Abstract
AbstractMooring and glider observations and a high‐resolution satellite sea surface temperature image reveal features of a transient submesoscale front in a typical mid‐ocean region of the Northeast Atlantic. Analysis of the observations suggests that the front is forced by downfront winds and undergoes symmetric instability, resulting in elevated upper‐ocean kinetic energy, restratification, and turbulent dissipation. The instability is triggered as downfront winds act on weak upper‐ocean vertical stratification and strong lateral stratification produced by mesoscale frontogenesis. The instability's estimated rate of kinetic energy extraction from the front accounts for the difference between the measured rate of turbulent dissipation and the predicted contribution from one‐dimensional scalings of buoyancy‐ and wind‐driven turbulence, indicating that the instability underpins the enhanced dissipation. These results provide direct evidence of the occurrence of symmetric instability in a quiescent open‐ocean environment and highlight the need to represent the instability's restratification and dissipative effects in climate‐scale ocean models.
Highlights
The ocean surface boundary layer controls exchanges of tracers such as heat, momentum, and carbon between the atmosphere and the ocean interior and plays an important role in global tracer budgets and Earth's climate
The OSMOSIS mooring and glider measurements provide a unique observational data set that enables a dynamical description of upper‐ocean forced submesoscale instabilities with horizontal scales down to O (1 km)
We conduct a case study of a symmetric instability (SI) event in this mid‐ocean setting, based on four lines of direct observational evidence: (i) Upper‐ocean kinetic energy is enhanced during the SI event (Figure 2b), as expected from the active development of a submesoscale instability; (ii) the event is associated with downfront winds and mesoscale frontogenesis (Figures 1b and 2d), conditions that favor the onset of SI and that are regularly met in mid‐ocean environments; (iii) a shoaling of the mixed layer is observed (Figure 2a), consistent with theoretical predictions for SI; and (iv) dissipation is elevated in a manner quantitatively consistent with SI extracting kinetic energy from the background flow, which is broadly in geostrophic balance (Figure 4)
Summary
The ocean surface boundary layer controls exchanges of tracers such as heat, momentum, and carbon between the atmosphere and the ocean interior and plays an important role in global tracer budgets and Earth's climate. Previous observational studies show that SI in the upper ocean may provide an effective exchange pathway between the mixed layer and pycnocline and play a significant role in the ocean circulation's energy balance While these investigations were mostly conducted in regions with strong, persistent fronts and surface forcing, such as the Gulf Stream (Thomas et al, 2013; 2016), the Kuroshio (D'asaro et al, 2011), and the Antarctic Circumpolar Current (Adams et al, 2017; Viglione et al, 2018), a growing body of work from numerical models (e.g., Brannigan, 2016; Skyllingstad et al, 2017) and, more indirectly, from observations (e.g., du Plessis et al, 2019; Thompson et al, 2016) suggests that SI may be important in more quiescent open‐ocean environments, which extend across the bulk of the global ocean.
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