Deep Ocean Mixing Research Articles

Seasonal variation of turbulent diapycnal mixing in the northwestern Pacific stirred by wind stress

Profiles of potential density obtained from CTD measurements along the northwestern Pacific section at 137°E are used to estimate both mean and seasonal variations of turbulent diapycnal mixing based on both a Thorpe‐scale method and a finescale parameterization method. In general, away from the bottom, the estimated mean value along this section is about O(10−4 m2/s), indicating much stronger mixing in the Northwestern Pacific than in the stratified mid‐latitude interior ocean. It is further found that the diffusivity can even exceed 10−4 m2/s around the rough topography. The turbulent diapycnal mixing in the upper ocean also displays a distinct seasonal variation with the amplitude decreasing with depth, bearing a statistically significant correlation with surface wind stress forcing. Our study here provides evidence for the important role of energy input from wind stress in sustaining the deep ocean mixing.

Inertia-gravity waves beyond the inertial latitude. Part 1. Inviscid singular focusing

The paper is concerned with analytical study of inertia-gravity waves in rotating density-stratified ideal fluid confined in a spherical shell. It primarily aims at clarifying the possible role of these motions in deep ocean mixing. Recently, it was found that on the ‘non-traditional’ β-plane inertia-gravity internal waves can propagate polewards beyond their inertial latitude, where the wave frequency equals the local Coriolis parameter, by turning into subinertial modes trapped in the narrowing waveguides around the local minima of buoyancy frequencyN. The behaviour of characteristics was established: wave horizontal and vertical scales decrease as the wave advances polewards and tend to zero at a latitude corresponding to an attractor of characteristics. However, the basic questions about wave evolution, its quantitative description and the possibility of its reflection from the critical latitude remain open. The present work addresses these issues by studying the linear inviscid evolution of finite bandwidth wavepackets on the ‘non-traditional’ β-plane past the inertial latitude for generic oceanic stratification. Beyond the inertial latitude, the wave field is confined in narrowing waveguides of three distinct generic types around different local minima of the buoyancy frequency. In the oceanic context, the widest is adjacent to the flat bottom, the thinnest is the upper mixed layer, and the middle one is located between the seasonal and main thermocline. We find explicit asymptotic solutions describing the wave field in the WKB approximation. As a byproduct, the conservation of wave action principle is explicitly formulated for all types of internal waves on the ‘non-traditional’ β-plane. The wave velocities and vertical shear tend to infinity and become singular at the attractor latitude or its vicinity for both monochromatic and finite bandwidth packets. We call this phenomenonsingular focusing. These WKB solutions are shown to remain valid up to singularity for the bottom and mid-ocean waveguides. The main conclusion is that even in the inviscid setting the wave evolution towards smaller and smaller horizontal and vertical scales is irreversible: there is no reflection. For situations typical of deep ocean, a simultaneous increase in wave amplitude and decrease of vertical scale causes a sharp increase of vertical shear, which may lead to wave breaking and increased mixing.

A mechanism of deep-ocean mixing due to near-inertial waves generated by flow over bottom topography

This article describes a remarkable effect of the horizontal component of the Coriolis vector, which is traditionally neglected in geophysical fluid wave theory, for the generation of inertio-gravity waves caused by flow over topography. Earlier an initial-value approach was used to calculate the evolutions of internal waves generated by atmospheric forcing in the linear Boussinesq model as done by many investigators for the study of near-inertial waves in the ocean. In this study, however, we focused on the mechanism of generating near-inertial waves by a bottom forcing with various periods. It is shown that the non-traditional Coriolis effects can significantly enhance the generation of near-inertial waves. Moreover, the intensity of generated near-inertial waves increases as the period of bottom forcing approaches to the local inertial period, suggesting the importance of tidal flows for generating near-inertial waves in deep ocean near critical latitudes. It is inferred that such near-inertial waves may contribute to a mechanism of deep-ocean mixing.

Deep-ocean mixing on the basin scale: Inference from North Atlantic transects

The measurements of thermohaline fine structure in the North Atlantic along four WOCE zonal transect and two post-WOCE sections were analyzed. The statistics of the near-bottom mixed-layer height H B as well as the Ellison L E and Thorpe L Th scales in the abyss were evaluated in relation to a roughness parameter z r, defined in terms of the rms of bathymetric irregularities of horizontal lengthscales less than ∼20–25 km. Within the near-bottom mixed layer, the variations of potential temperature did not exceed 0.005 °C. A combined probability distribution of H B was perfectly lognormal with the median 46.1 m, irrespective of the latitude. On the average, H B is about 1% of the ocean depth D for the depth range ∼1.5–5.5 km. In shallower regions over the continental slopes ( D<1.5–2 km), the ratio H B/ D is as high as 10%. The same tendency of increasing H B/ D with D was observed for D>5.5 km, in the abyss. The reduced Ellison scale ( L E− L Eo) correlated with the Thorpe scale, L E− L Eo=0.5 L Th, and L E at two post-WOCE transects showed a dependence on z r, L E− L Eo= Az r, where A=O(10 −2). Above the regions of enhanced z r, larger values of Ellison and Thorpe scales were found at larger distances ζ from the ocean floor. The thermohaline fine structure and mixing in abyssal waters are influenced by the topography up to hundreds of meters above the bottom, the latter increasing with z r. The correlations between ( L E− L Eo), L Th, and z r as well as H B and z r can be used to evaluate turbulent lengthscales l tr in the abyssal boundary layer. An example of the calculation of l tr is given for a post-WOCE transatlantic equatorial transect.

Internal gravity waves: parametric instability and deep ocean mixing

The Boussinesq approximation provides a convenient framework to describe the dynamics of stably-stratified fluids. A fundamental motion in these fluids consists of internal gravity waves, whatever the strength of the stratification. These waves may be unstable through parametric instability, which results in turbulence and mixing. After a brief review of the main properties of internal gravity waves, we show how the parametric instability of a monochromatic internal gravity wave organizes itself in space and time, using energetics arguments and a simple kinematic model. We provide an example, in the deep ocean, where such instability is likely to occur, as estimates of mixing from in situ measurements suggest. We eventually discuss the fundamental role of internal gravity wave mixing in the maintenance of the abyssal thermal stratification. To cite this article: C. Staquet, C. R. Mecanique 335 (2007).

Progress in the determination of 227Ac in sea water

227Ac is a naturally occurring radioisotope with a unique combination of properties that make it suitable for the determination of deep ocean mixing and upwelling rates. Here, we present a method for the determination of 227Ac in sea water on sample sizes of 20–80 L. The measurement is based on co-precipitation of 227 Ac with MnO 2, followed by chemical isolation of actinium in the presence of an artificial Ac isotope. Actinium is then electrodeposited onto silver discs. In two alpha-spectrometric counting periods, first the artificial 225Ac isotope is counted, then after > 100 days five daughters of 227Ac. The first counting period gives a total yield for the procedure, integrating chemical recovery and detector efficiency. The total yield was found here to be on average 15 ± 5%, the chemical yield on average about 50%. The counting of five decay products of 227Ac in the second period makes the method particularly sensitive. Using appropriate decay corrections, the initial 227Ac activity can be determined to better than 10% relative error for concentrations < 10,000 atoms/L. We compare data acquired by the new method to a data set from in-situ pumps, from a parallel sampling campaign in the Eastern Weddell Gyre, and we can show excellent agreement. Repeated determinations of 227Ac in a uranium reference material (UREM-11) demonstrate the accuracy of the method.

Shear at the critical diurnal latitude

[1] Around latitudes ∣ϕ∣ ≈ 30° where diurnal D1 equals the local inertial frequency f = 2Ωsinϕ, Ω denoting the Earth's rotational vector, several mechanisms can enhance shear at f due to a reduction in vertical scales. This would imply locally enhanced deep-ocean mixing. Here, recent 1.5 years of acoustic Doppler current profiler (ADCP) observations from the Canary Basin demonstrate largest kinetic energy at semidiurnal tides (D2), but a complete absence of D2-shear. Instead, shear is peaking at subinertial 0.97 ± 0.01f and terdiurnal 3f(≈D2 + f ≈ D3 here), and vertical scales Δz(f) < 0.1Δz(D2). However, the f-band is broader than deterministic tidal frequencies and the smallest vertical scales, organizing shear in thin layers, are found at the lower inertio-gravity wave limit, which equals 0.97f for the weakest stratification observed (N = 6f, using Δz = 10 m). Hence, besides possibly subharmonic resonance, other mechanisms must be involved in enhancing f-shear, including non-linear harmonic interactions and wave trapping at the critical latitude's poleward shift.

Hotspots of deep ocean mixing on the Oregon continental slope

Two deep ocean hotspots of turbulent mixing were found over the Oregon continental slope. Thorpe‐scale analyses indicate time‐averaged turbulent energy dissipation rates of ε &gt; 10−7 W/kg and eddy diffusivities of Kρ ∼ 10−2 m2/s at both hotspots. However, the structure of turbulence and its generation mechanism at each site appear to be different. At the 2200‐m isobath, sustained &gt;100‐m high turbulent overturns occur in stratified fluid several hundred meters above the bottom. Turbulence shows a clear 12.4‐h periodicity proposed to be driven by flow over a nearby 100‐m tall ridge. At the 1300‐m isobath, tidally‐modulated turbulence of similar intensity is confined within a stratified bottom boundary layer. Along‐slope topographic roughness at scales not resolved in global bathymetric data sets appears to be responsible for the bulk of the turbulence observed. Such topography is common to most continental slopes, providing a mechanism for turbulence generation in regions where barotropic tidal currents are nominally along‐isobath.

Iron (II) distribution and oxidation kinetics in hydrothermal plumes at the Kairei and Edmond vent sites, Indian Ocean

Deep-sea hydrothermal activity cycles the entire volume of the global ocean through deep-sea hydrothermal plumes at least every 4–8 × 10 3 a, a rapid timescale that is comparable to global deep-ocean mixing. An important process within hydrothermal plumes is the oxidation of dissolved iron discharged with vent fluids, leading to the co-precipitation of many other vent-sourced metals, thus, modifying gross hydrothermal fluxes to the deep ocean, and acting as a net sink in the ocean chemical budgets of certain key tracers. Here, we report direct measurements of dissolved Fe(II) oxidation rates from two recently discovered Indian Ocean vent sites that are intermediate between Pacific and Atlantic values, thus demonstrating that the rate of iron oxidation and formation of scavenging Fe(III) solid phases, in hydrothermal plumes varies systematically along the global deep-ocean “conveyor.” The average pseudo first-order rate constant for the oxidation reaction is 0.303 h −1 and the corresponding half-life 2.31 h. The rate plots show some curvature, however, indicating additional processes, possibly interactions with organic matter, may also influence the net oxidation/removal rate. Different rates of oxidation of iron in hydrothermal plumes in different ocean basins have important implications for the dispersion of produced oxide phases, which in turn may impact on uptake of other metals from seawater and deposition into underlying ridge-crest and ridge flank sediments.

Speculations on a schematic theory of the Younger Dryas

The possible implications of a change in deep ocean mixing generated by the abrupt coverage of continental shelves during deglaciation is explored schematically. A simple Cartesian continental shelf/deep ocean tidal model is used to mimic the behavior of tidal dissipation at a variety of frequencies relative to a basin-near-resonance. The actual estimated sea level curve is then used to calculate a representative deep water tidal dissipation through time, which is then translated by means of a one-dimensional loop flow to explore the implications of a change in deep ocean mixing. It is possible to pick parameters for the loop model, which is analogous to the Stommel two-state box model, such that interesting transitions occur when diffusion alone is changed, including oscillatory responses. To the extent that the box model is believed to mimic the ocean circulation, it can produce shifts analogous to the re-glaciation and subsequent de-glaciation characterizing the Younger Dryas interval. That changing topography and winds would likely also have a profound influence on mixing by mesoscale disturbances is noted but not pursued.

EPIC2001 and the Coupled Ocean–Atmosphere System of the Tropical East Pacific

Abstract Coupled global ocean–atmosphere models currently do a poor job of predicting conditions in the tropical east Pacific, and have a particularly hard time reproducing the annual cycle in this region. This poor performance is probably due to the sensitivity of the east Pacific to the inadequate representation of certain physical processes in the modeled ocean and atmosphere. The representations of deep cumulus convection, ocean mixing, and stratus region energetics are known to be problematic in such models. The U.S. Climate Variability and Predictability (CLIVAR) program sponsored the field experiment East Pacific Investigation of Climate Processes in the Coupled Ocean–Atmosphere System 2001 (EPIC2001), which has the goal of providing the observational basis needed to improve the representation of certain key physical processes in models. In addition to physical processes, EPIC2001 research is directed toward a better understanding and simulation of the effects of short-term variability in the east ...

Climate Simulations for 1951–2050 with a Coupled Atmosphere–Ocean Model

The authors simulate climate change for 1951‐2050 using the GISS SI2000 atmospheric model coupled to HYCOM, a quasi-isopycnal ocean model (‘‘ocean E’’), and contrast the results with those obtained using the same atmosphere coupled to a passive Q-flux ocean model (‘‘ocean B’’) and the same atmosphere driven by observed SST (‘‘ocean A’’). All of the models give reasonable agreement with observed global temperature change during 1951‐2000, but the quasi-isopycnal ocean E mixes heat more deeply and hence sequesters heat more effectively on the century timescale. Global surface warming in the next 50 yr is only 0.38‐0.48C with this ocean in simulations driven by an ‘‘alternative scenario’’ climate forcing (1.1 W m 22 in the next 50 yr), only half as much as with ocean B. From the different models the authors estimate that the earth was out of radiation balance by about 0.18 W m22 in 1951 and is now out of balance by about 0.75 W m22. This energy imbalance, or residual climate forcing, a consequence of deep ocean mixing of heat anomalies and the history of climate forcings, is a crucial measure of the state of the climate system that should be precisely monitored with full-ocean temperature measurements.

Aspects of Deep Ocean Mixing

The turbulent motions responsible for ocean mixing occur on scales much smaller than those resolved in numerical simulations of oceanic flows. Great progress has been made in understanding the sources of energy for mixing, the mechanisms, and the rates. On the other hand, we still do not have adequate answers to first order questions such as the extent to which the thermohaline circulation of the ocean, and hence the earth's climate, is sensitive to the present mixing rates in the ocean interior. Internal waves, generated by either wind or flow over topography, appear to be the principle cause of mixing. Mean and eddy flows over topography generate internal lee waves, while tidal flows over topography generate internal tides. The relative importance of these different internal wave sources is unknown. There are also great uncertainties about the spatial and temporal variation of mixing. Calculations of internal tide generation are becoming increasingly robust, but we do not know enough about the subsequent behavior of internal tides and their eventual breakdown into turbulence. It does seem, however, that most internal tide energy flux is radiated away from generation sites as low modes that propagate over basin scales. The mechanisms of wave-wave interaction and topographic scattering both act to transfer wave energy from low modes to smaller dissipative scales.

Deep ocean mixing

Deep ocean mixing

An efficient and accurate representation of complex oceanic and biospheric models of anthropogenic carbon uptake

Establishing the link between atmospheric CO 2 concentration and anthropogenic carbon emissions requires the development of complex carbon cycle models of the primary sinks, the ocean and terrestrial biosphere. Once such models have been developed, the potential exists to use pulse response functions to characterize their behaviour. However, the application of response functions based on a pulse increase in atmospheric CO 2 to characterize oceanic uptake, the conventional technique, does not yield a very accurate result due to nonlinearities in the aquatic carbon chemistry. Here, we propose the use of an ocean mixed-layer pulse response function that characterizes the surface to deep ocean mixing in combination with a separate equation describing air-sea exchange. The use of a mixed-layer pulse response function avoids the problem arising from the nonlinearities of the carbon chemistry and gives therefore more accurate results. The response function is also valid for tracers other than carbon. We found that tracer uptake of the HILDA and Box-Diffusion model can be represented exactly by the new method. For the Princeton 3-D model, we find that the agreement between the complete model and its pulse substitute is better than 4% for the cumulative uptake of anthropogenic carbon for the period 1765–2300 applying the IPCC stabilization scenarios S450 and S750 and better than 2% for the simulated inventory and surface concentration of bomb-produced radiocarbon. By contrast, the use of atmospheric response functions gives deviations up to 73% for the cumulative CO 2 uptake as calculated with the Princeton 3-D model. We introduce the use of a decay response function for calculating the potential carbon storage on land as a substitute for terrestrial biosphere models that describe the overturning of assimilated carbon. This, in combination with an equation describing the net primary productivity permits us to exactly characterize simple biosphere models. As the time scales of biospheric overturning are one key aspect to determine the amount of anthropogenic carbon which might be sequestered by the biosphere, we suggest that decay response functions should be used as a simple and standardized measure to compare different models and to improve understanding of their behaviour. We provide analytical formulations for mixed-layer and terrestrial biosphere decay pulse response functions which permit us to easily build a substitute for the ‘Bern’ carbon cycle model (HILDA). Furthermore, mixed-layer response functions for the Box-Diffusion, a 2-D model, and the Princeton 3-D model are given. DOI: 10.1034/j.1600-0889.1996.t01-2-00006.x

The deep ocean disposal of carbon dioxide

Once released to the environment, carbon dioxide equilibrates within the ocean-atmosphere pool. Because the mixing of surface seawater is relatively slow, release to the atmosphere causes a transient peak in atmospheric carbon dioxide concentration. If the release could be transferred to the ocean, this transient might be avoided. In addition, interaction with the large carbon pool contained within the ocean sediment would be accelerated. The resultant increase in ocean alkalinity would tend to reduce equilibrium P CO2 values. The dynamics of the processes involved are complex and not yet understood in detail. This paper reviews deep ocean carbonate chemistry and mixing, and estimates their relative importance for the prediction of atmospheric carbon dioxide concentrations resulting from deep ocean release. Predictions based on mathematical modelling suggest that disposal into the surface ocean (< 1 km depth) would permit equilibration with the atmosphere within a few years to decades and would therefore offer little advantage. Disposal into ocean basins greater than 3km in depth would delay equilibration with the atmosphere for several hundred years, eliminating the atmospheric concentration transient. Resultant interaction with calcite-rich sediments could probably reduce the long-term ( > 2000 year) atmospheric enrichment by a significant amount (∼50%)

Ocean ventilation during the last 12,000 years: Hypothesis of counterpoint deep water production

Available radiocarbon data on selected planktonic foraminifera species suggest that differential dissolution reduces the 14 C C ratio in planktonic tests, removing 14C-rich specimens from assemblages of surface-dwelling species. The effect is analogous to that observed for stable isotope composition in planktonic foraminifera. Other disturbing factors apparently affect benthic radiocarbon data, as well. Thus, radiocarbon stratigraphies are difficult to interpret in terms of history of deep-water ages, that is, deep-ocean mixing. The best data at hand (South China Sea ∗ ∗ See Andree et al., 1986. ), after corrections for differential dissolution and benthic (vital?) deviations, suggest a high variability of deep-water mass ages in the late glacial period, which is explained in terms of vertical motion of deep-water mass boundaries across the recording sites. The fluctuations are ascribed to sporadic increases in deep-water production in the North Pacific during cold periods and to the presence of a glacial bottom-water mass which resisted mixing during deglaciation.

Entrainment and vertical transport of deep-ocean water by buoyant hydrothermal plumes

Submarine hydrothermal vents produce effluent plumes in the water column which can be easily detected with tracers such as 3He, manganese and methane1–3. Comparison of the tracer concentrations and temperature anomalies in such plumes with direct measurements of the pure vent waters shows that the plumes are typically mixtures of 1 part vent water with 104 parts ambient sea water1. However, the extent to which hydrothermal plumes entrain and transport sea water and contribute to deep-ocean mixing is not known. We present here hydrographie data collected in the vicinity of an active hydrothermal field on the Endeavour segment of the Juan de Fuca Ridge where a well-defined effluent layer resides in the water column ∼200 m above the depth of the vent field. The temperature and salinity signature of this layer indicates that a small volume of hot vent water can be very efficient at entraining ambient sea water. Directly above the Endeavour vent field, the effluent layer is composed of 0.01% vent water, 30% ambient water normally found at that depth, and ∼70% entrained water which has been transported from deeper in the water column.

Interior Reflections of a Periodically Forced Equatorial Kelvin Wave

Abstract A fully three-dimensional, wind-forced equatorial model is used to study the effects of the strong near- surface equatorial pycnocline on energy transmission into the deep ocean. The equatorial Kelvin waves forced by a patch of zonal wind oscillating at the annual period are isolated from the complete response, and their energy transmission into the deep ocean is investigated as a function of forcing geometry, pycnocline structure, and the amplitude of deep-ocean mixing. Solutions form well-defined beams of energy that propagate through realistic pycnoclines with surprisingly little reflection. Vertical mixing damps the beams in the direction of their propagation and stretches their longitudinal extent. For sufficiently strong mixing the solutions low their beamline character and appear as surface-trapped signals. This result may help to resolve the differences between the solutions found in previous investigations.