Seasonality of submesoscale flows in the ocean surface boundary layer

Abstract

A signature of submesoscale flows in the upper ocean is skewness in the distribution of relative vorticity. Expected to result for high Rossby number flows, such skewness has implications for mixing, dissipation, and stratification within the upper ocean. An array of moorings deployed in the Northeast Atlantic for 1 year as part of the experiment of the Ocean Surface Mixing, Ocean Submesoscale Interaction Study (OSMOSIS) reveals that relative vorticity is positively skewed during winter even though the scale of the Rossby number is less than 0.5. Furthermore, this skewness is reduced to zero during spring and autumn. There is also evidence of modest seasonal variations in the gradient Rossby number. The proposed mechanism by which relative vorticity is skewed is that the ratio of lateral to vertical buoyancy gradients, as summarized by the inverse gradient Richardson number, restricts its range during winter but less so at other times of the year. These results support recent observations and model simulations suggesting that the upper ocean is host to a seasonal cycle in submesoscale turbulence.

Open-Ocean Submesoscale Motions: A Full Seasonal Cycle of Mixed Layer Instabilities from Gliders

“…The prevalence of these motions in more quiescent regions of the ocean remains unresolved. The results presented in this study provide direct observational evidence, complimentary to Buckingham et al (2016), that submesoscale motions are active throughout the ocean, at least during certain times of the year.…”

“…This approximation is justified because the horizontal length scale resolved by the glider is on the order of 4 km, and recent analyses of high-resolution numerical models (Brannigan et al 2015) show that the flow is in geostrophic balance down to scales of 5 km. Furthermore, Buckingham et al (2016) show from the mooring data that the Rossby number associated with these scales, calculated from the inner moorings, has values jRoj , 0.5. The application of these approximations allows q bc to be written in a more compact form:…”

“…This offers a higher degree of granularity than recent studies that have analyzed bulk seasonal differences in flow characteristics, which are then used to infer submesoscale activity. Both numerical models (Mensa et al 2013;Sasaki et al 2014) and observations (Callies et al 2015;Swart et al 2015;Buckingham et al 2016) provide strong evidence that there are seasonal modulations in submesoscale activity. In general, these reflect a steepening of the kinetic energy spectra during summer, typically with slopes around k…”

“…1). Moored observations from the OSMOSIS region show a seasonal cycle in the vorticity skewness at scales smaller than 5 km, indicative of temporal changes in the intensity of submesoscale turbulence (Buckingham et al 2016). Moorings were complemented by measurements from ocean gliders that provided continuous sampling from the surface to a depth of 1000 m, obtaining measurements of temperature, salinity, dissolved oxygen, fluorescence, optical backscatter, and photosynthetically available radiation (PAR).…”

Submesoscale Turbulence over a Topographic Slope

“…Both of these mechanisms could explain the results of Capet et al [12] and Klein et al [3] who demonstrated in numerical simulations of the California Current system and a baroclinically-unstable zonal flow respectively, that submesoscale processes favor the generation of cyclonic vortices with Ro larger than one. These results also hold in observations for open ocean regimes in which the fluid motion does not feel any additional constraints due to changes in the water column depth [37]. Here we explore the robustness of these asymmetries in topographically-controlled flows.…”