Observations of Elevated Mixing and Periodic Structures Within Diurnal Warm Layers

Abstract

Surface drifters (SWIFTs) equipped with down-looking high-resolution acoustic doppler current profilers (ADCPs) were used to estimate the turbulent kinetic energy (TKE) dissipation rate $\left(ϵ\right)$ within highly stratified diurnal warm layers (DWLs) in the Southern California Bight. Over a 10-day period, five instances of DWLs were observed with strong surface temperature anomalies up to 3°C and velocity anomalies up to 0.3 m $s^{-1}$. Profiles of $ϵ$ in the upper 5 m suggest turbulence is strongly modulated by the DWL stratification. Burst-averaged (8.5 min) $ϵ$ is stronger than predicted by law-of-the-wall boundary layer scaling within the DWLs and suppressed below. Predictions for $ϵ$ within the DWLs are improved by a shear-production scaling using observed shear and linearly decaying turbulent stress. However, $ϵ$ is still under-predicted. Examination of the un-averaged acoustic backscatter data suggests elevated $ϵ$ is related to the presence of turbulent structures in the DWLs which span the layer height and strongly modulate TKE. Evolution in the bulk Richardson number each day suggests the DWLs become unstable to layer-scale overturning and entrainment each afternoon, thus the turbulent structures may result from shear-driven instability. This interpretation is supported by a conditional average of the data during a burst characterized by strongly periodic structures. The structures resemble high-frequency internal waves with strong asymmetry in the along-flow direction (steepening) which suggests they are unstable. Coincident asymmetric patterns in upwelling/downwelling and corresponding regions of strong vertical convergence/divergence suggest that both vertical transport and local TKE generation are plausible sources of elevated $ϵ$ in the DWLs.