Exchange Between the Mixed Layer and Thermocline Induced by Mixed-Layer Instability
Abstract
Submesoscale turbulence in the upper ocean consists of fronts, filaments, and vortices that have horizontal scales between 100 m and 100 km. High-resolution numerical simulations have suggested that submesoscale turbulence is associated with strong vertical motion that could substantially enhance the vertical exchange between the mixed layer and the thermocline, which may have a profound impact on marine ecosystems and climate. Theoretical, numerical, and observational work indicates that submesoscale turbulence is energized primarily by a baroclinic instability in the mixed layer, which is most vigorous in winter. In this study, we demonstrate how such mixed-layer baroclinic instabilities induce vertical exchange between the mixed layer and thermocline, and we propose a scaling law for the dependence of the exchange on environmental parameters. From linear stability analysis and nonlinear simulations, we show that the exchange, quantified by how much thermocline water is entrained into the mixed layer, is inversely proportional to the thermocline Richardson number. Our results imply that the tracer exchange between the mixed layer and thermocline is more efficient when the mixed layer is deeper, the lateral buoyancy gradient is larger, and the thermocline stratification is weaker. The scaling suggests vigorous exchange between deep mixed layers and the permanent thermocline in winter.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2022
- Bibcode:
- 2022AGUFMOS45E1235L