Seasonality of eddy kinetic energy in an eddy permitting global climate model

•Examined the seasonal cycle of upper-ocean mesoscale turbulence in a high resolution climate model simulation.•Seasonality in ocean mesoscale turbulence was quantified by isotropic wavenumber spectra of surface eddy kinetic energy.•The small-scale turbulence was most energetic during winter time at scales below 30 km.•Showed a global picture of seasonality in baroclinic available potential energy conversion rates.•The dominant cause for seasonality was baroclinic instability due to reduced stratification in the winter mixed layer.

We examine the seasonal cycle of upper-ocean mesoscale turbulence in a high resolution CESM climate simulation. The ocean model component (POP) has 0.1° resolution, mesoscale resolving at low and middle latitudes. Seasonally and regionally resolved wavenumber power spectra are calculated for sea-surface eddy kinetic energy (EKE). Although the interpretation of the spectral slopes in terms of turbulence theory is complicated by the strong presence of dissipation and the narrow inertial range, the EKE spectra consistently show higher power at small scales during winter throughout the ocean. Potential hypotheses for this seasonality are investigated. Diagnostics of baroclinc energy conversion rates and evidence from linear quasigeostrophic stability analysis indicate that seasonally varying mixed-layer instability is responsible for the seasonality in EKE. The ability of this climate model, which is not considered submesoscale resolving, to produce mixed layer instability although damped by dissipation, demonstrates the ubiquity and robustness of this process for modulating upper ocean EKE.