Statistics of Submesoscale Turbulence in the Upper Ocean

The submesoscale variability of the upper ocean is composed of balanced motion and internal waves. Applying statistical tools to observations, we attempt to understand how energy is partitioned between these two classes of motion. We present wavenumber spectra of kinetic energy, potential energy and passive-tracer variance inferred from SeaSoar tows in the subtropical North Pacific. These spectra, taken at different depths, all have slopes close to -2. Balanced dynamics dominate at large scales, internal-wave dynamics dominate at small scales below the mixed layer. The part of the spectra associated with balanced dynamics does not show any change in slope with depth as predicted by quasi-geostrophic turbulence theories. A quasi-geostrophic stability analysis reveals that in this region baroclinic instability injects energy directly in the submesoscale, dominating any transfer of energy through a turbulent cascade. Our analysis shows that submesoscale energy is dominated by balanced motions at scales larger than a few kilometers. In some regions, an inertial range is present and quasi-geostrophic turbulence theory may be a useful framework. In other regions, however, small-scale instabilities directly inject variability in the submesoscale.