Global Estimate of Seasonality in Scales of Oceanic Turbulence

Wavenumber spectral analysis is a powerful method for characterizing the properties of ocean turbulence. Here we calculate seasonally and regionally resolved wavenumber power spectra of sea-surface temperature (SST), sea-surface height (SSH), and surface eddy kinetic energy (EKE) from the high resolution ocean component of a CESM global climate model. Until now there has not been a comprehensive analysis of ocean mesoscale turbulence in this new category of model. Furthermore, this study provides a test bed for future work on infrared satellite observations. The ocean component model (POP) has 0.1° degree resolution, mesoscale resolving at most latitudes. We have found seasonality in the spectra, which indicates the possibility of different turbulent schemes for each season. Although the spatial resolution of the model is not considered submesoscale resolving, we see that the seasonality originates in the submesoscale range (below 50km) in the power level of the spectra. On the other hand, it is difficult to extract physical meanings from the actual values of spectral slopes since the slopes depend on the wavelength range we fit the spectra due to numerical viscosity. Thus, we propose the possibility of mixed-layer instability (e.g. Callies et al. (2014)) playing an important role in the seasonality of submeso/mesoscale turbulence, and power levels are a more robust criteria in detecting seasonality than spectral slopes. We also compare the spectral analysis with structure function analysis. The strength of structure functions is that they can characterize scaling properties of turbulence even when the data has gaps or missing data as in infrared satellite observations of SST.