The Contribution of Surface and Submesoscale Processes to Turbulence in the Open Ocean Surface Boundary Layer

Abstract

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...

Spatiotemporal Characteristics and Generation Mechanisms of Submesoscale Currents in the Northeastern South China Sea Revealed by Numerical Simulations

“…Given its importance in energy cascade and vertical material/heat transport, the submesoscales, especially their kinematic characteristics and generation/dissipation mechanisms, have been widely studied in the past decade (see a review by McWilliams, 2016). Examples include those studies in the open oceans (e.g., Buckingham et al, 2019;Callies et al, 2015;du Plessis et al 2017;Sasaki et al, 2014;Erickson & Thompson, 2018;Gula et al, 2014), but even more studies focus on coastal regions and marginal seas, such as the California Current region (e.g., Capet et al, 2008aCapet et al, , 2008bMolemaker et al, 2015;Renault et al, 2018), the Gulf of Mexico (e.g., Barkan et al, 2017;Luo et al, 2016;Zhong & Bracco, 2013), and the Mediterranean Sea (e.g., Damien et al, 2017;Pascual et al, 2017).…”

Effects of <sup>238</sup>U variability and physical transport on water column <sup>234</sup>Th downward fluxes in the coastal upwelling system off Peru

“…Generally, a deficit of 234 Th relative to 238 U is observed in the surface ocean and reflects net removal of 234 Th due to particle sinking, whereas secular equilibrium between 234 Th and 238 U is observed for intermediate and deep waters. Integrating this surface 234 Th deficit with depth yields the sinking flux of 234 Th and, if elemental : 234 Th ratios are known, the sinking flux of elements such as C, N, P, Si and trace metals (e.g., Bhat et al, 1968;Buesseler et al, 1998Buesseler et al, , 1992Buesseler et al, , 2006Coale and Bruland, 1987;Weinstein and Moran, 2005;Owens et al, 2015;Black et al, 2019;Puigcorbé et al, 2020).…”

“…This assumption is typically appropriate in openocean settings where 234 Th fluxes due to advection and diffusion are small relative to the downward fluxes of 234 Th associated with particle sinking. However, in upwelling regions such as the equatorial Pacific and coastal systems, advective and diffusive 234 Th fluxes may become increasingly important (e.g., Bacon et al, 1996;Buesseler et al, 1998Buesseler et al, , 1995Dunne and Murray, 1999). For example, in the equatorial Pacific, strong upwelling post El Niño could account for ∼ 50 % of the total 234 Th fluxes (Bacon et al, 1996;Buesseler et al, 1995).…”

“…Accurate measurements of current velocities and diffusivities are however challenging, and thus direct observations of the effects of physical processes on 234 Th distributions in coastal regions are scarce. Limited studies have incorporated advection and diffusion in the nearshore zones of the Arabian Sea (Buesseler et al, 1998), Gulf of Maine (Gustafsson et al, 1998;Benitez-Nelson et al, 2000), South China Sea (Cai et al, 2008) and Peruvian oxygen minimum zone (OMZ) (Black et al, 2018). In the Arabian Sea, coastal upwelling during the southwest monsoon season could account for over 50 % of the total 234 Th flux (Buesseler et al, 1998).…”

“…Limited studies have incorporated advection and diffusion in the nearshore zones of the Arabian Sea (Buesseler et al, 1998), Gulf of Maine (Gustafsson et al, 1998;Benitez-Nelson et al, 2000), South China Sea (Cai et al, 2008) and Peruvian oxygen minimum zone (OMZ) (Black et al, 2018). In the Arabian Sea, coastal upwelling during the southwest monsoon season could account for over 50 % of the total 234 Th flux (Buesseler et al, 1998). Horizontal advection has been shown to be substantial in the inner Casco Bay of the Gulf of Maine (Gustafsson et al, 1998), whereas offshore advection and diffusion are only important in late summer (Benitez-Nelson et al, 2000).…”