The ocean surface boundary layer is a critical interface across which momentum, heat, and trace gases are exchanged between the oceans and atmosphere. Surface processes (winds, waves, and buoyancy forcing) are known to contribute significantly to fluxes within this layer. Recently, studies have suggested that submesoscale processes, which occur at small scales (0.1-10 km, hours to days) and therefore are not yet represented in most ocean models, may play critical roles in these turbulent exchanges. While observational support for such phenomena has been demonstrated in the vicinity of strong current systems and littoral regions, relatively few observations exist in the open-ocean environment to warrant representation in Earth system models. We use novel observations and simulations to quantify the contributions of surface and submesoscale processes to turbulent kinetic energy (TKE) dissipation in the open-ocean surface boundary layer. Our observations are derived from moorings in the North Atlantic, December 2012 to April 2013, and are complemented by atmospheric reanalysis. We develop a conceptual framework for dissipation rates due to surface and submesoscale processes. Using this framework and comparing with observed dissipation rates, we find that surface processes dominate TKE dissipation. A parameterization for symmetric instability is consistent with this result. We next employ simulations from an ocean front-resolving model to reestablish that dissipation due to surface processes exceeds that of submesoscale processes by 1-2 orders of magnitude. Together, these results suggest submesoscale processes do not dramatically modify vertical TKE budgets, though such dynamics may be climatically important owing to their ability to remove energy from the ocean. Key Points:• We present a multimonth record of OSBL turbulence in the open ocean • The contribution of surface and submesoscale processes is examined • Dissipation rates due to surface processes dominate those of submesoscale processes Supporting Information:• Supporting Information S1 • Movie S1 Figure 1. (a) Surface and (b) submesoscale processes believed to be the dominant mechanisms for turbulence generation in the open-ocean environment. These are the expectations at the OSMOSIS observation site. (a) Winds drive waves and currents in the upper ocean. This creates turbulence through the effects of breaking waves, current shear, and Langmuir motions caused by the interaction of the Stokes shear with the background vorticity field.Similarly, buoyancy loss at the ocean surface reduces vertical stratification and permits upright convection (i.e., gravitational instability). (b) Stirring and straining by the mesoscale eddy field generates pronounced lateral gradients in density. Winds oriented downfront (i.e., in the direction of the geostrophic shear at the front) transport dense water into areas of less dense (more buoyant) waters. Termed the Ekman buoyancy flux, B e , this flux reduces vertical stratification and admits symmetric instability (SI) within t...

Ocean circulation is dominated by turbulent geostrophic eddy fields with typical scales ranging from 10 to 300 km. At mesoscales (>50 km), the size of eddy structures varies regionally following the Rossby radius of deformation. The variability of the scale of smaller eddies is not well known due to the limitations in existing numerical simulations and satellite capability. Nevertheless, it is well established that oceanic flows (<50 km) generally exhibit strong seasonality. In this study, we present a basin-scale analysis of coherent structures down to 10 km in the North Atlantic Ocean using two submesoscale-permitting ocean models, a NEMO-based North Atlantic simulation with a horizontal resolution of 1/60 (NATL60) and an HYCOM-based Atlantic simulation with a horizontal resolution of 1/50 (HYCOM50). We investigate the spatial and temporal variability of the scale of eddy structures with a particular focus on eddies with scales of 10 to 100 km, and examine the impact of the seasonality of submesoscale energy on the seasonality and distribution of coherent structures in the North Atlantic. Our results show an overall good agreement between the two models in terms of surface wave number spectra and seasonal variability. The key findings of the paper are that (i) the mean size of ocean eddies show strong seasonality; (ii) this seasonality is associated with an increased population of submesoscale eddies (10-50 km) in winter; and (iii) the net release of available potential energy associated with mixed layer instability is responsible for the emergence of the increased population of submesoscale eddies in wintertime. Plain Language Summary The ocean is dominated by circular currents of water in swirling motion called oceanic eddies. This class of motion is by far the largest reservoir of oceanic kinetic energy. Much is known about this oceanic eddies at scale >50 km while we are yet to fully comprehend their distribution in terms of size and dynamics at scales <50 km. This is due to the lack of sufficient observational data sets at these scales in the ocean. In this study, we use two kilometric-resolving models of the North Atlantic ocean to investigate the spatial and temporal variability of oceanic eddies down to 10-km scale. Our results show that the distribution of oceanic eddies at spatial scale <100 km undergo strong seasonality and that this seasonality is as a result of an increased population of smaller eddies (10-50 km) often called submesoscales eddies in wintertime. We found that submesoscale turbulence (a class of oceanic turbulence at fine scale) is responsible for the increase in smaller-scale eddy distribution in winter. Improving our knowledge of the scale of eddy structures is key to several applications in physical oceanography. The interaction between the eddy field and large-scale flow is one of the main drivers of ocean circulation. This interaction is presently parameterized in noneddy-resolving ocean and climate models, and

• We used two submesoscale permitting ocean models of the North Atlantic Ocean to investigate kinetic energy exchanges at fine-scales. • KE fluxes at fine-scales are strongly impacted by submesoscale turbulence with 10 a stronger forward cascade in winter within the mixed-layer. 11 • Not accounting for ageostrophic motions yields a significant underestimation of 12 the forward cascade.

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The ocean is a turbulent fluid with a broad range of energetic scales, ranging from large ∼O(1,000 km) to centimeter scales. The ocean kinetic energy is mostly concentrated in the quasigeostrophic mesoscale eddy field with scales ∼O(100 km) (Stammer & Böning, 1992). Due to nonlinear interactions among different length scales, energy can be transferred both from large to small (forward, or direct cascade) and from small to large scale (inverse cascade). Understanding the distribution of kinetic energy (KE) and variance across scales in oceanic flows is, therefore, key to our knowledge of ocean circulation (Ferrari & Wunsch, 2009). To estimate the variance and energy associated with eddy motions at different scales, velocity wavenumber power spectral density has proven to be very efficient (

An important characteristic of geophysically turbulent flows is the transfer of energy between scales. Balanced flows pass energy from smaller to larger scales as part of the well-known upscale cascade while submesoscale and smaller scale flows can transfer energy eventually to smaller, dissipative scales. Much effort has been put into quantifying these transfers, but a complicating factor in realistic settings is that the underlying flows are often strongly spatially heterogeneous and anisotropic. Furthermore, the flows may be embedded in irregularly shaped domains that can be multiply connected. As a result, straightforward approaches like computing Fourier spatial spectra of nonlinear terms suffer from a number of conceptual issues. In this paper, we develop a method to compute cross-scale energy transfers in general settings, allowing for arbitrary flow structure, anisotropy and inhomogeneity. We employ a Green's function approach to the kinetic energy equation to relate kinetic energy at a point to its Lagrangian history. A spatial filtering of the resulting equation naturally decomposes kinetic energy into length scale dependent contributions and describes how the transfer of energy between those scales takes place. The method is applied to a doubly periodic simulation of vortex merger, resulting in the demonstration of the expected upscale energy cascade. Somewhat novel results are that the energy transfers are dominated by pressure work, rather than kinetic energy exchange, and dissipation is a noticeable influence on the larger scale energy budgets. We also describe, but do not employ here, a technique for developing filters to use in complex domains.

We used two submesoscale permitting ocean models of the North Atlantic Ocean to investigate kinetic energy exchanges at fine-scales.• KE fluxes at fine-scales are strongly impacted by submesoscale turbulence with 10 a stronger forward cascade in winter within the mixed-layer. 11• Not accounting for ageostrophic motions yields a significant under-estimation of 12 the forward cascade.

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