Characterization of ocean submesoscale turbulence regimes from satellite observations of Sea Surface Temperatures

Abstract

Satellite infrared radiometers can be used not only for the evaluation of Sea Sur-face Temperature (SST), but have also the potential of characterizing the distinct turbulence regimes present in the upper ocean. These regimes, own to submesoscale, have different contributions on the vertical transport of nutrients, heat, and gases between the oceanic up- per layers and the ocean interior, and contain key information about the dynamics of the mixed layer. However, the different regimes do not always leave a clear footprint on sur- face scalars as SST, unless an adequate data processing technique based in the theory of turbulence is applied on them. In this study, we will present evidence that infrared SST provided by the AATSR sensor (Envisat) can be processed to characterize the underlying turbulence regime. As a first step, data have been divided into granules adapted to the observed cloud coverage with a maximum size of 512 by 512 pixels (i.e. 512 km by 512 km). These granules have been classified into two groups: one characterized by the presence of submesoscale instabilities ( 5-10 km) and one characterized by the presence of vortices and filaments. Then, for all the granules of each group we have analyzed classical statistical descriptors such as the spectral slope associated to power spectra, and also new descriptors emerging from the use of singularity analysis (curvature of singularity fronts, singularity spectra), which is based on multifractal theory of turbulent flows. Our results show that both groups of images are characterized by similar spectral slopes, what would indicate a similar cascade process in both cases. On the contrary, the singularity spectra and the characteristics of front curvatures in both groups were neatly different, what allows to separate both dynamical regimes. We will discuss the implications of our work under the perspective of assessing submesoscale ocean turbulence at global scale