Dynamics and impact of diurnal warm layers in the ocean

Thin Diurnal Warm Layers (DWLs) form near the surface of the ocean on days of large solar radiation,
weak to moderate winds, and small surface waves. DWLs are characterized by complex dynamics,
and are relevant to the ocean especially by modifying surface-layer mixing and atmosphere-ocean
fluxes. Here, we use idealized Large Eddy Simulations (LES) and second-moment turbulence
modelling, both including the effects of Langmuir turbulence, to identify the key non-dimensional
parameters of the problem, and explore DWL properties and dynamics across a wide parameter
space. Comparison of LES and the second-moment turbulence models shows that the latter provide
an accurate representation of the DWL structure and dynamics. We find that, for equilibrium wave
conditions, Langmuir effects are significant only in the Stokes layer very close to the surface. While
we see pulses in the turbulent stresses and shear in the LES, there are no relevant effects of
Langmuir turbulence on DWL bulk properties and total entrainment. Results of the parameter space
analysis agree with the midday scaling by Pollard et al. (1986), however, with modified model
coefficients and deviations of up to 30% especially at high-latitudes. We develop non-dimensional
expressions for the strength and timing of the DWL temperature peak in the afternoon, and discuss
the mixing efficiency and energetics of DWLs in the presence of Langmuir turbulence.