Ocean Mixing: Drivers, Mechanisms and Impacts

Ocean Mixing
Copyright
Contents
List of contributors
Editors' biographies
Acknowledgements
1 Ocean mixing: oceanography at a watershed
References
2 The role of ocean mixing in the climate system
2.1 Introduction
2.2 The role of ocean mixing in shaping the contemporary climate mean state
2.2.1 Meridional overturning circulation and heat transport
2.2.2 Southern Ocean
2.2.3 Mixing in exchanges between marginal seas and the open ocean
2.2.4 Mixing and marine ecosystems
2.3 Ocean mixing and transient climate change
2.3.1 Ocean anthropogenic heat and carbon uptake
2.3.2 Contemporary and future sea level rise
2.3.3 Changes in nutrient fluxes
2.3.4 Changes in ocean mixing sources
2.3.4.1 Tides
2.3.4.2 Lee waves
2.4 Ocean mixing in past climate states
2.4.1 The Early Pliocene
2.4.2 The Last Glacial Maximum (LGM)
2.5 Summary and conclusion
References
3 The role of mixing in the large-scale ocean circulation
3.1 Introduction
3.2 Flavours of mixing
3.3 Non-dissipative theories of ocean circulation
3.3.1 Ekman pumping
3.3.2 Momentum redistribution by geostrophic turbulence
3.4 How can mixing shape circulation?
3.4.1 By altering surface wind and buoyancy forcing
3.4.2 By altering density gradients
3.4.3 By producing and consuming water masses
3.5 Where is mixing most effective at shaping circulation?
3.5.1 Isotropic mixing, from top to bottom
3.5.2 Mesoscale stirring, from top to bottom
3.6 Some impacts on basin-scale overturning circulation
3.6.1 Abyssal overturning cell
3.6.2 North Atlantic Deep Water circulation
3.6.3 Southern Ocean upwelling: adiabatic or diabatic?
3.6.4 The return flow to the North Atlantic
3.6.5 Shallow hemispheric cells
3.7 Some impacts on basin-scale horizontal circulation
3.7.1 Upper-ocean gyres
3.7.2 The Stommel and Arons circulation
3.7.3 The Antarctic Circumpolar Current
3.8 Conclusions
References
4 Ocean near-surface layers
4.1 Introduction
4.2 Mixing layers and mixed layers in theory
4.2.1 Mixing and surface layers: Monin–Obukhov scaling
4.2.1.1 Monin–Obukhov scaling
4.2.1.2 Consistent drag laws
4.2.2 Near-surface distinctions from M–O theory and each other
4.2.2.1 Salinity and humidity
4.2.2.2 Free convection layers and velocities
4.2.2.3 Penetrating radiation
4.2.2.4 Budget equations
4.2.2.5 Time-varying forcing
4.2.2.6 Lateral variation
4.2.2.7 Waves
4.2.2.8 Waves and momentum
4.2.2.9 Sea ice
4.2.2.10 Bubbles, spray, whitecaps, & foam
4.2.2.11 Shallow water
4.2.3 Mixed layers: boundary layer memory
4.2.3.1 Entrainment
4.2.3.2 Restratification
4.2.4 A home for submesoscales
4.3 Observing the surface layers and their processes
4.3.1 Observing mixing
4.3.1.1 Defining mixing from mean profiles
4.3.1.2 Estimating turbulent fluxes
4.3.1.3 Dissipation and loss of shear and temperature variance
4.3.2 Wave-driven turbulence
4.3.3 Laboratory experiments
4.4 Modelling surface layers and their processes
4.4.1 Large eddy simulations
4.4.2 1D boundary layer models
4.4.2.1 Slab mixed layers
4.4.2.2 K-profile parameterisations
4.4.2.3 Second-moment closures
4.4.2.4 Bulk mixed layers
4.4.2.5 Prognostic vs. diagnostic boundary layer depth
4.4.2.6 Non-breaking wave mixing
4.4.3 Ocean and climate models
4.5 Global perspective
4.5.1 Energy and forcing
4.5.2 Surface layers, weather, and climate
4.6 Outlook
Acknowledgements
References
5 The lifecycle of surface-generated near-inertial waves
5.1 Introduction
5.2 Generation of near-inertial waves at the surface
5.3 Propagation of near-inertial waves out of the mixed layer
5.3.1 Refraction
5.3.2 Straining
5.3.3 Interaction with frontal vertical circulations
5.4 Interactions of near-inertial waves with variable stratification, other internal waves, and mean flows in the interior
5.4.1 Variable stratification
5.4.2 Interactions with other internal waves
5.4.3 Interactions with mean flows
5.4.3.1 Near inertial wave trapping and amplification in anticyclones and fronts
5.4.3.2 Energy exchange with mean flows
5.5 Dissipation of near-inertial waves
5.5.1 Near-surface dissipation
5.5.2 Interior dissipation
5.5.3 Near-bottom dissipation
5.6 Discussion
5.6.1 Vertical mixing
5.6.2 Lateral mixing
5.7 Conclusions and outstanding questions
Acknowledgements
References
6 The lifecycle of topographically-generated internal waves
6.1 Introduction
6.2 Generation
6.2.1 Internal tides
6.2.1.1 Small amplitude topography
6.2.1.2 Finite amplitude topography
6.2.1.3 Global distribution
6.2.2 Quasi-steady lee waves
6.2.2.1 Small amplitude topography
6.2.2.2 Finite amplitude topography
6.2.2.3 Global distribution
6.3 Internal tide propagation and an integral estimate of decay
6.4 Wave-wave interactions
6.4.1 Theoretical background
6.4.2 Parametric subharmonic instability of the internal tide
6.4.3 Wave-wave interactions in finestructure methods, mixing parameterisations, and numerical simulations
6.4.4 Global perspective
6.5 Wave-mean flow interactions
6.5.1 Theoretical background
6.5.2 Mean-flow effects on wave propagation
6.5.3 Mean-flow effects on wave energy
6.5.4 Global perspective
6.6 Wave-topography interaction
6.6.1 Theoretical background & observational estimates
6.6.1.1 Abyssal hills
6.6.1.2 Continental margins
6.6.2 Global distribution
6.7 Conclusions and outstanding questions
Acknowledgements
References
7 Mixing at the ocean's bottom boundary
7.1 Introduction
7.2 Common ground
7.2.1 Equations
7.2.2 Boundary conditions
7.2.3 Coordinate transformations and the one-dimensional model
7.2.4 Integration
7.2.5 Energetics and mixing
7.2.5.1 Energetics
7.2.5.2 Metrics of mixing
7.3 Implications of the bottom intensification of ocean mixing for upwelling: buoyancy budgets for bottom-intensified mixing
7.3.1 Abyssal ocean circulation models are sensitive to bottom topography
7.3.2 One-dimensional solutions for flow near a sloping bottom boundary
7.3.3 Expressions for the upwelling in the BBL and downwelling in the SML
7.3.4 How much larger is the upwelling in the BBL than the net upwelling?
7.3.5 Net upwelling in the abyss depends mainly on the shape of the ocean floor
7.3.6 What can be learned from purposefully released tracers?
7.3.7 Implications for the circulation of the abyssal ocean
7.3.8 Summary remarks
7.4 Production mechanisms for boundary mixing
7.4.1 Internal wave reflection / internal tide generation
7.4.2 Sub-inertial flow and topography
7.4.3 Friction and sub-inertial flows
7.5 Discussion
Acknowledgements
References
8 Submesoscale processes and mixing
8.1 Introduction
8.2 Life-cycle of submesoscale fronts
8.2.1 Frontogenesis
8.2.2 Instability of surface boundary layer fronts
8.2.2.1 Mixed-layer baroclinic instability
8.2.2.2 Symmetric instability
8.2.2.3 Forced symmetric instability
8.2.3 Submesoscale processes at the bottom of the ocean
8.2.3.1 Bottom boundary layer baroclinic instability
8.2.3.2 Bottom injection of PV and submesoscale instabilities
8.2.3.3 Topographic wakes
8.2.4 The influence of vertical mixing on the evolution of a submesoscale front
8.2.5 Frontal arrest and routes to dissipation
8.3 Redistribution of density and restratification at the submesoscale
8.3.1 Restratification induced by submesoscale processes
8.3.2 Competition between destratification and restratification of a front
8.3.3 Bottom boundary layer mixing and restratification
8.4 Redistribution of passive tracers and particles
8.4.1 Conservative tracers
8.4.2 Mixing and transport of reactive tracers
8.4.3 Impacts on the dispersion of buoyant material
8.4.4 Dispersion by the deep submesoscale currents
8.5 Conclusion and future directions
Acknowledgements
References
9 Isopycnal mixing
9.1 Introduction
9.2 Background concepts
9.2.1 What is mixing?
9.2.2 What is isopycnal?
9.2.3 What then is isopycnal mixing?
9.2.4 Lateral mixing near boundaries
9.3 Mechanisms of isopycnal stirring and dissipation
9.3.1 Mesoscale turbulence
9.3.2 Transport by coherent structures
9.3.3 Chaotic advection
9.3.4 Shear-driven mixing
9.3.5 Additional submesoscale isopycnal mixing processes
9.3.6 Diapycnal dissipation of isopycnal tracer variance
9.3.7 Frontogenesis and loss of balance
9.4 Frameworks for thinking about isopycnal mixing
9.4.1 Reynolds-averaged tracer equations
9.4.2 Mixing-length theory
9.4.3 Spectral-space view of turbulence and mixing
9.4.4 Isopycnal mixing in numerical models
9.5 Observational estimates of isopycnal mixing
9.5.1 Tracer-based methods
9.5.1.1 Natural vs. anthropogenic tracers
9.5.1.2 Tracer release experiments
9.5.1.3 Tracer variance and mixing length
9.5.2 Drifter and float-based methods
9.5.2.1 Single-particle dispersion and diffusivity
9.5.2.2 Relative particle dispersion and diffusivity
9.6 Simulation-based estimates
9.6.1 Inverse methods
9.6.2 Direct simulation
9.6.2.1 Lagrangian trajectory simulation
9.6.2.2 Tracer-based methods
9.7 Impacts of isopycnal mixing
9.7.1 Physical circulation
9.7.2 Passive tracers
9.7.3 Isopycnal mixing and ocean biogeochemical cycles
9.8 Summary and future directions
Acknowledgements
References
10 Mixing in equatorial oceans
10.1 Introduction
10.2 Ocean turbulence peaks at the equator, or does it?
10.3 Mixing in the cold tongues: diurnal forcing of turbulence below the mixed layer
10.4 The concepts of marginal instability and self-organised criticality and how they apply to mixing in the cold tongues
10.5 The importance of inertia-gravity waves and flow instabilities
10.6 Westerly wind bursts in the Indian Ocean and western Pacific
10.7 Variations on subseasonal, seasonal and interannual timescales
10.8 Equatorial mixing in large-scale models
10.9 Shortcomings, surprises and targets for future investigation
References
11 Mixing in the Arctic Ocean
11.1 Introduction
11.2 Foundations
11.3 Key findings
11.3.1 Ice-ocean interactions
11.3.2 Tidal mixing
11.3.3 Near-inertial motions and the internal wave continuum
11.3.4 Eddies
11.3.5 Double diffusion
11.3.6 Thermohaline intrusions
11.4 Grand challenges
11.5 Conclusions
Acknowledgements
References
12 Mixing in the Southern Ocean
12.1 Introduction
12.2 Large-scale context: foundations
12.3 Upper cell: mixed-layer transformations
12.3.1 Foundations and setting
12.3.2 Mixing in the surface boundary layer and connection to subsurface adiabatic stirring
12.3.2.1 Surface water mass transformation
12.3.2.2 Submesoscale-induced mixing and stratification
12.3.2.3 Interactions between sea ice and Southern Ocean mixing
12.4 Interior mixing: regional and mesoscale processes
12.4.1 Foundations: Southern Ocean eddy pathways
12.4.2 Mixing and coherent structures: adiabatic recipes for Southern Ocean mixing
12.5 Interior mixing: closing the budgets through turbulence at the smallest scales
12.5.1 Foundations
12.5.2 Recent findings: sub-surface diapycnal mixing pathways in the Southern Ocean
12.6 Grand challenges
Acknowledgements
References
13 The crucial contribution of mixing to present and future ocean oxygen distribution
13.1 Introduction
13.2 Role of mixing in oxygen minimum zones
13.3 Role of mixing on global deoxygenation
13.4 Response of OMZ to global warming
13.5 Conclusions and grand challenges
Acknowledgements
References
14 New technological frontiers in ocean mixing
14.1 Introduction
14.2 Current and historical measurements of mixing
14.3 Recent technological developments: novel methods
14.3.1 Shear microstructure on AUVs
14.3.2 Temperature microstructure on new platforms
14.3.3 Finescale parameterisations from autonomous platforms
14.3.4 Large-eddy method using autonomous platforms and moorings
14.4 Future outlook
14.5 Conclusions
References
Index