Observations of elevated mixing and periodic structures within diurnal
warm layers
Abstract
A fleet of surface drifters (SWIFTs) equipped with down-looking
high-resolution ADCPs were used to estimate the turbulent kinetic energy
(TKE) dissipation rate (ε) 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 meters suggest turbulence is strongly
modulated by the DWL stratification. Burst-averaged (8.5 minutes) ε is
stronger than predicted by law-of-the-wall boundary layer scaling within
the DWL and suppressed below. Predictions for ε within the DWL 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 DWL
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
both vertical transport and local TKE generation are plausible sources
of elevated ε in the DWL.