Anisotropic Turbulence and Subsurface Heat Fluxes in the Diurnal Warm Layer

Kenneth Hughes, Oregon State University, Corvallis, OR, United States, Jim Moum, Oregon State University, College of Earth Ocean & Atmospheric Sciences, Corvalis, OR, United States and Emily L Shroyer, Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, United States
The daily stratifying effect of penetrating solar radiation modifies the dynamics of near-surface ocean turbulence, causing it to be anistropic. We investigate this near-surface turbulence using horizontal temperature spectra from thermistors mounted on a new surface-following platform. Spectra display inertial range behavior over several decades in wavenumber, with the largest horizontal scales being at least an order of magnitude larger than possible for isotropic turbulence, and larger than the distance to the free surface. We therefore analyze our the spectra in the framework of an anisotropic energy cascade. Deviations from the canonical -5/3 spectral slope (i) provide estimates of the outer horizontal length scales of turbulence under different wind speeds, (ii) are consistent with existing simulations of anisotropic turbulence, and (iii) let us infer the evolution of turbulence from its largest to smallest scales. These analyses improve both our qualitative understanding of turbulence in the diurnal warm layer, and make quantitative estimates more robust. We take advantage of these improvements to develop in situ estimates of turbulent subsurface heat fluxes and the consequences for sea surface temperature.