Turbulence Observations and Energetics of Diurnal Warm Layers

Diurnal warm layers (DWLs) develop under relatively weak winds and strong solar radiation and have important consequences for ocean surface transport and air-sea interactions. In this paper we investigate DWLs during three consecutive days in the subtropical South Atlantic using observations from an underwater glider equipped with a turbulence microstructure package, a series of drifters at two different depths, and a 1-D turbulence model. The observations and modeling show that the DWLs create a near-surface stratification that partially decouples the surface current from the mixed layer below. However, we find that turbulent entrainment of momentum from below the DWL is important in the evolution of the surface current. We further derive buoyancy, potential and kinetic energy budgets, and identify the dominant terms. The upper ocean potential energy budget is dominated by the incoming solar radiation, with only a small contribution from turbulent mixing. Turbulent shear production, however, is found to be an important influence on the upper ocean mean kinetic energy, receiving a similar fraction of the wind work as the acceleration of the diurnal jet. The DWL evolution and energy budgets are corroborated with simulations from a freely-evolving 1-D turbulence model, which additionally shows that the exchange of mean kinetic energy with the surface wave field due to rotation is of minor importance to the development of the DWLs we observe.