Seasonal oceanic variability on meso- and submesoscales: a turbulence perspective

Seasonal variability of the upper ocean on meso- and submesoscales is investigated in the framework of the quasi-normal scale elimination theory, or QNSE. The longitudinal and transverse velocity spectra in this theory have a bi-component structure comprised of the Coriolis and Kolmogorov-like branches that are identified with meso- and submesoscales, respectively. For the former, spectral amplitudes are determined by the Coriolis parameter, f, while for the latter, the amplitudes are quantified in terms of the energy flux, π_ε, proceeding from larger to smaller scales. This flux can be identified with the effective submesoscale dissipation. The Kolmogorov and Coriolis subranges are delineated at a length scale L_c that marks a crossover between the respective spectra. The theoretical spectra agree well with those obtained in many observational campaigns. In phase with the seasonal variations of the intensities of instabilities and turbulence, the magnitudes of π_ε and L_c increase in winter and decrease in summer. Mirroring these changes, the bi-component structure of the kinetic energy spectra changes with seasons and renders meaningless the characterization of their seasonal variability in terms of a single slope. The theoretical results are validated against the data collected in Oleander, LatMix and North-Western Pacific observations.