Submesoscale Turbulence in the Upper Ocean

Jörn Callies, Ph.D., 2016
Raffaele Ferrari, Advisor

The dynamics of submesoscale flows, ocean currents 1–100 km in horizontal extent, have received considerable theoretical attention, but observational constraints are largely missing. We diagnose from observations that in the thermocline the dynamics transition from geostrophic turbulence at large scales to inertia–gravity waves at small scales. A similar transition is shown to occur in the atmosphere, suggesting intriguing similarities between atmospheric and oceanic dynamics. We furthermore diagnose from observations a seasonal cycle in the surface ocean: submesoscale fronts and currents are more energetic in the deep wintertime mixed layer than in the summertime seasonal thermocline. This hints at the importance of baroclinic mixed layer instabilities in energizing submesoscale turbulence. To better understand this energization, three aspects of baroclinic mixed layer instabilities are investigated. First, we formulate a quasi-geostrophic model that captures the essence of how the submesoscale range is energized in winter. Second, we investigate how baroclinic instabilities are affected by atmospherically forced convection. We find that baroclinic instabilities are remarkably resilient to the presence of even vigorous convection. Third, we show that the restratification induced by baroclinic mixed layer instabilities depends on characteristics of the baroclinic eddies themselves, a dependence not captured by a previously proposed parameterization.