Abstract
The authors present inferences of diapycnal diffusivity from a compilation of over 5200 microstructure profiles. As microstructure observations are sparse, these are supplemented with ...indirect measurements of mixing obtained from (i) Thorpe-scale overturns from moored profilers, a finescale parameterization applied to (ii) shipboard observations of upper-ocean shear, (iii) strain as measured by profiling floats, and (iv) shear and strain from full-depth lowered acoustic Doppler current profilers (LADCP) and CTD profiles. Vertical profiles of the turbulent dissipation rate are bottom enhanced over rough topography and abrupt, isolated ridges. The geography of depth-integrated dissipation rate shows spatial variability related to internal wave generation, suggesting one direct energy pathway to turbulence. The global-averaged diapycnal diffusivity below 1000-m depth is O(10−4) m2 s−1 and above 1000-m depth is O(10−5) m2 s−1. The compiled microstructure observations sample a wide range of internal wave power inputs and topographic roughness, providing a dataset with which to estimate a representative global-averaged dissipation rate and diffusivity. However, there is strong regional variability in the ratio between local internal wave generation and local dissipation. In some regions, the depth-integrated dissipation rate is comparable to the estimated power input into the local internal wave field. In a few cases, more internal wave power is dissipated than locally generated, suggesting remote internal wave sources. However, at most locations the total power lost through turbulent dissipation is less than the input into the local internal wave field. This suggests dissipation elsewhere, such as continental margins.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract
Midocean ridge fracture zones channel bottom waters in the eastern Brazil Basin in regions of intensified deep mixing. The mechanisms responsible for the deep turbulent mixing inside the ...numerous midocean fracture zones, whether affected by the local or the nonlocal canyon topography, are still subject to debate. To discriminate those mechanisms and to discern the canyon mean flow, two moorings sampled a deep canyon over and away from a sill/contraction. A 2-layer exchange flow, accelerated at the sill, transports 0.04–0.10-Sv (1 Sv ≡ 10
6
m
3
s
−1
) up canyon in the deep layer. At the sill, the dissipation rate of turbulent kinetic energy
ε
increases as measured from microstructure profilers and as inferred from a parameterization of vertical kinetic energy. Cross-sill density and microstructure transects reveal an overflow potentially hydraulically controlled and modulated by fortnightly tides. During spring to neap tides,
ε
varies from
O
(10
−9
) to
O
(10
−10
) W kg
−1
below 3500 m around the 2-layer interface. The detection of temperature overturns during tidal flow reversal, which almost fully opposes the deep up-canyon mean flow, confirms the canyon middepth enhancement of
ε
. The internal tide energy flux, particularly enhanced at the sill, compares with the lower-layer energy loss across the sill. Throughout the canyon away from the sill, near-inertial waves with downward-propagating energy dominate the internal wave field. The present study underlines the intricate pattern of the deep turbulent mixing affected by the mean flow, internal tides, and near-inertial waves.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The Bay of Bengal has a complex upper-ocean temperature and salinity structure that is, in places, characterized by strong salinity stratification and multiple inversions in temperature. Here, two ...short time series from continuously profiling floats, equipped with microstructure sensors to measure subsurface mixing, are used to highlight implications of complex hydrography on upper-ocean heat content and the evolution of sea surface temperature. Weak mixing coupled with the existence of subsurface warm layers suggest the potential for storage of heat below the surface mixed layer over relatively long time scales. On the diurnal time scale, these data demonstrate the competing effects of surface heat flux and subsurface mixing in the presence of thin salinity-stratified mixed layers with temperature inversions. Pre-existing stratification can amplify the sea surface temperature response through control on the vertical extent of heating and cooling by surface fluxes. In contrast, subsurface mixing entrains relatively cool water during the day and relatively warm water during the night, damping the response to daytime heating and nighttime cooling at the surface. These observations hint at the challenges involved in improving monsoon prediction at longer, intraseasonal time scales as models may need to resolve upper-ocean variability over short time and fine vertical scales.
Buoyancy exchange between the deep and the upper ocean, which is essential for maintaining global ocean circulation, mainly occurs through turbulent mixing. This mixing is thought to result primarily ...from instability of the oceanic internal wave field, but internal waves tend to radiate energy away from the regions in which they are generated rather than dissipate it locally as turbulence and the resulting distribution of turbulent mixing remains unknown. Another, more direct, mixing mechanism involves the generation of turbulence as strong flows pass through narrow passages in topography, but the amount of turbulence generated at such locations remains poorly quantified owing to a lack of direct measurements. Here we present observations from the crest of the Mid-Atlantic Ridge in the subtropical North Atlantic Ocean that suggest that passages in rift valleys and ridge-flank canyons provide the most energetic sites for oceanic turbulence. Our measurements show that diffusivities as large as 0.03 m2 s-1 characterize the mixing downstream of a sill in a well-stratified boundary layer, with mixing levels remaining of the order of 10-4 m2 s-1 at the base of the main thermocline. These mixing rates are significantly higher than the diffusivities of the order of 10-5 m2 s-1 that characterize much of the global thermocline and the abyssal ocean. Our estimates suggest that overflows associated with narrow passages on the Mid-Atlantic Ridge in the North Atlantic Ocean produce as much buoyancy flux as has previously been estimated for the entire Romanche fracture zone, a large strait in the Mid-Atlantic Ridge that connects the North and South Atlantic basins. This flux is equivalent to the interior mixing that occurs in the entire North Atlantic basin at the depth of the passages, suggesting that turbulence generated in narrow passages on mid-ocean ridges may be important for buoyancy flux at the global scale.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Abstract Data from three midlatitude, month-long surveys are examined for evidence of enhanced vertical mixing associated with the transition layer (TL), here defined as the strongly stratified layer ...that exists between the well mixed layer and the thermocline below. In each survey, microstructure estimates of turbulent dissipation were collected concurrently with fine-structure stratification and shear. Survey-wide averages are formed in a “TL coordinate” zTL, which is referenced around the depth of maximum stratification for each profile. Averaged profiles show characteristic TL structures such as peaks in stratification N2 and shear variance S2, which fall off steeply above zTL = 0 and more gradually below. Turbulent dissipation rates ɛ are 5–10 times larger than those found in the upper thermocline (TC). The gradient Richardson number Ri = N2/S2 becomes unstable (Ri < 0.25) within ~10 m of the TL upper boundary, suggesting that shear instability is active in the TL for zTL > 0. Ri is stable for zTL ≤ 0. Turbulent dissipation is found to scale exponentially with depth for zTL ≤ 0, but the decay scales are different for the TL and upper TC: ɛ scales well with either N2 or S2. Owing to the strong correlation between S2 and N2, existing TC scalings of the form ɛ ~ |S|p|N|q overpredict variations in ɛ. The scale dependence of shear variance is not found to significantly affect the scalings of ɛ versus N2 and S2 for zTL ≤ 0. However, the onset of unstable Ri at the top of the TL is sensitively dependent to the resolution of the shears.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
The traditional model of tidal dissipation is based on a frictional bottom boundary layer, in which the work done by bottom drag is proportional to a drag coefficient and the velocity cubed. However, ...away from shallow, coastal regions the tidal velocities are small, and the work done by the bottom boundary layer can account for only weak levels of dissipation. In the deep ocean, the energy flux carried by internal waves generated over rough topography dominates the energy transfer away from barotropic flow. A parameterization for the internal wave drag over rough topography is included as a dissipative mechanism in a model for the barotropic tides. Model results suggest that the inclusion of this dissipation mechanism improves hydro‐dynamical models of the ocean tide. It also substantially increases the amount of modeled tidal dissipation in the deep ocean, bringing dissipation levels there into agreement with recent estimates from TOPEX/Poseidon altimetry data.
Observations of turbulent dissipation above rough bathymetry in the abyssal Brazil Basin are presented. Relative to regions with smooth bathymetry, dissipation is markedly enhanced above rough ...topography of the Mid-Atlantic Ridge with levels above bathymetric slopes exceeding levels observed over crests and canyon floors.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Internal wave theory is used to examine the generation, radiation, and energy dissipation of internal tides in the deep ocean. Estimates of vertical energy flux based on a previously developed model ...are adjusted to account for the influence of finite depth, varying stratification, and two-dimensional topography. Specific estimates of energy flux are made for midocean ridge topography. Weakly nonlinear theory is applied to the wave generation at idealized topography to examine finite amplitude corrections to the linear theory. Most internal tide energy is generated at low modes associated with spatial scales from roughly 20 to 100 km. The Richardson number of the radiated internal tide typically exceeds unity for these motions, and so direct shear instability of the generated waves is not the dominant energy transfer mechanism. It also seems that wave-wave interactions are ineffective at transferring energy from the large wavelengths that dominate the energy flux. Instead, it appears that most of the internal tide energy is radiated over O(1000 km) distances. A small fraction of energy flux, less than 30%, is generated at smaller spatial scales, and this energy flux may dissipate locally. Estimates along the Mid-Atlantic Ridge in the South Atlantic suggest that the vertical energy flux of M2 internal tides is 3-5 mW m2, with 1-2 mW m2 likely contributing to local mixing. Along the East Pacific Rise, bathymetry is more smooth and tides are weaker, and estimates suggest internal tide energy flux is negligible. Radiated low modes are likely influenced by topographic scattering, though general topography scatters less than 10% of the low-mode energy to higher wavenumbers. Thus, low-mode internal tides may contribute to mixing at locations far away from their generation sites
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Hot vent fluid enters the ocean at high-temperature hydrothermal vents, also known as black smokers. Because of the large temperature difference between the vent fluid and oceanic near-bottom waters, ...the hydrothermal effluent initially rises as a buoyant plume through the water column. During its rise, the plume engulfs and mixes with background ocean water. This process, called entrainment, gradually reduces the density of the rising plume until it reaches its level of neutral buoyancy, where the plume density equals that of the background water, and it begins to spread along a surface of constant density. (For a much more detailed discussion of buoyant hydrothermal plumes, see Di Iorio et al., 2012, in this issue).
In this study, the energy flux and energy dissipation of deep ocean internal tides are examined. Properties of the internal tide from two distinct generation regions are contrasted: the Mid-Atlantic ...Ridge (MAR) and the Hawaiian Ridge. Considerable differences are noted for the baroclinic energy flux,
〈
u
′
p
′
〉
, radiated from each site. Radiation from the MAR is relatively rich in high modes, with an energy flux spectral peak at mode 5 and modes 10 and greater accounting for 40% of the total flux. In contrast, Hawaiian Ridge radiation is dominantly composed of modes 1 and 2, with modes 10 and greater accounting for less than 5% of the total flux. Depth integrated energy flux levels are
O
(
1
)
kW
m
-
1
at the MAR site, and
O
(
10
)
kW
m
-
1
at the Hawaiian Ridge. Despite these differences, observed turbulent dissipation rates at these sites are similar in magnitude and depth dependence. Decay scales, estimated as
L
ε
=
(
∫
-
H
0
u
′
p
′
d
z
)
/
(
∫
-
H
0
ρ
0
ε
d
z
)
, range from
O
(
100
)
km
to
O
(
1000
)
km
. The mean decay scale based on the MAR data is 230
km, a factor of 3 smaller than at the Hawaiian Ridge site. We demonstrate that the dissipation level scales with the energy flux available in the high modes, which is comparable at both sites, rather than the total energy flux.
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