Inhibited vertical mixing and seasonal persistence of a thin cyanobacterial layer in a stratified lake

AbstractHarmful blooms of the filamentous cyanobacteria Planktothrix rubescens have become common in many lakes as they have recovered from eutrophication over the last decades. These cyanobacteria, capable of regulating their vertical position, often flourish at the thermocline to form a deep chlorophyll maximum. In Lake Zurich (Switzerland), they accumulate during stratified season (May–October) as a persistent metalimnetic thin layer (~2 m wide). This study investigated the role of turbulent mixing in springtime layer formation, its persistence over the summer, and its breakdown in autumn. We characterised seasonal variation of turbulence in Lake Zurich with four surveys conducted in April, July and October of 2018 and September of 2019. Surveys included microstructure profiles and high-resolution mooring measurements. In July and October, the thin layer occurred within a strong thermocline ($$N \gtrsim 0.05$$ N ≳ 0.05 s$$^{-1}$$ - 1 ) and withstood significant turbulence, observed as turbulent kinetic energy dissipation rates ($$\varepsilon \approx 10^{-8}$$ ε ≈ 10 - 8 W kg$$^{-1}$$ - 1 ). Vertical turbulent overturns –monitored by the Thorpe scale– went mostly undetected and on average fell below those estimated by the Ozmidov scale ($$L_O \approx 1$$ L O ≈ 1 cm). Consistently, vertical diffusivity was close to molecular values, indicating negligible turbulent fluxes. This reduced metalimnetic mixing explains the persistence of the thin layer, which disappears with the deepening of the surface mixed layer in autumn. Bi-weekly temperature profiles in 2018 and a nighttime microstructure sampling in September 2019 showed that nighttime convection serves as the main mechanism driving the breakdown of the cyanobacterial layer in autumn. These results highlight the importance of light winds and convective mixing in the seasonal cycling of P. rubescens communities within a strongly stratified medium-sized lake.

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Quasigeostrophic Diagnosis of Mixed Layer Dynamics Embedded in a Mesoscale Turbulent Field

Journal of Physical Oceanography ◽

10.1175/jpo-d-14-0178.1 ◽

2016 ◽

Vol 46 (1) ◽

pp. 275-287 ◽

Cited By ~ 6

Author(s):

Cédric P. Chavanne ◽

Patrice Klein

Keyword(s):

Surface Layer ◽

High Resolution ◽

Mixed Layer ◽

Finite Thickness ◽

Vertical Mixing ◽

Three Dimensional ◽

Upper Ocean ◽

Surface Mixed Layer ◽

Deep Interior ◽

Quasigeostrophic Model

AbstractA quasigeostrophic model is developed to diagnose the three-dimensional circulation, including the vertical velocity, in the upper ocean from high-resolution observations of sea surface height and buoyancy. The formulation for the adiabatic component departs from the classical surface quasigeostrophic framework considered before since it takes into account the stratification within the surface mixed layer that is usually much weaker than that in the ocean interior. To achieve this, the model approximates the ocean with two constant stratification layers: a finite-thickness surface layer (or the mixed layer) and an infinitely deep interior layer. It is shown that the leading-order adiabatic circulation is entirely determined if both the surface streamfunction and buoyancy anomalies are considered. The surface layer further includes a diabatic dynamical contribution. Parameterization of diabatic vertical velocities is based on their restoring impacts of the thermal wind balance that is perturbed by turbulent vertical mixing of momentum and buoyancy. The model skill in reproducing the three-dimensional circulation in the upper ocean from surface data is checked against the output of a high-resolution primitive equation numerical simulation.

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

Biogeosciences ◽

10.5194/bg-11-6927-2014 ◽

2014 ◽

Vol 11 (23) ◽

pp. 6927-6937 ◽

Cited By ~ 15

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.

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Impact of episodic vertical fluxes on sea surface pCO 2

Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences ◽

10.1098/rsta.2010.0340 ◽

2011 ◽

Vol 369 (1943) ◽

pp. 2009-2025 ◽

Cited By ~ 20

Author(s):

A. Mahadevan ◽

A. Tagliabue ◽

L. Bopp ◽

A. Lenton ◽

L. Mémery ◽

...

Keyword(s):

Dissolved Inorganic Carbon ◽

Vertical Mixing ◽

Analytical Framework ◽

Relative Magnitude ◽

Sea Surface ◽

Surface Mixed Layer ◽

Episodic Events ◽

Subsurface Waters ◽

The Individual ◽

Episodic Mixing

Episodic events like hurricanes, storms and frontal- and eddy-driven upwelling can alter the partial pressure of CO 2 (pCO 2 ) at the sea surface by entraining subsurface waters into the surface mixed layer (ML) of the ocean. Since pCO 2 is a function of total dissolved inorganic carbon (DIC), temperature ( T ), salinity and alkalinity, it responds to the combined impacts of physical, chemical and biological changes. Here, we present an analytical framework for assessing the relative magnitude and sign in the short-term perturbation of surface pCO 2 arising from vertical mixing events. Using global, monthly, climatological datasets, we assess the individual, as well as integrated, contribution of various properties to surface pCO 2 in response to episodic mixing. The response depends on the relative vertical gradients of properties beneath the ML. Many areas of the ocean exhibit very little sensitivity to mixing owing to the compensatory effects of DIC and T on pCO 2 , whereas others, such as the eastern upwelling margins, have the potential to generate large positive/negative anomalies in surface pCO 2 . The response varies seasonally and spatially and becomes more intense in subtropical and subpolar regions during summer. Regions showing a greater pCO 2 response to vertical mixing are likely to exhibit higher spatial variability in surface pCO 2 on time scales of days.

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Hydrography, Circulation, and Mixing at the Calypso Deep (the Deepest Mediterranean Trough) during 2006–09

Journal of Physical Oceanography ◽

10.1175/jpo-d-15-0198.1 ◽

2016 ◽

Vol 46 (4) ◽

pp. 1255-1276 ◽

Cited By ~ 4

Author(s):

H. Kontoyiannis ◽

V. Lykousis ◽

V. Papadopoulos ◽

S. Stavrakakis ◽

E. G. Anassontzis ◽

...

Keyword(s):

Bottom Topography ◽

Vertical Mixing ◽

Average Error ◽

Surrounding Water ◽

Local Maxima ◽

Vertical Diffusivity ◽

Station Grid ◽

Vertical Scale ◽

Bottom Depth ◽

Lens Center

AbstractThe mass and flow fields from June 2006 to May 2009 in the Calypso Deep (bottom depth ~5.2 km) are investigated using eddy-resolving surface-to-bottom hydrography (station grid spacing ~0.2°) and two tall moorings yielding current-meter records at depths from 700 m to near bottom. A salty warm lens (excess core salinity and temperature are ~0.01 and 0.025°C relative to the surrounding water) of Cretan Deep Water with a core at ~3000 m and a horizontal (vertical) scale of ~50 km (1.5 km) is identified in June 2006 to be locked over the trough. The lens coincides with local maxima in dissolved oxygen. In October 2006 the salinity content of the lens and of all deeper layers is increased; the oxygen maxima are shifted to the bottom layers, indicating an episodic intrusion of higher-density ventilated Adriatic water. The circulation changes from anticyclonic at all depths in June 2006 to cyclonic below ~2.5 km in October 2006, whereas after January 2007 it is cyclonic at all instrumented depths. The measured currents are weak (mean speeds < 5 cm s−1) and persistent in direction, being mostly along the bottom topography at all current-meter depths. After October 2006, the lens erodes due to salt/heat loss caused predominantly by lateral (intrusive) mixing, which works from the outside toward the lens center. The horizontal diffusivity is on the order of ~10 m2 s−1, near the center of the lens, and ~102 to 103 m2 s−1, at its periphery, with an average error ~15 times the diffusivity value. In the deepest part of the trough and in periods of predominance of vertical mixing the vertical diffusivity at 4400 m is ~(4 ± 3) × 10−3 m2 s−1.

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Upper-Ocean Response to Hurricane Frances (2004) Observed by Profiling EM-APEX Floats*

Journal of Physical Oceanography ◽

10.1175/2010jpo4313.1 ◽

2011 ◽

Vol 41 (6) ◽

pp. 1041-1056 ◽

Cited By ~ 108

Author(s):

Thomas B. Sanford ◽

James F. Price ◽

James B. Girton

Keyword(s):

Drag Coefficient ◽

Vertical Mixing ◽

Ocean Model ◽

Upper Ocean ◽

Surface Mixed Layer ◽

Wind Speeds ◽

Sst Cooling ◽

Ocean Response ◽

Upper Ocean Response ◽

The Right

Abstract Three autonomous profiling Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats were air deployed one day in advance of the passage of Hurricane Frances (2004) as part of the Coupled Boundary Layer Air–Sea Transfer (CBLAST)-High field experiment. The floats were deliberately deployed at locations on the hurricane track, 55 km to the right of the track, and 110 km to the right of the track. These floats provided profile measurements between 30 and 200 m of in situ temperature, salinity, and horizontal velocity every half hour during the hurricane passage and for several weeks afterward. Some aspects of the observed response were similar at the three locations—the dominance of near-inertial horizontal currents and the phase of these currents—whereas other aspects were different. The largest-amplitude inertial currents were observed at the 55-km site, where SST cooled the most, by about 2.2°C, as the surface mixed layer deepened by about 80 m. Based on the time–depth evolution of the Richardson number and comparisons with a numerical ocean model, it is concluded that SST cooled primarily because of shear-induced vertical mixing that served to bring deeper, cooler water into the surface layer. Surface gravity waves, estimated from the observed high-frequency velocity, reached an estimated 12-m significant wave height at the 55-km site. Along the track, there was lesser amplitude inertial motion and SST cooling, only about 1.2°C, though there was greater upwelling, about 25-m amplitude, and inertial pumping, also about 25-m amplitude. Previously reported numerical simulations of the upper-ocean response are in reasonable agreement with these EM-APEX observations provided that a high wind speed–saturated drag coefficient is used to estimate the wind stress. A direct inference of the drag coefficient CD is drawn from the momentum budget. For wind speeds of 32–47 m s−1, CD ~ 1.4 × 10−3.

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Efficiency of Mixing Forced by Unsteady Shear Flow

Journal of Physical Oceanography ◽

10.1175/2008jpo3927.1 ◽

2009 ◽

Vol 39 (5) ◽

pp. 1150-1166 ◽

Cited By ~ 13

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.

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

Biogeosciences Discussions ◽

10.5194/bgd-11-12137-2014 ◽

2014 ◽

Vol 11 (8) ◽

pp. 12137-12157 ◽

Cited By ~ 6

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.

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The stratification maxima of the seasonally varying Surface layer in the Arctic Ocean’s Beaufort Gyre

10.1575/1912/27522 ◽

2021 ◽

Author(s):

◽

Peter A. Roemer

Keyword(s):

Mixed Layer ◽

Vertical Mixing ◽

Heat Energy ◽

The Arctic ◽

Small Scale ◽

Geographic Variability ◽

Vertical Diffusivity ◽

Beaufort Gyre ◽

The Arctic Ocean ◽

Initial Evolution

The Beaufort Gyre region of the Arctic Ocean is strongly stratified at the base of the wintertime mixed layer, which impedes the vertical transport of heat, energy, and other tracers. Ice-Tethered Profiler observations during 2004-2018 were used to characterize and investigate the seasonal and interannual variability of the strength, depth, density, and thickness of this highly stratified layer at the base of the mixed layer. This includes investigating the remnant stratification maximum, which formed when the summer mixed layer shoaled. Seasonally, the stratification maximum was never in a steady state. It was largest in October (4.8 × 10−3 rad2/sec2) and decreased during all winter months (to 2.3 × 10−3rad2/sec2 in June), indicating that surface forcing and interior vertical mixing were never in equilibrium during the year. Interannually, the period from 2011-2018 had a higher stratification maximum than then the period from 2005-2010 regardless of the season. The remnant stratification maximum was consistently weaker than the winter stratification maximum from which it formed. The initial evolution of the remnant stratification maximum is used to estimate an effective vertical diffusivity of order 10−6m2/s. No significant geographic variability was found, in part due to high temporal and small scale variability of the stratification maximum layer. Implications for heat transport through to the sea ice cover are discussed.

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Subsurface iron accumulation and rapid aluminum removal in the Mediterranean following African dust deposition

Biogeosciences ◽

10.5194/bg-18-6435-2021 ◽

2021 ◽

Vol 18 (24) ◽

pp. 6435-6453

Author(s):

Matthieu Bressac ◽

Thibaut Wagener ◽

Nathalie Leblond ◽

Antonio Tovar-Sánchez ◽

Céline Ridame ◽

...

Keyword(s):

Mediterranean Sea ◽

Comprehensive Evaluation ◽

Vertical Mixing ◽

Western Mediterranean ◽

Surface Mixed Layer ◽

Dust Deposition ◽

Near Surface ◽

Surface Ocean ◽

Basin Scale ◽

The Mediterranean

Abstract. Mineral dust deposition is an important supply mechanism for trace elements in the low-latitude ocean. Our understanding of the controls of such inputs has been mostly built on laboratory and surface ocean studies. The lack of direct observations and the tendency to focus on near-surface waters prevent a comprehensive evaluation of the role of dust in oceanic biogeochemical cycles. In the frame of the PEACETIME project (ProcEss studies at the Air-sEa Interface after dust deposition in the MEditerranean sea), the responses of the aluminum (Al) and iron (Fe) cycles to two dust wet deposition events over the central and western Mediterranean Sea were investigated at a timescale of hours to days using a comprehensive dataset gathering dissolved and suspended particulate concentrations, along with sinking fluxes. Dissolved Al (dAl) removal was dominant over dAl released from dust. The Fe / Al ratio of suspended and sinking particles revealed that biogenic particles, and in particular diatoms, were key in accumulating and exporting Al relative to Fe. By combining these observations with published Al / Si ratios of diatoms, we show that adsorption onto biogenic particles, rather than active uptake, represents the main sink for dAl in Mediterranean waters. In contrast, systematic dissolved Fe (dFe) accumulation occurred in subsurface waters (∼ 100–1000 m), while dFe input from dust was only transient in the surface mixed layer. The rapid transfer of dust to depth, the Fe-binding ligand pool in excess to dFe in subsurface (while nearly saturated in surface), and low scavenging rates in this particle-poor depth horizon are all important drivers of this subsurface dFe enrichment. At the annual scale, this previously overlooked mechanism may represent an additional pathway of dFe supply for the surface ocean through diapycnal diffusion and vertical mixing. However, low subsurface dFe concentrations observed at the basin scale (

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Baroclinic Instability with a Simple Model for Vertical Mixing

Journal of Physical Oceanography ◽

10.1175/jpo-d-18-0270.1 ◽

2019 ◽

Vol 49 (12) ◽

pp. 3273-3300 ◽

Cited By ~ 2

Author(s):

Matthew N. Crowe ◽

John R. Taylor

Keyword(s):

Growth Rate ◽

Stability Analysis ◽

Simple Model ◽

Numerical Simulations ◽

Mixed Layer ◽

Numerical Models ◽

Baroclinic Instability ◽

Vertical Mixing ◽

Small Scale ◽

Surface Mixed Layer

AbstractHere, we examine baroclinic instability in the presence of vertical mixing in an idealized setting. Specifically, we use a simple model for vertical mixing of momentum and buoyancy and expand the buoyancy and vorticity in a series for small Rossby numbers. A flow in subinertial mixed layer (SML) balance (see the study by Young in 1994) exhibits a normal mode linear instability, which is studied here using linear stability analysis and numerical simulations. The most unstable modes grow by converting potential energy associated with the basic state into kinetic energy of the growing perturbations. However, unlike the inviscid Eady problem, the dominant energy balance is between the buoyancy flux and the energy dissipated by vertical mixing. Vertical mixing reduces the growth rate and changes the orientation of the most unstable modes with respect to the front. By comparing with numerical simulations, we find that the predicted scale of the most unstable mode matches the simulations for small Rossby numbers while the growth rate and orientation agree for a broader range of parameters. A stability analysis of a basic state in SML balance using the inviscid QG equations shows that the angle of the unstable modes is controlled by the orientation of the SML flow, while stratification associated with an advection/diffusion balance controls the size of growing perturbations for small Ekman numbers and/or large Rossby numbers. These results imply that baroclinic instability can be inhibited by small-scale turbulence when the Ekman number is sufficiently large and might explain the lack of submesoscale eddies in observations and numerical models of the ocean surface mixed layer during summer.

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