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# Insights from a 3D temperature sensors mooring on stratified ocean turbulence

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A unique small-scale 3-D mooring array has been designed consisting of five parallel lines, 100 m long and 4 m apart, and holding up to 550 high-resolution temperature sensors. It is built for quantitative studies on the evolution of stratified turbulence by internal wave breaking in geophysical flows at scales which go beyond that of a laboratory. Here we present measurements from above a steep slope of Mount Josephine, NE Atlantic where internal wave breaking occurs regularly. Vertical and horizontal coherence spectra show an aspect ratio of 0.25-0.5 near the buoyancy frequency, evidencing anisotropy. At higher frequencies, the transition to isotropy (aspect ratio of 1) is found within the inertial subrange. Above the continuous turbulence spectrum in this subrange, isolated peaks are visible that locally increase the spectral width, in contrast with open ocean spectra. Their energy levels are found to be proportional to the tidal energy level.
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Insights from a 3-D temperature sensors mooring
on stratied ocean turbulence
Hans van Haren
1
, Andrea A. Cimatoribus
1
, Frédéric Cyr
1,2
, and Louis Gostiaux
3
1
NIOZ Royal Netherlands Institute for Sea Research and Utrecht University, Den Burg, Netherlands,
2
Now at Institut
Méditerranéen dOcéanologie (MIO), Marseille CEDEX 9, France,
3
Laboratoire de Mécanique des Fluides et dAcoustique,
UMR CNRS 5509, École Centrale de Lyon, Université de Lyon, Écully CEDEX 9, France
Abstract A unique small-scale 3-D mooring array has been designed consisting of ve parallel lines, 100 m
long and 4 m apart, and holding up to 550 high-resolution temperature sensors. It is built for quantitative
studies on the evolution of stratied turbulence by internal wave breaking in geophysical ows at scales
which go beyond that of a laboratory. Here we present measurements from above a steep slope of Mount
Josephine, NE Atlantic where internal wave breaking occurs regularly. Vertical and horizontal coherence spectra
show an aspect ratio of 0.250.5 near the buoyancy frequency, evidencing anisotropy. At higher frequencies,
the transition to isotropy (aspect ratio of 1) is found within the inertial subrange. Above the continuous
turbulence spectrum in this subrange, isolated peaks are visible that locally increase the spectral width, in
contrast with open ocean spectra. Their energy levels are found to be proportional to the tidal energy level.
1. Introduction
In the stratied ocean, internal waves and turbulent mixing are three-dimensional 3-Dphysical processes
that are enhanced above sloping topography [Thorpe, 1987]. Knowledge about turbulent mixing properties
is important for the (re)distribution of energy, suspended matter, and nutrients, which are indispensable for
life. According to near-bottom ocean observations, more than 50% of the turbulent mixing in terms of energy
dissipation rate and eddy diffusivity can occur in less than 5% of a dominant wave cycle during the arrival of
an upslope propagating and highly nonlinear turbulent bore [van Haren and Gostiaux, 2012]. Such a bore
generates a trail of short internal waves close to the local, thin-layer buoyancy frequency. According to
numerical and laboratory modeling [e.g., Aghsaee et al., 2010], the bore front may be 2-D in appearance as
an interfacial wave or at least have different scales in cross-slope and along-slope dimensions. The same scale
difference occurs in turbulence when it is hampered in the vertical by the stable density stratication. Thus far,
this has been mainly established for low Reynolds number ows [Gargett, 1988]. In higher Reynolds number
ows, especially those associated with internal wave breaking, turbulent overturn development from the largest
to the smallest, dissipative scales is expected to be an essentially 3-D process (similar scales in all three Cartesian
coordinates). This raises questions about traditional in situ observations of internal wave turbulence that have so
far been made using 1-D moored instrumented lines equipped with high-resolution temperature sensors.
In the past, few experiments have been set up in the ocean with two or more instrumented mooring lines at hor-
izontal distances of less than 10 km, roughly resolving the correlation length scale of the relevant internal waves.
Exceptions were the formidable one-time pyramid experiment of the Internal Wave EXperiment (IWEX) in the
open West Atlantic Ocean resolving scales of 6 m [Briscoe, 1975], and, at considerably larger scales O(110 km),
the deep-sea conventional mooring experiment above the Madeira Basin abyssal plain in the East Atlantic
[Saunders, 1983].
The IWEX experiment showed high correlation in the internal wave band between neighboring sensors, but
even at the 6 m horizontal scale, a drop in coherence was observed near the local buoyancy frequency of the
small-scale internal waves. This drop was not attributed to instrumental deciency but to nonlinear wave
effects [Briscoe, 1975]. A set of 2 Hz sampled acoustic Doppler current proler (ADCP) observations from half-
way up the continental slope in the Bay of Biscay conrmed these decorrelation scales for highly nonlinear
turbulent bores that dominate internal wave breaking above sloping topography [van Haren, 2007].
A recent three moorings free-fall dropping experiment done by our team in 2000 m water depth failed in its
aim of forming a triangle with horizontal sides of 75 m. The lines were rather at horizontal distances of 200,
180, and 20 m, with an estimated error of 20 m in location determination. The observations proved that
VAN HAREN ET AL. THE 3-D MOORING STRATIFIED OCEAN TURBULENCE 4483
PUBLICATION
S
Geophysical Research Letters
RESEARCH LETTER
10.1002/2016GL068032
Key Points:
Successful deployment of 3-D
temperature sensors mooring in the
deep ocean
Demonstrating the transition from
anisotropic to isotropic turbulence
and showing coherent portions in the
inertial subrange
Facilitating a better understanding of
internal wave breaking and turbulence
development which is not easily
achievable in the laboratory
Correspondence to:
H. van Haren,
hans.van.haren@nioz.nl
Citation:
van Haren, H., A. A. Cimatoribus, F. Cyr,
and L. Gostiaux (2016), Insights from a
3-D temperature sensors mooring on
stratied ocean turbulence, Geophys.
Res. Lett.,43, 44834489, doi:10.1002/
2016GL068032.
Accepted 8 APR 2016
Accepted article online 11 APR 2016
Published online 14 MAY 2016
turbulent bore motions certainly decorrelate at the 200 m scale and in the smaller details also at 20 m
(unpublished results). This experiment led to the construction of a single multiple-line 3-D mooring array
of temperature sensors resolving 1 m vertical and 4 m horizontal scales. After several years of development,
trials, and modications, these innovative measurements are presented here focusing on statistics of the
internal wave turbulence band.
2. Technical Details
For transportation and handling in port, the entire mooring is fold up (Figure 1a). This 6 m tall, high-grade
aluminum structure consists of two sets of four arms 3 m long. We mounted 5 × 95 = 475 NIOZ4high-
resolution temperature (T)-sensors in tubes taped at 1.0 m intervals to ve 104 m long, 0.0032 m diameter
thin nylon-coated steel cables. Four cables connect the corner tips of the upper and lower sets of arms; a
central instrumented line connects the upper and lower inner frames. After the structure is lifted overboard
into the sea by a crane, the arms are unfolded and lines stretched by lowering the central weight. Upon being
fully stretched (Figure 1b), the mooring is released into free fall to the seaoor.
The four corner lines are spaced 4.0 m from the central line and 5.6 m between them. Each line is kept under
about 1000 N tension distributed from two top buoys totaling 5000 N net buoyancy. This tension and large
net buoyancy maintain a relatively stiff mooring, with little motion under current drag. Vertical tilt was small
(<1°). Heading information showed commonly <1° variations of compass data around their mean values,
except brief <10° variations during three strong (~0.22 m s
1
) current speed events. The T-sensors were
located between 5 and 99 m above the bottom. They sampled at 1 Hz, with a precision better than
5×10
4
°C and a noise level of 1 × 10
4
°C [van Haren et al., 2009]. Every 4 h, all sensors were synchronized
to a single clock via induction. Due to various problems, 33 of the 475 sensors did not function properly.
Their data are not considered.
Figure 1. (a) Compacted 3-D mooring array of temperature sensors just before deployment in the ocean. (b) Model of the
unfolded array to scale.
Geophysical Research Letters 10.1002/2016GL068032
VAN HAREN ET AL. THE 3-D MOORING STRATIFIED OCEAN TURBULENCE 4484
This 3-D thermistor array was successfully moored at 37°00N, 013°48W, and 1740 m water depth on the eastern
ank of Mount Josephine, about 400km southwest of Lisbon (Portugal)in the NE Atlantic on 2 May 2015 (year-
day 121). The line orientation was directed southeast-northwest SE-NW (lines 1 and 4), NE-SW (lines 2 and 3) to
within ±5°, with lines 1 and 2 facing downslope (direction ~ESE). The average localbottom slope of about 10° is
more than twice as steep (supercritical) than the average slope of internal tides under local stratication condi-
tions [van Haren et al., 2015]. The site is also well below the Mediterranean Sea outow (between 1000 and
1400m), so that salinity compensated apparent density inversions in temperature are expected to be minimal.
The 38 days of T-data are converted into Conservative(~potential) Temperature data Θ[IOC,SCOR,IAPSO,
2010]. Theyare used as a tracer for density anomaly referenced to 1600m (σ
1.6
) variations following the relation
δσ
1.6
=αδΘ,α=0.044 ± 0.005 kg m
3
°C
1
for the depth range [1500 2000] m, where αdenotes the apparent
thermal expansion coefcient under local conditions [van Haren et al., 2015]. This relation is established from
nearby shipborne Conductivity-Temperature-Depth prole data. The high spatial (1 m in the vertical) and tem-
poral (1 H z) resolu tion o f the T-sensors, in combination with their low noise level, allows the estimation of the
vertical averages of the dissipation rate of turbulent kinetic energy <ε>and vertical eddy diffusivity <K
z
>via
reordering [Thorpe, 1977]. Vertical averaging is denoted by <>. Although this method has been used
frequently in the past, mostly from shipborne or free-fall proling data, its assumptions have been debated
from recent numerical studies [e.g., Chalamalla and Sarkar, 2015; Mater et al., 2015]. The latter study shows that
instantaneous dissipation rates estimated from this method may be biased high by a factor of 4 during large
convective events but that this bias disappears after sufcient geometrical averaging. In the averaged εand
K
z
values reported below this bias is thus minimized (well within one standard deviation of error). Based on pre-
vious detailed analyses using data from a similar 100 m long but 1-D mooring nearby [Cimatoribus and van
Haren, 2015, 2016], we can condently say that while convection is an important process, in particular in some
parts of the water column and during the upslope tidal phase, shear-driven turbulence is still the dominant
process. This is because above such sloping topography internal waves breaking is observed to be a complex
mix of convective and shear-driven overturning over a wide range of scales.
Figure 2. High-resolution Conservative Temperature observations at the central line. (a) Entire 38 day time series, a mean
from upper (1641 m), middle (1688 m), and lower (1735 m) sensors. The periods of Figures 2b and 2c are indicated by blue
bars. (b) Four day period of relatively low tide amplitudes. (c) As in Figure 2b but for large tide amplitudes. The purple line
indicates a period with average turbulence levels, detailed in Figure 3.
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VAN HAREN ET AL. THE 3-D MOORING STRATIFIED OCEAN TURBULENCE 4485
3. Observations
The main periodicity of internal waves above Mount Josephine is the semidiurnal lunar tide (see temperature
sensor data from the central line, Figure 2). The tidal excursion well exceeds the 94 m range of thermistors as
none of the color contours can be followed uninterruptedly. This is observed during both a relatively weak
tide period (Figure 2b) and one with approximately twice the amplitude (Figure 2c). As usual above sloping
topography, fronts representing nonlinear bores and high-frequency internal waves are present, with the
sharper fronts when tides are larger (Figure 2c).
The vertically averaged turbulence parameter estimate values of the 1 Hz data have a range of about four orders
of magnitude. Mean values are relatively high. Over the displayed 4 days, 94 m full range of sensors and thus not
necessarily representing a particular overturn scale, means are as follows: [<ε>] = 1.3 ± 1 × 10
7
m
2
s
3
,[<K
z
>]
=1.6±1.2×10
2
m
2
s
1
, and [<N>] = 1.6 ± 0.4 × 10
3
s
1
for Figure 2b and [<ε>]=6±4×10
7
m
2
s
3
,
[<K
z
>] = 2.5 ± 1.5 × 10
2
m
2
s
1
,and[<N>] = 1.9 ± 0.4 × 10
3
s
1
for Figure 2c, where []denotes averaging with
time and the error one standard deviation. These mean turbulence levels are more than two orders of magni-
tude larger than the canonical value of K
z
=10
4
m
2
s
1
required to maintain the ocean stratied [Munk,
1966]. They are, within error, equal to the largest values observed above Mount Josephine previously, in
350 m deeper waters [van Haren et al., 2015]. They are three orders of magnitude larger than values observed
in the ocean interior [e.g., Gregg, 1989].
An example of 600s time-depth series showing a passage of fronts when turbulence levels are average is
given in Figure 3. It shows that incoherent motions can be studied in some detail between the ve lines.
These (upslope propagating) fronts are seen to rst hit at line 1 then 2, c(entral), and lines 3 and 4, the latter
about simultaneously. This is more or less visible in the same order for the off-bottom cool front at 1720 m,
day 141.4547, the near-bottom cold front at 1735 m, day 141.4575, and the warm depression at 1660 m, day
141.458. Noting that the time-tick marks are at 86 s, typical relative delays are 1020 s over 48 m, yielding
Figure 3. Ten minute detail of data at the purple line in Figure 2, here in the rst panel labeled cfor central line. Black
contour lines are drawn every 0.05°C. The other panels show the Conservative Temperature observations for the same
time window from the four corner lines, labeled 14. The black contours are repeated from the rst panel (central line).
Geophysical Research Letters 10.1002/2016GL068032
VAN HAREN ET AL. THE 3-D MOORING STRATIFIED OCEAN TURBULENCE 4486
an estimate of phase/propagation
speed of 0.4 m s
1
twice the particle/current speed
measured by a large-scale acous-
tic Doppler current proler on a
simultaneous mooring 1 km away.
The turbulence is found at fre-
quencies (σ)wellbeyondthe
100 m based (large-scale), buoy-
ancy frequency Nand the maxi-
mum 2 m based (small-scale)
buoyancy frequency N
s,max
.Two
power spectra P, obtained by aver-
aging all sensors over a 4 day per-
iod (Figure 4), demonstrate an
image of the internal wave and
turbulence band obtained from
442 independent temperature
records in an ocean water column
with a volume of 3000 m
3
.The
average spectra show that even
at the highest frequency (the
Nyquist frequency), the sensors
respond above their noise level. There, the width of spectral variation is small, due to the large number
of degrees of freedom (~15,000).
For lower frequencies σ~<3N
s,max
, however, this width of variation increases, in particular for σ<2N
s,max
,evi-
dencing larger contributions from coherent signals. Between N<σ<1000 and 2000 cpd (cycle(s) per day) we
expect this to be the turbulence inertial subrange. Roughly, this part of the spectrum follows a slope of σ
5/3
(green dashed line), particularly for the red spectrum. A 5/3 slope is expected for temperature in a turbulence
inertial subrange [Tennekes and Lumley, 1972] or a passive scalar under isotropic conditions [Warhaft, 2000].
Regularly, peaks, like around N
s,max
,signicantly extend above the inertial subrange background.
The two spectra from weak (blue) and strong (red) tides are not offset deliberately. Their difference in energy is
approximately a factor of 4 in the internal wave band f<σ<N,fthe inertial frequency, and a factor of about
4
2
=16 forσ>N. To within error, a factor of 4 is the ratio of the calculated mean dissipation rates and the ratio
of the tidal amplitudes squared. With less energy in the blue spectrum, its inertial subrange peaks are smaller
and shifted to lower frequencies consistent with smaller buoyancy frequencies. In contrast, the internal wave
band is relatively more energetic and its slope is steeper than 5/3 at low frequencies consistent with previous
observations under relatively weak turbulent conditions [Cimatoribus and van Haren, 2015].
The all-sensors, 4 day average coherence spectra are given in Figure 5. As expected, the coherence in the
range [0.11 0.99] decreases at any frequency with increasing separation distance, both in the vertical (Δz)
and in the horizontal (Δx,y). The lower tidal energy period (Figure 5a) shows smaller coherence at all levels
and intervals compared with the more energetic period (Figure 5b). For Δz= 1 m during the more
energetic period, the coherence attens to a low level at about 5000 cpd from lower frequencies. This
5000 cpd corresponds to the roll-off of the red power spectrum in Figure 4. At the high-frequency end
(10002000 cpd) the observed inertial subrange drops into noise for a vertical separation distance of
23 m. Like in the power spectra, several coherence peaks occur within the inertial subrange, foremost for
the tidally energetic period (Figure 5b). They coincide in frequency with the power spectral peaks extending
above the σ
5/3
slope and up to a σ
2
slope from the M
2
peak (as does, e.g., the peak around σ=N
s,max
in
Figure 5b). This implies approximately the same energy level in vertical motions at tidal frequencies and at
coherent inertial subrange peak frequencies. These inertial subrange peaks are consistent throughout the
coherence spectra, independently of the separation distance. They are least well visible for 1 m separation
distance, due to its highest coherence.
Figure 4. Temperature power spectra, averages of 442 near-raw periodograms
for the Kaiser window tapered time series of the 4 day periods in Figures 2b
(blue) and 2c (red). The data are subsampled at 0.5Hz for computational
reasons. Besides inertial (f) and semidiurnal lunar tidal (M
2
) frequencies several
buoyancy frequencies are indicated including 4 day large-scale mean Nand the
maximum small-scale N
s,max
.Thevalue5/3 indicates the spectral slope σ
5/3
.
The 95% signicance level is indicated by the small vertical bar.
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VAN HAREN ET AL. THE 3-D MOORING STRATIFIED OCEAN TURBULENCE 4487
Comparing the vertical and horizontal coherence spectra demonstrates the transition from anisotropy to
isotropy through the inertial subrange. At the high-frequency end of the internal wave band, σ=N, the coher-
ence level of horizontal separation distance Δx,y= 4 m coincides with that of vertical Δz=12 m, while Δx,
y= 8 m coincides with that of Δz=24 m. This suggests an anisotropic aspect ratio of 0.250.5, depending
on the tidal energy. When the frequency increases, this aspect ratio becomes equal to 1 near 2N
s,max
,as
Δx,y=4m(Δx,y= 8 m) nearly match that of Δz=4m(Δz= 8 m). Below this cutoff, the coherence drops below
the 95% signicance level. The coherence measurements hence capture the transition from anisotropy to
isotropy of the ow in a much clearer way than the power spectrum, although in the latter the σ
5/3
slope
is permanently reached for approximately 2N
s,max
<σ(<270 N).
4. Discussion
Ocean ows commonly create environments with high bulk Reynolds numbers (Re = UL/ν), with values
reaching Re = O(10
4
10
6
) in areas where internal wave breaking is imminent such as the one studied here.
Parameters used for this scaling are, U= O(0.010.1) m s
1
, the velocity scale, L= O(110) m, the length scale,
and, ν=10
6
m
2
s
1
, the kinematic viscosity. In spite of the relatively large turbulence levels generated by
such wave breaking, the ocean rapidly restraties in stable density layers. The present observations shed
some light on the anisotropic nature of stratied turbulence in such a deep-sea environment. Hereby, the
small-scale density layering plays an important role in determining the scale of separation between anisotro-
pic (lower frequencies) and isotropic (higher frequencies) motions. This study suggests that this separation
occurs at about twice the small-scale buoyancy frequency. This resembles the ndings by Gargett [1988], here
for higher Reynolds number ows. The frequencies at which isotropy is found likely relate with Froude num-
ber equal to 1 [Billant and Chomaz, 2001]. Unfortunately, we lacked adequate local current measurements to
verify this.
The results presented in this study are unique following years of attempts in performing three-dimensional
measurements of oceanic turbulence. To our knowledge, these data are the rst observational evidence of
the transition from anisotropy to isotropy in geophysical turbulence. Thus far, coherence computations
separated internal wave from turbulent motions at the large-scale buoyancy frequency N[e.g., Briscoe,
1975], without further detailing the inertial subrange as in the present data. The transition frequency
Figure 5. Average coherence spectra between all pairs of independent sensors for the labeled vertical (Δz) and horizontal
(Δx,y) distances. The low dashed lines indicate the approximate 95% signicance levels computed following Bendat and
Piersol [1986]. (a) Weak tide period of Figure 2b. (b) Strong tide period of Figure 2c.
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VAN HAREN ET AL. THE 3-D MOORING STRATIFIED OCEAN TURBULENCE 4488
between anisotropic and isotropic turbulence of about 2N
s,max
observed here informs about the particular
scale of motions at the separation. This separation is found at frequencies well beyond Nand is related to
the small-scale stratication. This stresses the larger importance of the formation of (thin) layering than
large-scale stratication for anisotropy. Perhaps, the observed inertial subrange peak frequencies indicate
an interaction between this small-scale layering and the larger-scale near-inertial and tidal internal wave
motions, but this requires further study.
The present array of ve 100 m long lines 4 and 5.6m apart seems an upper limit for the size of mooring to be
deployed in one action. For larger scales and more lines other solutions have to be found. One could lower
mooring lines via a winch cable several thousands of meters all the way to the bottom. Hereby, a precision
of a few meters horizontally can only be obtained using a short-baseline system to be deployed rst. This is
an elaborateadditional operation, performed only for an extensive set of moorings such as the ANTARES under-
water neutrino telescope off Toulon, France [Ageron et al., 2011]. However, the present ANTARES mooring line
separation is 90 mon average, which exceeds the required horizontal decorrelation scale resolution for internal
waves by one order of magnitude [Briscoe,1975;LeBlond and Mysak, 1978] and which narrowed to 4 m here.
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VAN HAREN ET AL. THE 3-D MOORING STRATIFIED OCEAN TURBULENCE 4489
Acknowledgments
This research was supported in part by
NWO, the Netherlands Organization for
the advancement of science. L.G. is
supported by the Agence Nationale de
la Recherche, ANR-13-JS09-0004-01
(STRATIMIX). We thank the captain and
crew of the R/V Pelagia and NIOZ-MTM
for their very helpful assistance during
deployment and recovery. We thank
J. van Heerwaarden, R. Bakker, and
M. Laan for all the work, discussions, and
trials during design and construction.
Data use requests can be directed to
hans.van.haren@nioz.nl.
... In contrast with the atmospheric observations of Frehlich et al. (2008) and with 30-s sampled deep-ocean temperature spectra resolving just the stratified turbulence range (Bouruet-Aubertot et al., 2010), recent deep-ocean high-resolution 1-s sampled temperature spectra from a small-scale five-line 3D mooring array above steep topography demonstrated a two orders of magnitude wide inertial subrange N <  < roll-off with distinctly different small-range variability in the low-and high-frequency parts (van Haren et al., 2016). The distinction was not found in the smooth DNS-spectra (e.g., Augier et al., 2015;Maffioli, 2017). ...
... For the latter to occur one needs a large slope, with spatial scales exceeding those of the carrier wave. The internal tide has a horizontal scale O(1 km), which may explain why the small ridge-crest site does not exhibit shear-convection, but highly shear-induced turbulence mainly: Its horizontal spatial scales match those of the internal tide (van Haren et al., 2016). The convection is not necessarily horizontally bounded, but the indirect effects of the topography are the wave steepening and breaking, which is expected to vary over the wave's horizontal scales that set a natural boundary. ...
Preprint
The inertial subrange of turbulence in a density stratified environment is the transition from internal waves to isotropic turbulence, but it is unclear how to interpret its extension to anisotropic stratified turbulence. Knowledge about stratified turbulence is relevant for the dispersal of suspended matter in geophysical flows, such as in most of the ocean. For studying internal-wave-induced ocean-turbulence moored high-resolution temperature (T-)sensors are used. Spectra from observations on episodic quasi-convective internal wave breaking above a steep slope of large seamount Josephine in the Northeast-Atlantic demonstrate an inertial subrange that can be separated in two parts: A large-scale part with relatively coherent portions adjacent to less coherent portions, and a small-scale part that is smoothly continuous (to within standard error). The separation is close to the Ozmidov frequency, and coincides with the transition from anisotropic/quasi-deterministic stratified turbulence to isotropic/stochastic inertial convective motions as inferred from a comparison of vertical and horizontal co-spectra. These observations contrast with T-sensor observations of shear-dominated internal wave breaking in an equally turbulent environment above the slope of a small Mid-Atlantic ridge-crest, which demonstrate a stochastic inertial subrange throughout.
... The mooring lines were 100 m tall but only 4 m apart, so that only LO was partially resolved in 3D. Nevertheless, statistical information was obtained on the transition from anisotropic 2D stratified turbulence to isotropic 3D turbulence through a non-smooth inertial subrange (van Haren et al. 2016a). Technically, this prototype of multiple parallel lines mooring array could only be deployed in a single overboard deployment operation, for which the lines were fold-up. ...
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The redistribution of matter in the deep-sea depends on water-flow currents and turbulent exchange, for which breaking internal waves are an important source. As internal waves and turbulence are essentially three-dimensional 3D, their dynamical development should ideally be studied in a volume of seawater. However, this is seldom done in the ocean where 1D-observations along a single vertical line are already difficult. We present the design, construction and successful deployment of a half-cubic-hectometer (480,000 m3) 3D-T mooring array holding 2925 high-resolution temperature sensors to study weakly density-stratified waters of the 2500-m deep Western Mediterranean. The stand-alone array samples temperature at a rate of 0.5 Hz, with precision <0.5 mK, noise level <0.1 mK and expected endurance of 3 years. The independent sensors are synchronized inductively every 4 h to a single standard clock. The array consists of 45 vertical lines 125 m long, at 9.5 m horizontally from their nearest neighbor. Each line is held under tension of 1.3 kN by a buoyancy element that is released chemically one week after deployment. All fold-up lines are attached to a grid of cables that is tensioned in a 70 m diameter ring of steel tubes. The array is build-up in harbor-waters, with air filling the steel tubes for floatation. The flat-form array is towed to the mooring site under favorable sea-state conditions. By opening valves in the steel tubes, the array is sunk and its free-fall is controlled by a custom-made drag-parachute reducing the average sinking speed to 1.3 m s-1 and providing smooth horizontal landing on the flat seafloor.
... Internal ocean tides have been observed by satellite remote sensing of sea surface elevation (Egbert & Ray, 2000;Dushaw, 2002;Egbert & Ray, 2003;Zhao et al., 2012;Zhao, 2014) and moored measurement of oceanographic quantities (Munk & Wunch, 1998;Kunze et al., 2002;Garrett & St. Laurent, 2002;Rudnick et al., 2003;Nash et al., 2004;Rainville & Pinkel, 2006;Martini et al., 2011;Kelly et al., 2012;van Haren et al., 2016). In contrast to these observations from above the sea surface and through water columns, attempts to detect internal tides from below (from the ocean bottom) are few. ...
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Barotropic tidal currents over bottom topography force density surfaces to oscillate vertically and thereby to act as quasi‐stationary internal tide sources. Deploying a seafloor pressure gauge array with an aperture of 30 km for a year (2014–2015), we detected the low‐mode semidiurnal internal tidal waves propagating with a horizontal phase speed of ~1 m/s in the onshore and offshore directions over the array along the eastern slope of Aogashima Island, south of Japan. The amplitudes of the offshore propagating waves were greater than those of the onshore propagating waves, and both were positively correlated with the amplitudes of the local semidiurnal tide, which peaked in September and March. A tide‐resolving ocean circulation model (JCOPE‐T) well reproduced the observed onshore and offshore internal tidal wave propagation. The model indicated a standing wave region on the slope, where offshore propagating waves interact with standing waves locally pinned to the slope. Along the same profile over a distance of 100 km, we conducted seismic–oceanographic analysis of the legacy multi‐channel seismic reflection data to retrieve vertical cross‐sections of the reflecting layers, which indicated sharp temperature changes in the ocean. Many of the slant reflecting layers were subparallel to the contour lines of the semidiurnal internal‐tide‐associated temperature anomalies in the JCOPE‐T model, suggesting a causal link between the fine reflection layering structure and the semidiurnal low‐mode internal tidal field.
... At the latter distance, where turbulent motions are less energetic and internal waves smoother, the current meter also shows an aspect ratio of 1 near the larger-scale buoyancy frequency. The transition to full 3D-motions, with aspect ratio of 1, is also observed in a recent five-line T-sensor mooring in the lower 100 m above a sloping bottom [25]. Those measurements were however made in considerably deeper waters of 1740 m with half the stratification. ...
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The turbulence regime near the crest of a biologically rich seamount of the Mid-Atlantic Ridge southwest of the Azores was registered in high spatial and temporal resolution. Internal tides and their higher harmonics dominate the internal wave motions, producing considerable shear-induced turbulent mixing in layers of 10–50 m thickness. This interior mixing of about 100 times open-ocean interior values is observed both at a high-resolution temperature sensor mooring-site at the crest, 770 m water depth being nearly 400 m below the top of the seamount, and a CTD-yoyo site at the slope off the crest 400 m horizontally away, 880 m water depth. Only at the mooring site, additionally two times higher turbulence is observed near the bottom, associated with highly non-linear wave breaking. The highest abundance of epifauna, notably sponges, are observed just below the crest and 100 m down the eastern slope (700–800 m) in a cross-ridge video-camera transect. This sponge belt is located in a water layer of depressed oxygen levels (saturation 63±2%) with a local minimum centered around 700 m. Turbulent mixing supplies oxygen to this region from above and below and is expected to mix nutrients away from this biodegraded layer towards the depth of highest abundance of macrofauna.
... The amount of kinetic energy available for dissipation at small scales in rotating stratified turbulence (RST) is a crucial quantity for sub-grid scale parameterizations in oceanic and climate models, and depends on the amount of energy that is transferred to these small scales in the presence of inertia-gravity waves, through breaking at small scales of the large-scale quasi-geostrophic (QG) balance (Lelong & Riley 1991;Staquet & Sommeria 2002;Riley & deBruynKops 2003), and a lowering of the Richardson number below some critical value. Wave-turbulence interactions allow for coupling to the mean flow (Finnigan 1999); they have been measured in the stratosphere as well as in the upper ocean and lead to vertical mixing and enhanced dissipation (see e.g., Klymak et al. (2008);van Haren et al. (2016)). Few studies have considered mixing in decaying rotating stratified flows. ...
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We present a simple model for the scaling properties of mixing in weakly rotating stratified flows characterized by their Rossby, Froude and Reynolds numbers Ro, Fr and Re. It is based on (i) equipartition between kinetic and potential modes; (ii) sub-dominant vertical velocity, w/urms smaller than 1; and (iii) lessening of the energy transfer to small scales as measured by a dissipation efficiency compared to its dimensional expression. We determine the domains of validity of such laws by analyzing a large numerical study of the unforced Boussinesq equations on grids of 1024 cube points, with an emphasis on atmospheric and oceanic parameters (Ro/Fr larger than 2.47), and with mostly constant Re around 10000; the Prandtl number is one, initial conditions are isotropic and at large scale for the velocity, and zero for the temperature fluctuations. Three regimes in Froude number, as for stratified flows, are observed: dominant waves, eddy-wave interactions and strong turbulence. A wave-turbulence balance for the transfer time leads to energy dissipation efficiency growing linearly with Fr (intermediate regime), with a saturation at larger Fr numbers. The mixing efficiency Gamma, putting together the three relationships of the model allows for the prediction of the scaling as the square of 1/Fr in the low/intermediate regimes for high Re, whereas for higher Fr, Gamma scales as the square-root of RB, as already observed: as turbulence strengthens, smaller buoyancy fluxes altogether correspond to a decoupling of velocity and temperature fluctuations which become passive.
Article
The impact of large atmospheric disturbances on deep benthic communities is not well known quantitatively. Observations are scarce but may reveal specific processes leading to turbulent disturbances. Here, we present high-resolution deep-ocean observations to study potential turbulent mixing by a large atmospheric disturbance. We deployed an array of 100-Hz sampling-rate geophysical broadband Ocean Bottom Seismometers (OBSs) on the seafloor. Within the footprint of this array we also deployed an oceanographic 0.5-Hz sampling-rate vertical temperature sensor string covering the water phase between 7 and 207 m above the seafloor at about 3000 m depth off eastern Taiwan between June 2017 and April 2018. In September 2017, all instruments recorded Category 4 cyclone Typhoon Talim’s passage northeast of the array one day ahead of the cyclone’s closest approach when the cyclone’s eye was more than 1,000 km away. For 10 days, a group of near-inertial motions appeared most clearly in temperature. The group contained the largest inertial amplitudes in the ten month time series, and which led to turbulence dissipation rates O(10⁻⁷ m² s⁻³). The observation reflects the importance of barotropic response to a cyclone and the propagation of inertio-gravity waves in weak density stratification. In addition to internal tides, these waves drove turbulent overturns larger than 200 m that were concurrently recorded by OBSs. The turbulent signals were neither due to seismic activity nor to ocean-surface wave action. Cyclones can generate not only microseisms and earth hums, as well as turbulence in the water column, producing additional ground motions. Quantified turbulence processes may help constrain models on sediment resuspension and its effect on deep-sea benthic life.
Article
van Haren, H., 2020. High-resolution temperature observations of a shallow lagoon in the South Pacific (Bora Bora). Journal of Coastal Research, 36(3), 536–544. Coconut Creek (Florida), ISSN 0749-0208. The daily cycle of heating and cooling of the near-surface ocean may be quite different in the open ocean compared with a shallow lagoon with a seafloor that is a few meters deep and can be heated directly by the sun. This solar radiation can affect the local benthic communities. To study the physical processes associated with the daily cycle of the South Pacific lagoon surrounding Bora Bora, a vertical string of five high-resolution temperature sensors is moored at a 2-m-deep site for 3 weeks. Besides standard ocean warming (mostly during the day) and cooling (mostly at night), the sensors show relatively highest temperature near the lagoon floor during the warming phase and weakly stable stratification toward the end of the cooling phase. During the warming phase, highly variable stratification is observed extending into the water column, but only under conditions of calm weather and turbid waters. Under trade winds and clear waters, the lowest sensor or sensors show consistently higher temperature variability than higher sensors with spectral slopes indicative of shear and/or convective turbulence. During the cooling phase, the lower sensor shows consistently very low variance (nonturbulent), while other sensors show a spectral slope around the buoyancy frequency, evidencing weakly stratified waters supporting internal waves. These observations contrast with open-ocean near-surface observations of stable stratification during the warming phase and turbulent free convection during the cooling phase. Thus, lagoons seem to resemble the atmosphere more than the ocean in daytime thermodynamics and possibly act as a natural solar pond with bottom conductive heating (when salinity compensates for unstable temperature variations).
Article
The inertial subrange of turbulence in a density stratified environment is the transition from internal waves to isotropic turbulence, but it is unclear how to interpret its extension to anisotropic “stratified” turbulence. Knowledge about stratified turbulence is relevant for the dispersal of suspended matter in geophysical flows, such as in most of the ocean. For studying internal-wave-induced ocean-turbulence, moored high-resolution temperature (T-)sensors are used. Spectra from observations on episodic quasiconvective internal wave breaking above a steep slope of large seamount Josephine in the Northeast-Atlantic demonstrate an inertial subrange that can be separated in two parts: A large-scale part with relatively coherent portions adjacent to less coherent portions and a small-scale part that is smoothly continuous (to within standard error). The separation is close to the Ozmidov frequency and coincides with the transition from anisotropic/quasideterministic stratified turbulence to isotropic/stochastic inertial convective motions as inferred from a comparison of vertical and horizontal cospectra. These observations contrast with T-sensor observations of shear-dominated internal wave breaking in an equally turbulent environment above the slope of a small Mid-Atlantic ridge-crest, which demonstrate a stochastic inertial subrange throughout.
Chapter
An overview is presented of high-resolution temperature observations above underwater topography in the deep, generally stably stratified ocean. The Eulerian mooring technique is used by typically distributing 100 sensors over lines between 40 and 400 m long. The independent sensors sample at a rate of 1 Hz for up to one year with a precision better than 0.1 mK. This precision and sampling rate are sufficient to resolve all of the internal waves and their breaking including the large, energy containing turbulent eddies above underwater topography. Under conditions of a tight temperature-density relationship, the data are used to quantify turbulent overturns. The turbulent diapycnal mixing is important for the redistribution of nutrients, heat (to maintain the stable stratification) and the resuspension of sediment. The detailed observations show two distinctive turbulence processes that are associated with different phases of large-scale carriers (which are mainly tidal but also inertial, internal gravity waves or a sub-inertial sloshing motion): (i) highly nonlinear turbulent frontal bores during the upslope propagating phase, and (ii) Kelvin-Helmholtz billows, at some distance above the slope, during the downslope phase. While the former may be associated in part with convective turbulent overturning following Rayleigh-Taylor instabilities preceding and sharpening the bores, the latter are mainly related to shear-induced instabilities. Under weaker stratified conditions, away from boundaries, free convective mixing appears more often, but a clear inertial subrange in temperature spectra is indicative of dominant shear-induced turbulence. Turbulence is seen to increase in dissipation rate and diffusivity all the way to the bottom while stratification remains constant, which challenges the idea of a homogeneous ‘well-mixed bottom boundary layer’. With a newly developed five-lines mooring the transition is demonstrated from isotropy (full turbulence) to anisotropy (stratified turbulence/internal waves).
Article
An ocean-bottom experiment consisting of an array of four ocean-bottom seismometers (OBS) was conducted off the coast of southeast Taiwan during May-July 2011. We develop comprehensive analyses of the space-time kinematics of the tidal signals recorded in the compact high-sensitivity temperature loggers (CHTL) and the OBS geophones at the ocean bottom with depths ranging from 1254 to 1610 m. The evidence suggests that internal tides are responsible for the recorded signals: baroclinic internal waves (mainly the M2 tide) are generated by barotropic tidal currents in the Luzon Strait. The internal tides exhibit gradual phase changing and irregularly fluctuating strength, leaving signatures in the CHTL as ambient temperature variations, signifying low-mode wave motions within the stratified water layers; and in OBS geophones as intermittent "tremor" agitations, signifying high-mode turbulent flows on the seafloor. The M2 internal tides across our array are found to propagate in the northeast direction at speeds ranging from 1 to 2+ m s-1. Furthermore, the internal tides are identified at the ocean-bottom based on an operational hydrodynamic hindcast/forecast model. The simulations show good agreement with the observed temperature variation on the seafloor and substantiate the vertical velocity and displacement of the water parcel driven by the internal tides. The joint detection of the temperature and tremor signals provides further information about the interactions of internal tides with the seafloor topography and the associated energy dissipation. Our results elucidate the space-time ubiquity of the internal tides at the ocean bottom, which is an important interface of dynamic oceanography.
Article
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Direct numerical simulation (DNS) and large-eddy simulation (LES) are employed to study the mixing brought about by convective overturns in a stratified, oscillatory bottom layer underneath internal tides. The phasing of turbulence, the onset and breakdown of convective overturns, and the pathway to irreversible mixing are quantified. Mixing efficiency shows a systematic dependence on tidal phase, and during the breakdown of large convective overturns it is approximately 0.6, a value that is substantially larger than the commonly assumed value of 0.2 used for calculating scalar mixing from the turbulent dissipation rate. Diapycnal diffusivity is calculated using the irreversible diapycnal flux and, for tall overturns of O(50) m, the diffusivity is found to be almost 1000 times higher than the molecular diffusivity. The Thorpe (overturn) length scale is often used as a proxy for the Ozmidov length scale and thus infers the turbulent dissipation rate from overturns. The accuracy of overturn-based estimates of the dissipation rate is assessed for this flow. The Ozmidov length scale LO and Thorpe length scale LT are found to behave differently during a tidal cycle: LT decreases during the convective instability, while LO increases; there is a significant phase lag between the maxima of LT and LO; and finally LT is not linearly related to LO. Thus, the Thorpe-inferred dissipation rates are quite different from the actual values. Interestingly, the ratio of their cycle-averaged values is found to be O(1), a result explained on the basis of available potential energy.
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The ANTARES Neutrino Telescope was completed in May 2008 and is the first operational Neutrino Telescope in the Mediterranean Sea. The main purpose of the detector is to perform neutrino astronomy and the apparatus also offers facilities for marine and Earth sciences. This paper describes the design, the construction and the installation of the telescope in the deep sea, offshore from Toulon in France. An illustration of the detector performance is given.
Article
The relation between the flux of temperature (or buoyancy), the vertical temperature gradient and the height above the bottom is investigated in an oceanographic context, using high-resolution temperature measurements. The model for the evolution of a stratified layer by Balmforth et al. ( J. Fluid Mech. , vol. 355, 1998, pp. 329–358) is reviewed and adapted to the case of a turbulent flow above a wall. Model predictions are compared with the average observational estimates of the flux, exploiting a flux estimation method proposed by Winters & D’Asaro ( J. Fluid Mech. , vol. 317, 1996, pp. 179–193). This estimation method enables the disentanglement of the dependence of the average flux on the height above the bottom and on the background temperature gradient. The classical N-shaped flux–gradient relation is found in the observations. The model and the observations show similar qualitative behaviour, despite the strong simplifications used in the model. The results shed light on the modulation of the temperature flux by the presence of the boundary, and support the idea of a turbulent flux following a mixing-length argument in a stratified flow. Furthermore, the results support the use of Thorpe scales close to a boundary, if sufficient averaging is performed, suggesting that the Thorpe scales are affected by the boundary in a similar way to the mixing length.
Article
Oceanic density overturns are commonly used to parameterize the dissipation rate of turbulent kinetic energy. This method assumes a linear scaling between the Thorpe length scale LT and the Ozmidov length scale LO. Historic evidence supporting LTLO has been shown for relatively weak shear-driven turbulence of the thermocline; however, little support for the method exists in regions of turbulence driven by the convective collapse of topographically influenced overturns that are large by open-ocean standards. This study presents a direct comparison of LT and LO, using vertical profiles of temperature and microstructure shear collected in the Luzon Strait-a site characterized by topographically influenced overturns up to O(100)m in scale. The comparison is also done for open-ocean sites in the Brazil basin and North Atlantic where overturns are generally smaller and due to different processes.Akey result is that LT/LO increases with overturn size in a fashion similar to that observed in numerical studies of Kelvin-Helmholtz (K-H) instabilities for all sites but is most clear in data from the Luzon Strait. Resultant bias in parameterized dissipation is mitigated by ensemble averaging; however, a positive bias appears when instantaneous observations are depth and time integrated. For a series of profiles taken during a spring tidal period in the Luzon Strait, the integrated value is nearly an order of magnitude larger than that based on the microstructure observations. Physical arguments supporting LT LO are revisited, and conceptual regimes explaining the relationship between LT/LO and a nondimensional overturn size ĽT are proposed. In a companion paper, Scotti obtains similar conclusions from energetics arguments and simulations.
Article
We present a detailed analysis of temperature statistics in an oceanographic observational dataset. The data are collected using a moored array of thermistors, $100~\text{m}$ tall and starting $5~\text{m}$ above the bottom, deployed during four months above the slopes of a Seamount in the north-eastern Atlantic Ocean. Turbulence at this location is strongly affected by the semidiurnal tidal wave. Mean stratification is stable in the entire dataset. We compute structure functions, of order up to 10, of the distributions of temperature increments. Strong intermittency is observed, in particular, during the downslope phase of the tide, and farther from the solid bottom. In the lower half of the mooring during the upslope phase, the temperature statistics are consistent with those of a passive scalar. In the upper half of the mooring, the temperature statistics deviate from those of a passive scalar, and evidence of turbulent convective activity is found. The downslope phase is generally thought to be more shear-dominated, but our results suggest on the other hand that convective activity is present. High-order moments also show that the turbulence scaling behaviour breaks at a well-defined scale (of the order of the buoyancy length scale), which is however dependent on the flow state (tidal phase, height above the bottom). At larger scales, wave motions are dominant. We suggest that our results could provide an important reference for laboratory and numerical studies of mixing in geophysical flows.
Article
Underwater topography like seamounts causes the breaking of large ‘internal waves’ with associated turbulent mixing strongly affecting the redistribution of sediment. Here, ocean-turbulence is characterized and quantified in the lowest 100 m of the water column at three nearby sites above the slope of a deep-ocean seamount. Moored high-resolution temperature sensors show very different turbulence generation mechanisms over 3 and 5 km horizontal separation distances. At the steepest slope, turbulence was 100-times more energetic than at the shallowest slope where turbulence was still ten times more energetic than found in the open-ocean, away from topography. The turbulence on this extensive slope is caused by slope steepness and nonlinear wave evolution, but not by bottom-friction, ‘critical’ internal tide reflection or lee-wave generation.
Article
A three-dimensional array of 20 current meters, temperature sensors, and vertical temperature gradient sensors was successfully deployed for 40 days in late 1973 in the main thermocline over the Hatteras Abyssal Plain southeast of Bermuda. Sensor spacings in the main array were 1.4-1600 m in the horizontal, 2.1-1447 m in the vertical. The minimum sampling interval was 225 s. The ultimate purpose of the experiment was to estimate a vector wave number-frequency spectrum of internal waves without the usual assumptions of simple modal structure, horizontal isotropy, and linearity. The purpose of this paper is to describe some of the early results. Autospectra from the array normalize quite well in depth according to the WKBJ 'high-mode' solutions. Spectra of vertical displacements show a significant contribution from the internal semidiurnal tide. Samples of 1760 cross spectra calculated (based on a 40-day averaging interval) suggest horizontal isotropy, vertical homogeneity, and a possible degradation of current coherences because of fine structure in the velocity profile. Coherence of vertical displacements (i.e., temperaure fluctuations) for measurements separated horizontally decays with increasing separation according to f1/2X=330 m.cph, where f1/2(cph) is the frequency at which the coherence falls to one half and X (m) is the horizontal separation. This empirical rule is based on 1600 m>X>140 m; for smaller X1/2 exceeds the local buoyancy frequency. Autospectra and cross spectra of vertical displacements sometimes show peaks at frequencies just less than the local buoyancy frequency; current spectra do not show such peaks. Inverse modeling of the internal wave field is in progress; expected results are a vector wave number-frequency spectrum and a description in parameter space that hopefully will permit future experiments to be less elaborate.
Article
An experiment to measure near-bottom currents on the Madeira Abyssal Plain is described. The moorings placed near 33°N, 22°W were separated by 5–40 km with instruments at 10, 100 and 600 m above the bottom (depth ∼5300 m). Rotor stalling occurred ¼ to ⅓ of the time but does not hinder the analysis which separated currents into tidal (3 cm s−1, inertial (1.3 cm s−1) and low frequency (2.5 cm s−1) components. The M2 tide is found to be principally barotropic with magnitude, ellipticity, orientation and phase adequately predicted by the tidal model of Schwiderski (1979). Oscillations of near-inertial frequency are found to be bottom intensified, have a wavelength of 100 km directed nearly due south and 3 km vertically: their phase velocity is directed downward suggesting the bottom as the source. The vertical group velocity is estimated at ∼150 m day−1 upward and corresponds to the 4–6 day lag observed between 10 and 600 m for the envelope of inertial amplitude. Low-frequency statistics are presente...