scholarly journals Validation of Thorpe-scale-derived vertical diffusivities against microstructure measurements in the Kerguelen region

2014 ◽  
Vol 11 (23) ◽  
pp. 6927-6937 ◽  
Author(s):  
Y.-H. Park ◽  
J.-H. Lee ◽  
I. Durand ◽  
C.-S. Hong
Keyword(s):  
Bottom Topography ◽  
Water Layer ◽  
Polar Front ◽  
Surface Mixed Layer ◽  
Kerguelen Islands ◽  
Seasonal Thermocline ◽  
Thorpe Scale ◽  
Microstructure Measurements ◽  
High Degree ◽  
O 10

Abstract. The Thorpe scale is an energy-containing vertical overturning scale of large eddies associated with shear-generated turbulence. This study investigates indirect estimates of vertical diffusivities from the Thorpe scale method in the polar front region east of the Kerguelen Islands based on fine-scale density profiles gathered during the 2011 KEOPS2 (KErguelen Ocean and Plateau compared Study 2) cruise. These diffusivities are validated in comparison with diffusivities estimated from the turbulence dissipation rate directly measured via a TurboMAP (Turbulence ocean Microstructure Acquisition Profiler) microstructure profiler. The results are sensitive to the choice of the diffusivity parameterization and the overturn ratio Ro, and the optimal results have been obtained from the parameterization by Shih et al. (2005) and the Ro = 0.25 criterion, rather than the parameterization by Osborn (1980) and the Ro = 0.2 criterion originally suggested by Gargett and Garner (2008). The Thorpe-scale-derived diffusivities in the KEOPS2 region show a high degree of spatial variability, ranging from a canonical value of O(10−5) m2 s−1 in the Winter Water layer and in the area immediately north of the polar front to a high value of O(10−4) m2 s−1 in the seasonal thermocline between the surface mixed layer and the Winter Water. The latter high diffusivities are found especially over the shallow plateau southeast of the Kerguelen Islands and along the polar front that is attached to the escarpment northeast of the islands. The interaction of strong frontal flow with prominent bottom topography likely causes the observed elevated mixing rates.

scholarly journals Validation of the Thorpe scale-derived vertical diffusivities against microstructure measurements in the Kerguelen region

2014 ◽  
Vol 11 (8) ◽  
pp. 12137-12157 ◽  
Author(s):  
Y.-H. Park ◽  
J.-H. Lee ◽  
I. Durand ◽  
C.-S. Hong
Keyword(s):  
Surface Waters ◽  
Water Layer ◽  
Polar Front ◽  
Surface Mixed Layer ◽  
Kerguelen Islands ◽  
Seasonal Thermocline ◽  
Thorpe Scale ◽  
Microstructure Measurements ◽  
High Degree ◽  
O 10

Abstract. The Thorpe scale is an energy containing vertical overturning scale of large eddies associated with shear generated turbulence. We make here indirect estimates of vertical diffusivities from the Thorpe scale method in the Polar Front region east of the Kerguelen Islands based on fine scale density profiles gathered during the 2011 KEOPS2 cruise. These are validated in comparison with diffusivities estimated from the turbulence dissipation rate directly measured via a TurboMAP microprofiler. The results are sensitive to the choice of the diffusivity parameterization and the Gargett and Garner's (2008) overturn ratio Ro, with the optimal results showing an agreement within a factor of 4, on average, having been obtained from the parameterization by Shih et al. (2005) and the Ro = 0.25 criterion. The Thorpe scale-derived diffusivities in the KEOPS2 region show a high degree of spatial variability, ranging from a canonical value of O(10−5 m2 s−1) in the Winter Water layer and in the Subantarctic surface waters immediately north of the Polar Front to a high value of O(10−4 m2 s−1) in the seasonal thermocline just below the surface mixed layer. The latter values are found especially over the shallow plateau southeast of the Kerguelen Islands and in the Antarctic surface waters associated with the Polar Front attached to the escarpment northeast of the islands.

scholarly journals Use of Ra isotopes to deduce rapid transfer of sediment-derived inputs off Kerguelen

2015 ◽  
Vol 12 (5) ◽  
pp. 1415-1430 ◽  
Author(s):  
V. Sanial ◽  
P. van Beek ◽  
B. Lansard ◽  
M. Souhaut ◽  
E. Kestenare ◽  
...  
Keyword(s):  
Phytoplankton Bloom ◽  
Chemical Elements ◽  
Continuous Process ◽  
Polar Front ◽  
Global Ocean ◽  
Data Set ◽  
Radium Isotopes ◽  
Heterogeneous Distribution ◽  
Kerguelen Islands ◽  
Rapid Transfer

Abstract. The Southern Ocean is known to be the largest high-nutrient, low-chlorophyll (HNLC) region of the global ocean due to iron limitation. However, a large phytoplankton bloom develops annually downstream of the Kerguelen Islands, a bloom which is sustained partly by iron released from the sediments deposited onto the shelves. In the framework of the KEOPS-2 project, we used radium isotopes (224Ra, T1/2 = 3.66 d; 223Ra, T1/2 = 11.4 d; 228Ra, T1/2 = 5.75 yr) to provide information on the origin of iron fertilization and on the timescales of the transfer of sediment-derived inputs (including iron and other micronutrients) towards offshore waters. Significant 224Ra and 223Ra activities were found in the near vicinity of the Kerguelen Islands, in agreement with the short half-lives of these isotopes. Significant 224Ra and 223Ra activities were also detected up to 200 km downstream of the islands and more unexpectedly in offshore waters south of the polar front. These observations thus clearly indicate (i) that the sediment-derived inputs are rapidly transferred towards offshore waters (on timescales on the order of several days up to several weeks) and (ii) that the polar front is not a physical barrier for the chemical elements released from the sediments of the Kerguelen Plateau. The Ra data set suggests that iron and other micronutrients released by the shelves of the Kerguelen Islands may contribute to fueling the phytoplankton bloom downstream of the islands, despite the presence of the polar front. However, the heterogeneous distribution of the 224Ra and 223Ra activities in surface waters suggests that this supply across the front is not a continuous process but rather a process that is highly variable in space and time.

Efficiency of Mixing Forced by Unsteady Shear Flow

2009 ◽  
Vol 39 (5) ◽  
pp. 1150-1166 ◽  
Author(s):  
Ryuichiro Inoue ◽  
William D. Smyth
Keyword(s):  
Equations Of Motion ◽  
Mixing Efficiency ◽  
Coordinate Frame ◽  
Random Disturbance ◽  
Time Varying ◽  
Deceleration Phase ◽  
Ozmidov Scale ◽  
Thorpe Scale ◽  
The Mean ◽  
Microstructure Measurements

Abstract The dependence of mixing efficiency on time-varying forcing is studied by direct numerical simulation (DNS) of Kelvin–Helmholtz (KH) instability. Time-dependent forcing fields are designed to reproduce a wavelike oscillation by solving the equations of motion in a tilted coordinate frame and allowing the tilt angle to vary in time. Mixing efficiency Γc is defined as the ratio of potential energy gain to dissipation, both averaged over one forcing cycle and first examined via parameters characterizing waves: the minimum Richardson number Rimin and the normalized frequency of the forcing ω/N. The effect of Reynolds number Re0 and the initial random disturbance amplitude b are also examined. In the experiments presented, Γc varies between 0.21 and 0.36 and is controlled by the timing of two events: the emergence of KH billows and the arrival of the deceleration of the mean shear by the wavelike forcing. Here, Γc is higher than a canonical value of 0.2 when the deceleration phase of the forcing suppresses the less efficient turbulence after breakdown of KH billows. However, when Rimin and ω/N are small, KH billows start to develop before Rimin is achieved. Therefore, the forcing accelerates the mean shear and thereby sustains turbulence after the breakdown of KH billows. The canonical value is then reproduced in the DNS. Although larger values of Re0 and b intensify the development of KH billows and modify Γc, this effect is less significant when forcing fields act to sustain turbulence. The time-averaged Thorpe scale and Ozmidov scale are also used to see how mixing is modified by forcing fields and compared with past microstructure measurements. It is found that DNS also corresponds to past observations if the forcing accelerates the mean shear to sustain turbulence.

scholarly journals Fe and C co-limitation of heterotrophic bacteria in the naturally fertilized region off Kerguelen Islands

2014 ◽  
Vol 11 (11) ◽  
pp. 15733-15752 ◽  
Author(s):  
I. Obernosterer ◽  
M. Fourquez ◽  
S. Blain
Keyword(s):  
Trace Metal ◽  
Heterotrophic Bacteria ◽  
Surface Mixed Layer ◽  
Kerguelen Islands ◽  
Uptake Rates ◽  
Fe Uptake ◽  
Limiting Nutrient ◽  
Heterotrophic Metabolism ◽  
Heterotrophic Production

Abstract. It has univocally been shown that iron (Fe) is the primary limiting nutrient for phytoplankton metabolism in High Nutrient Low Chlorophyll (HNLC) oceans, yet, the question of how this trace metal affects heterotrophic microbial activity is far less understood. We investigated the role of Fe for bacterial heterotrophic production and growth at three contrasting sites in the naturally Fe-fertilized region east of Kerguelen Islands and at one site in HNLC waters during the KEOPS2 (Kerguelen Ocean and Plateau Compared Study 2) cruise in spring 2011. We performed dark incubations of natural microbial communities amended either with iron (Fe, as FeCl3), or carbon (C, as trace-metal clean glucose), or a combination of both, and followed bacterial abundance and heterotrophic production for up to 7 days. Our results show that single and combined additions of Fe and C stimulated bulk and cell-specific bacterial production at all sites, while bacterial growth was enhanced only in two out of four occasions. The extent of stimulation of bulk bacterial heterotrophic production by single Fe or C additions increased with increasing in situ bacterial Fe uptake rates in the surface mixed layer. Our results provide evidence that both Fe and C are present at limiting concentrations for bacterial heterotrophic activity, in HNLC and fertilized regions, in spring. The observation that the extent of stimulation by both elements was related to Fe-uptake rates highlights the tight interaction between the C- and Fe-cycles through bacterial heterotrophic metabolism in the Southern Ocean.

scholarly journals Use of Ra isotopes to deduce rapid transfer of sediment-derived inputs off Kerguelen

2014 ◽  
Vol 11 (9) ◽  
pp. 14023-14061 ◽  
Author(s):  
V. Sanial ◽  
P. van Beek ◽  
B. Lansard ◽  
M. Souhaut ◽  
E. Kestenare ◽  
...  
Keyword(s):  
Phytoplankton Bloom ◽  
Chemical Elements ◽  
Continuous Process ◽  
Polar Front ◽  
Global Ocean ◽  
Radium Isotopes ◽  
Heterogeneous Distribution ◽  
Kerguelen Islands ◽  
High Nutrient ◽  
Rapid Transfer

Abstract. The Southern Ocean is known as the largest High-Nutrient, Low-Chlorophyll (HNLC) region of the global ocean due to iron limitation. However, a large phytoplankton bloom develops annually downstream of the Kerguelen Islands, which is sustained partly by iron released from the sediments deposited onto the margins. In the framework of the KEOPS-2 project, we used radium isotopes (224Ra, T1/2 = 3.66 d; 223Ra, T1/2 = 11.4 d; 228Ra, T1/2 = 5.75 yr) to provide information on the origin of iron fertilization and on the timescales of the transfer of sediment-derived inputs (including iron and other micronutrients) towards offshore waters. Significant 223Ra and 223Ra activities were found in the near vicinity of the Kerguelen Islands, in agreement with the short half-lives of these isotopes. Significant 224Ra and 223Ra activities were also detected up to 200 km downstream of the islands and more unexpectedly in offshore waters south of the Polar Front. These observations thus clearly indicate (i) that the sediment-derived inputs are rapidly transferred towards offshore waters (on timescales in the order of several days up to several weeks) and (ii) that the Polar Front is not a physical barrier for the chemical elements released from the sediments of Kerguelen Plateau. The Ra dataset suggests that iron and other micronutrients released by the shallow sediments of the Kerguelen margins may contribute to fuel the phytoplankton bloom downstream of the islands, despite the presence of the Polar Front. However, the heterogeneous distribution of the 224Ra and 223Ra activities in surface waters suggests that this supply across the front is not a continuous process, but rather a process that is highly variable in space and time.

scholarly journals Transformation and mixing of North Pacific Water Mass in Sangihe-Talaud in August 2019

2021 ◽  
Vol 944 (1) ◽  
pp. 012053
Author(s):  
I Y Sani ◽  
A S Atmadipoera ◽  
A Purwandana ◽  
F Syamsudin
Keyword(s):  
Turbulent Mixing ◽  
North Pacific ◽  
Water Mass ◽  
Bottom Topography ◽  
Mass Composition ◽  
Intermediate Water ◽  
Interior Layer ◽  
Maritime Continent ◽  
Pacific Water ◽  
O 10

Abstract Along the pathway, ITF water is considered to be transformed due to strong diapycnal mixing. This study aims to describe the structure of ITF water and to estimate turbulent mixing. The number of 6 CTD casts and 9 repeated CTD “yoyo” measurements were obtained from the “Years of Maritime Continent” YMC cruise (a joint cruise between BPPT/IPB/UNUD-Univ. Tokyo/JAMSTEC) and onboard R.V. Baruna Jaya IV in August 2019. The CTD datasets are processed with SBE Data Processing and analyzed for water mass composition, as well as turbulent mixing with Thorpe method. The results showed that thermocline water of NPSW with S-max, and intermediate water of NPIW with S-min from North Pacific origin are dominant. Transformation of NPSW and NPIW along their pathway can be identified from decreasing S-max of NPSW and increasing S-min of NPIW. Estimates of ϵ and Kρ are O(10−5) m2s−2 and 10−2 m2 s−1, respectively. High mixing occur also in the interior layer with the e and the Kp O(10−6) m2s−2 and O(10−1) m2 s−1, respectively. This is related to barotropic tidal activity that interacts with the bottom topography where there are many sills, causing the formation of strong baroclinic tides.

Turbulence and Mixing in the Arctic Ocean’s Amundsen Gulf

2021 ◽  
Vol 51 (1) ◽  
pp. 169-186
Author(s):  
Benjamin Scheifele ◽  
Stephanie Waterman ◽  
Jeffrey R. Carpenter
Keyword(s):  
Turbulent Mixing ◽  
Molecular Diffusion ◽  
Mesoscale Eddy ◽  
Water Layer ◽  
Energy Dissipation Rate ◽  
Heat Fluxes ◽  
The Arctic ◽  
Diapycnal Mixing ◽  
Order Of Magnitude ◽  
Microstructure Measurements

AbstractThis study uses CTD and microstructure measurements of shear and temperature from 348 glider profiles to characterize turbulence and turbulent mixing in the southeastern Beaufort Sea, where turbulence observations are presently scarce. We find that turbulence is typically weak: the turbulent kinetic energy dissipation rate ε has a median value (with 95% confidence intervals in parentheses) of 2.3 [2.2, 2.4] × 10−11 W kg−1 and is less than 1.0 × 10−10 W kg−1 in 68% of observations. Variability in ε spans five orders of magnitude, with indications that turbulence is bottom enhanced and modulated in time by the semidiurnal tide. Stratification is strong and frequently damps turbulence, inhibiting diapycnal mixing. Buoyancy Reynolds number estimates suggest that turbulent diapycnal mixing is unlikely in 93% of observations; however, a small number of strongly turbulent mixing events are disproportionately important in determining net buoyancy fluxes. The arithmetic mean diapycnal diffusivity of density is 4.5 [2.3, 14] × 10−6 m2 s−1, three orders of magnitude larger than that expected from molecular diffusion. Vertical heat fluxes are modest at O(0.1) W m−2, of the same order of magnitude as those in the Canada Basin double-diffusive staircase, however, staircases are generally not observed. Despite significant heat present in the Pacific Water layer in the form of a warm-core mesoscale eddy and smaller, O(1) km, temperature anomalies, turbulent mixing was found to be too low to release this heat to shallower depths.

scholarly journals High variability in dissolved iron concentrations in the vicinity of the Kerguelen Islands (Southern Ocean)

2015 ◽  
Vol 12 (12) ◽  
pp. 3869-3883 ◽  
Author(s):  
F. Quéroué ◽  
G. Sarthou ◽  
H. F. Planquette ◽  
E. Bucciarelli ◽  
F. Chever ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Sediment Resuspension ◽  
Polar Front ◽  
High Variability ◽  
Eastern Coast ◽  
Water Release ◽  
Kerguelen Islands ◽  
The North ◽  
North Eastern ◽  
Lateral Advection

Abstract. Dissolved Fe (dFe) concentrations were measured in the upper 1300 m of the water column in the vicinity of the Kerguelen Islands as part of the second KErguelen Ocean Plateau compared Study (KEOPS2). Concentrations ranged from 0.06 nmol L−1 in offshore, Southern Ocean waters to 3.82 nmol L−1 within Hillsborough Bay, on the north-eastern coast of the Kerguelen Islands. Direct island runoff, glacial melting and resuspended sediments were identified as important inputs of dFe that could potentially fertilise the northern part of the plateau. A significant deep dFe enrichment was observed over the plateau with dFe concentrations increasing up to 1.30 nmol L−1 close to the seafloor, probably due to sediment resuspension and pore water release. Biological uptake was shown to induce a significant decrease in dFe concentrations between two visits (28 days apart) at a station above the plateau. Our work also considered other processes and sources, such as lateral advection of enriched seawater, remineralisation processes, and the influence of the polar front (PF) as a vector for Fe transport. Overall, heterogeneous sources of Fe over and off the Kerguelen Plateau, in addition to strong variability in Fe supply by vertical or horizontal transport, may explain the high variability in dFe concentrations observed during this study.

scholarly journals Persistent Turbulence in the Samoan Passage

2019 ◽  
Vol 49 (12) ◽  
pp. 3179-3197 ◽  
Author(s):  
Jesse M. Cusack ◽  
Gunnar Voet ◽  
Matthew H. Alford ◽  
James B. Girton ◽  
Glenn S. Carter ◽  
...  
Keyword(s):  
Time Scales ◽  
Hot Spot ◽  
Water Layer ◽  
Shear Instability ◽  
Global Ocean ◽  
Bottom Water Layer ◽  
Thorpe Scale ◽  
Long Time ◽  
Density Band ◽  
Flow Through

AbstractAbyssal waters forming the lower limb of the global overturning circulation flow through the Samoan Passage and are modified by intense mixing. Thorpe-scale-based estimates of dissipation from moored profilers deployed on top of two sills for 17 months reveal that turbulence is continuously generated in the passage. Overturns were observed in a density band in which the Richardson number was often smaller than ¼, consistent with shear instability occurring at the upper interface of the fast-flowing bottom water layer. The magnitude of dissipation was found to be stable on long time scales from weeks to months. A second array of 12 moored profilers deployed for a shorter duration but profiling at higher frequency was able to resolve variability in dissipation on time scales from days to hours. At some mooring locations, near-inertial and tidal modulation of the dissipation rate was observed. However, the modulation was not spatially coherent across the passage. The magnitude and vertical structure of dissipation from observations at one of the major sills is compared with an idealized 2D numerical simulation that includes a barotropic tidal forcing. Depth-integrated dissipation rates agree between model and observations to within a factor of 3. The tide has a negligible effect on the mean dissipation. These observations reinforce the notion that the Samoan Passage is an important mixing hot spot in the global ocean where waters are being transformed continuously.

scholarly journals Spatial and Temporal Patterns of Small-Scale Mixing in Drake Passage

2007 ◽  
Vol 37 (3) ◽  
pp. 572-592 ◽  
Author(s):  
Andrew F. Thompson ◽  
Sarah T. Gille ◽  
J. A. MacKinnon ◽  
Janet Sprintall
Keyword(s):  
Wind Stress ◽  
Mesoscale Eddy ◽  
Drake Passage ◽  
Polar Front ◽  
Small Scale ◽  
Spatial And Temporal Patterns ◽  
Near Surface ◽  
Local Maxima ◽  
Vertical Strain ◽  
O 10

Abstract Temperature and salinity profiles obtained with expendable CTD probes throughout Drake Passage between February 2002 and July 2005 are analyzed to estimate turbulent diapycnal eddy diffusivities to a depth of 1000 m. Diffusivity values are inferred from density/temperature inversions and internal wave vertical strain. Both methods reveal the same pattern of spatial variability across Drake Passage; diffusivity estimates from inversions exceed those from vertical strain by a factor of 3 over most of Drake Passage. The Polar Front (PF) separates two dynamically different regions. Strong thermohaline intrusions characterize profiles obtained north of the PF. South of the PF, stratification is determined largely by salinity, and temperature is typically unstably stratified between 100- and 600-m depth. In the upper 400 m, turbulent diapycnal diffusivities are O(10−3 m2 s−1) north of the PF but decrease to O(10−4 m2 s−1) or smaller south of the PF. Below 400 m diffusivities typically exceed 10−4 m2 s−1. Diffusivities decay weakly with depth north of the PF, whereas diffusivities increase with depth and peak near the local temperature maximum south of the PF. The meridional pattern in near-surface mixing corresponds to local maxima and minima of both wind stress and wind stress variance. Near-surface diffusivities are also found to be larger during winter months north of the PF. Wind-driven near-inertial waves, strong mesoscale eddy activity, and double-diffusive convection are suggested as possible factors contributing to observed mixing patterns.

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