Mixed-Layer Instability As a Source of Surface Kinetic Energy in the Seasonal Cycle in a Global Climate Model

We examine the seasonal cycle of upper-ocean mesoscale turbulence in a high resolution Climate Earth Science Model (CESM) climate simulation. The ocean model component (POP) has 0.1° degree resolution, mesoscale resolving at low and middle latitudes. Seasonally and regionally resolved wavenumber power spectra are calculated for sea-surface eddy kinetic energy (KE), sea-surface temperature (SST) and sea-surface salinity (SSS). Although the spectral slopes themselves are not very informative (due to the strong presence of biharmonic diffusion), the KE spectra consistently show higher power at small scales during winter throughout the ocean. Potential hypotheses for the seasonality are investigated. Linear quasigeostrophic stability analysis suggests that the seasonality originates in the submesoscale range (on the scales of the mixed-layer Rossby wavelength) and seasonally varying mixed-layer instability is proposed as the driver. The ability of this climate model, which is not considered submesoscale resolving, to produce mixed layer instability demonstrates the ubiquity and robustness of this process for modulating upper ocean energy. As it is still impractically computationally expensive to run submesoscale-resolving global climate models, we believe our results provide insights into how mesoscale-resolving models reproduce seasonal variations and implications for future model runs.