Abstract
The seasonal cycle of submesoscale flows in the upper ocean is investigated in an idealised model domain analogous to mid-latitude open ocean regions. Submesoscale processes become much stronger as the resolution is increased, though with limited evidence for convergence of the solutions. Frontogenetical processes increase horizontal buoyancy gradients when the mixed layer is shallow in summer, while overturning instabilities weaken the horizontal buoyancy gradients as the mixed layer deepens in winter. The horizontal wavenumber spectral slopes of surface temperature and velocity are steep in summer and then shallow in winter. This is consistent with stronger mixed layer instabilities developing as the mixed layer deepens and energising the submesoscale. The degree of geostrophic balance falls as the resolution is made finer, with evidence for stronger non-linear and high-frequency processes becoming more important as the mixed layer deepens. Ekman buoyancy fluxes can be much stronger than surface cooling and are locally dominant in setting the stratification and the potential vorticity at fronts, particularly in the early winter. Up to 30% of the mixed layer volume in winter has negative potential vorticity and symmetric instability is predicted inside mesoscale eddies as well as in the frontal regions outside of the vortices.
Highlights
The upper ocean stratification is an important control on the transfer of momentum and tracers between the atmosphere and ocean interior
The development of upper ocean stratification has historically been viewed as a one-dimensional process driven by surface buoyancy and frictional fluxes, with allowance for shear-driven mixing at the base of the mixed layer
Attention has since focused on the role a number of other processes play in setting upper ocean stratification such as geostrophic adjustment (Dale et al, 2008; Tandon and Garrett, 1994), frontogenesis (Gula et al, 2014; Hoskins and Bretherton, 1972; Lapeyre et al, 2006; Shakespeare and Taylor, 2013), surface waves and Langmuir turbulence (Belcher et al, 2012; Grant and Belcher, 2009; Hamlington et al, 2014; Haney et al, Subm. to JPO; McWilliams and Fox-Kemper, 2013; Sutherland et al, 2014), Ekman buoyancy fluxes, symmetric and inertial instabilities
Summary
The upper ocean stratification is an important control on the transfer of momentum and tracers between the atmosphere and ocean interior. The development of upper ocean stratification has historically been viewed as a one-dimensional process driven by surface buoyancy and frictional fluxes, with allowance for shear-driven mixing at the base of the mixed layer. These ideas are encapsulated in a number of one-dimensional parameterisation schemes for the surface boundary layer The domain used here is analogous to an open ocean region rather than an eastern boundary current region (Capet et al, 2008a; 2008b; 2008c) or a western boundary current region (Gula et al, 2014; Mensa et al, 2013) This experiment is carried out in an idealised configuration intended to be analogous to the OSMOSIS (Ocean Surface Mixing Ocean Submesoscale Interaction Study) observation site in the North Atlantic.
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