Especially the transition from warming to cooling phase of the large-scale carrier
wave, associated with an upslope moving frontal bore, provides large turbulence
peaking at ε = 10
−5
− 10
−4 m
2 s
−3 and K z = 10
−1
− 10
0 m
2 s
−1 (Fig. 3b). The
reordered stable stratification is characterized by very thin layering down to the
lowest vertical resolution, in this case Δz = 0.5 m and much smaller than ≈
20 m. The layering approaches the bottom to within a meter just prior to a frontal
passage by large interior overturns associated with relatively strong (0.1 m s
−1 )
downward motions [32]. The large convective overturns preceding the front are not
so efficient in their mixing (m 1 ≈ 0.05), but the near-bottom stratified layer forming
the frontal bore is more efficient (m 1 ≈ 0.3 ≤ 0.36) than average [57]. The
downdraught seems part of sharpening the turbulent bore moving up the slope.
The frontal bore itself is basically the only large overturn extending from the
bottom upward, thus being important for sediment resuspension governed by
0.1 m s
−1 upward vertical current speeds [23]. Secondary instabilities are observed
along its fringe, contributing to effective mixing [58]. Although turbulence associated with upslope propagating bores is always large, the intensity and precise
extent vary every cycle of the carrier wave. The variations depend on the exact
direction of bore propagation with respect to the main bottom-slope direction, the
more upslope the stronger turbulent, and on the timing of arrival which varies over
about 10% of the carrier wave period [59]. This variation in arrival time was
attributed to variations in the background stratification which modify the paths of
internal waves, and apparently also large-scale sub-inertial motions. Behind the
front the trailing near-N waves also turn-over due to internal wave shear across the
sharp interface, but in rapidly decreasing magnitude.
Averaged over time of a two-week period and over depth over the range of
T-sensors gives [< ε>] = 3 ± 2 × 10
−7 m
2 s
−3 , [< K z >] = 6 ± 4 × 10
−3 m
2
s
−1 for [< N>] = 5 ± 3 × 10
−3 s
−1 above steep ‘supercritical’ slopes larger than
the average internal tidal slope [32, 60]. These turbulence values are more than two
orders of magnitude larger than estimated from ocean interior observations. Above
less steep slopes, values are smaller, but even above slopes well less than the
internal tide slope values are still about one order of magnitude larger than ocean
interior values [60]. In the vertical above a supercritical slope, the latter low values
are not observed up to 400 m above the bottom (Fig. 4) possibly due to the large
extent of tidal motions that have top-trough excursions exceeding 100 m [61].
These observations tend to support microstructure profiler observations of the
decrease of turbulence intensity to ocean interior values over a range of some
1000 m above sloping topography [31].
Interfacial Shear Instability Observations
Although near sloping bottoms moored high-resolution T-sensors show largest
turbulent overturning commonly in the form of frontal bores around the transition
between warming and cooling phases of the large-scale carrier wave, time series of
High-Resolution Observations of Internal Wave Turbulence …
137
wave, associated with an upslope moving frontal bore, provides large turbulence
peaking at ε = 10
−5
− 10
−4 m
2 s
−3 and K z = 10
−1
− 10
0 m
2 s
−1 (Fig. 3b). The
reordered stable stratification is characterized by very thin layering down to the
lowest vertical resolution, in this case Δz = 0.5 m and much smaller than
20 m. The layering approaches the bottom to within a meter just prior to a frontal
passage by large interior overturns associated with relatively strong (0.1 m s
−1 )
downward motions [32]. The large convective overturns preceding the front are not
so efficient in their mixing (m 1 ≈ 0.05), but the near-bottom stratified layer forming
the frontal bore is more efficient (m 1 ≈ 0.3 ≤ 0.36) than average [57]. The
downdraught seems part of sharpening the turbulent bore moving up the slope.
The frontal bore itself is basically the only large overturn extending from the
bottom upward, thus being important for sediment resuspension governed by
0.1 m s
−1 upward vertical current speeds [23]. Secondary instabilities are observed
along its fringe, contributing to effective mixing [58]. Although turbulence associated with upslope propagating bores is always large, the intensity and precise
extent vary every cycle of the carrier wave. The variations depend on the exact
direction of bore propagation with respect to the main bottom-slope direction, the
more upslope the stronger turbulent, and on the timing of arrival which varies over
about 10% of the carrier wave period [59]. This variation in arrival time was
attributed to variations in the background stratification which modify the paths of
internal waves, and apparently also large-scale sub-inertial motions. Behind the
front the trailing near-N waves also turn-over due to internal wave shear across the
sharp interface, but in rapidly decreasing magnitude.
Averaged over time of a two-week period and over depth over the range of
T-sensors gives [< ε>] = 3 ± 2 × 10
−7 m
2 s
−3 , [< K z >] = 6 ± 4 × 10
−3 m
2
s
−1 for [< N>] = 5 ± 3 × 10
−3 s
−1 above steep ‘supercritical’ slopes larger than
the average internal tidal slope [32, 60]. These turbulence values are more than two
orders of magnitude larger than estimated from ocean interior observations. Above
less steep slopes, values are smaller, but even above slopes well less than the
internal tide slope values are still about one order of magnitude larger than ocean
interior values [60]. In the vertical above a supercritical slope, the latter low values
are not observed up to 400 m above the bottom (Fig. 4) possibly due to the large
extent of tidal motions that have top-trough excursions exceeding 100 m [61].
These observations tend to support microstructure profiler observations of the
decrease of turbulence intensity to ocean interior values over a range of some
1000 m above sloping topography [31].
Interfacial Shear Instability Observations
Although near sloping bottoms moored high-resolution T-sensors show largest
turbulent overturning commonly in the form of frontal bores around the transition
between warming and cooling phases of the large-scale carrier wave, time series of
High-Resolution Observations of Internal Wave Turbulence …
137