-state balance of and reservoirs is analyzed with a regional formulation of the Lorenz Energy Cycle (Lorenz, 1955): K K′ K = 1 2 ρ0(u2 + v2) and K′￼= 1 2 ρ0(u′￼2 + v′￼2) (u, v) ρ0 ( . ) ( . )′ K K′ 2. Theory P′ P K K′ Swind(K) Sb. drag(K) Ssubgrid diss(K) T(K) −C(K, P) C(K′ , P′ ) Ssubgrid diss(K′ ) Sb. drag(K′ ) Swind(K′ ) T(K′ ) −TV(K, K′ ) −TH(K, K′ ) −CV(K, K′ ) −CH(K, K′ ) CH(K′ , K) +CV(K′ , K) Energy rates of change related to: • : wind stress, bottom drag, subgrid dissipation. • : non-local effects being spatial redistribution due to pressure work and advection. • : conversion between energy reservoirs. • : local and non-local eddy-mean energy conversions. • : energy conversions related to buoyancy flux. S( . ) T( . ) C( . , . ) −C(K, K′￼); C(K′￼, K) C(K, P); C(K′￼, P′￼) Scheme of ocean energy pathways as described by the Lorenz Energy Cycle (Lorenz, 1955). Dashed arrows denote non-local effects. Terms are defined positive in the direction of the arrow. Adapted from Capo et al., (2018). 1. Context What is the leading-order energy balance of the subtropical gyre of the North Atlantic? Which flows does it rely on, and how does it vary in space? • The North Atlantic is a key region in the global ocean circulation and plays a central role in regulating our climate (Ganachaud and Wunsch, 2000). • Its response to climate change is constrained by energy transfers between basin-scale time-averaged currents and smaller-scale (hundreds to the order of a kilometer) time-varying currents, including low- frequency mesoscale and submesoscale currents and high-frequency internal waves. • Although understanding the energy pathways of North Atlantic’s subtropical gyre is crucial for accurately representing its role in climate models, we lack (1) a description of energy pathways at the basin-scale (2) based on a dataset simultaneously resolving the different time-varying flows. Snapshots of surface (right) temperature and (left) normalised relative vorticity. (Right) Streamlines show 4-year averaged surface currents. Black square denotes the subtropical gyre region. 4. (time-mean) and (time-varying) budgets for tidal scenario K K′ (Top) Vertically-integrated averaged over a 4-year period. The four sub-regions: Coastal Gulf Stream and separation, Gulf stream’s extension, Interior and Eastern boundary are shown as squared areas. Streamlines show 4-year averaged surface currents. (Left and right) Frequency-wavenumber spectra and scheme of ocean energy pathways for the four sub-regions. In the schemes, amounts have been computed from time-averaged and vertically-integrated terms and then horizontally-averaged [10-3 m3 s-2]. K + K′ K′ : main energy balances : weak energy pathways (x <.>): ratio between net contributions in tidal and non-tidal scenarios Subtropical gyre’s interior • is larger than the reservoir by a factor of 6 and is dominated by submesoscale currents and internal waves. • and budgets are insensitive to tidal forcing. • budget follows a Sverdrup-like balance: only region with a net export provided by the wind. • reservoir is mainly energised by wind and tides. K′ K K K′ K K K′ • State-of-the-art