Transcript Ei dian otsikkoa - International Seabed Authority

The physical environment of cobalt-rich
ferromanganese crusts deposits, the potential impact
of exploration and mining on this environment, and
data required to establish environmental baselines
(HYDRODYNAMIC PROCESSES AT SEAMOUNTS)
Aike Beckmann
Division of Geophysics
University of Helsinki
OUTLINE
1 Introduction
seamounts in the world ocean, hydrodynamics of flow past a cylinder
2 Processes and Phenomena
- seamounts in (quasi-) steady flows
- seamounts in alternating (tidal) flows
- seamount induced turbulence
3 Consequences for material transport
- closed circulation cells, retention, sediment patterns
4 Parameter dependencies
- geometry, rotation, stratification, forcing
5 Monitoring requirements
- observational and modeling
(main examples: Fieberling Guyot and Great Meteor Seamount)
INTRODUCTION
seamounts in the world ocean:
> 30000 in the Pacific alone
Foundation Seamount Chain
HYDRODYNAMICS
OF FLOW PAST A CYLINDER
closed circulation cell
in a “technical” setting
(without rotation)
in a geophysical setting
(with rotation)
THE GEOPHYSICAL PROBLEM
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•
•
•
•
rotation
subsurface summit
steep (>30%) but finite slopes
stratification
variable currents
PROCESSES AND PHENOMENA
•
seamounts in (quasi-) steady flows:
Taylor columns, generation of meanders, eddy shedding
•
seamounts in alternating (tidal) flows:
seamount trapped waves, resonant amplification,
rectified flows, radiating waves
•
seamount induced turbulence:
internal waves, internal tides, vertical mixing
•
consequences for material transport:
closed circulation cells, retention, sediment patterns
•
seamount chains, seamount clusters
SEAMOUNTS
IN (QUASI-) STEADY FLOWS
“SPIN-UP”
(numerical simulation)
(a) trapped wave
(b) vortex generation
(c) eddy shedding
SEAMOUNTS
IN ALTERNATING (TIDAL) FLOWS
•
seamount trapped waves, resonant amplification
•
rectified flows, radiating waves
•
horizontal currents easily reach 0.5m/s, vertical currents 1000 m/day
OVERVIEW:
SEAMOUNT FLOW REGIMES
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“butterfly patterns”
closed circulations
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seamount trapped waves
rectified flows
•
overturning motion
SEAMOUNT-INDUCED TURBULENCE
internal waves, internal tides, vertical mixing
fluctuations
diffusivity
- 1
- 2
log K sc =
- 3
- 4
- 5
- 6
- 7
log(/ 2(dT/ dz)2 ) , (m2/ s)
0
50
Depth , (m)
•
100
150
200
60
54
48
42
36
30
24
Distance (miles)
Ampere Seamount
18
12
6
0
SEAMOUNT-INDUCED TURBULENCE
- 6.0
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•
•
vertical mixing
turbulent dissipation
- 7.0
- 7.5
- 8.0
- 8.5
- 9.0
log  (W/ kg)
in a relatively thin
bottom boundary layer
- 50
diffusivities averaged over the
summit of Ampere Seamount are
as high as (1-2)10-3 m2/s
the averaged mixing rate in the
region surrounding Ampere is
30-60 times the far field value
Depth (m)
•
- 6.5
- 100
- 150
significant contribution
to global vertical mixing
- 200
0
1000
2000
3000
4000
Distance from the summit of Ampere seamount (m)
CONSEQUENCES
FOR MATERIAL TRANSPORT
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closed circulation cells, retention, strong currents, upwelling, mixing
•
chlorophyll a at Great Meteor Bank (M. Kaufmann, pers. comm.)
•
observed biological fields
METEOR SEAMOUNT
TRACER STUDIES (I)
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passive tracer at Great Meteor Bank (combined tidal mean flow forcing)
numerical simulation of a summit source (Mohn and Beckmann, 2002)
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after 50 days: substantial retention despite strong currents,
especially below the surface mixed layer
METEOR SEAMOUNT
TRACER STUDIES (II)
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passive tracer at Great Meteor Bank (combined tidal mean flow forcing)
numerical simulation of a surface source (Mohn and Beckmann, 2002)
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after 50 days: substantial upwelling above the seamount,
mixed layer thickness (air-sea interaction) change
METEOR SEAMOUNT
PARTICLE STUDIES
• particles at Great Meteor Bank (combined tidal mean flow forcing)
numerical simulation of a surface layer trajectories
2002)
(Beckmann and Mohn,
• 50 days tracks: indication of retention for passive, less for active
particles
CONSEQUENCES
FOR SEDIMENT TRANSPORT
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characteristic deep sea sedimentation patterns
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seamount summit areas are often sediment depleted (due to strong currents)
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high sediment loads may lead to downslope turbidity currents
SEAMOUNT CHAINS AND CLUSTERS
interaction between neighboring seamounts is small, as long as distance is larger
than diameter (exception: circulation near the seamount base)
not investigated in detail:
PARAMETER DEPENDENCIES
the ingredients are the same, their relative contribution depends on:
SEAMOUNT
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geometry
o
height
o
diameter
o
steepness
o
base shape
o
degree of symmetry
o
smoothness
o
…
AMBIENTE
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rotation and stratification
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forcing
RESONANCE AS A FUNCTION OF
SEAMOUNT GEOMETRY
resonance phenomena:
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seamount trapped waves
rectified (time-mean) flow
given a particular environmental setting, resonances occurs for certain seamount geometries
upper curves: wave amplitude (current strength);
lower curves: time-mean flow (retention potential)
RESONANCE AS A FUNCTION OF
STRATIFICATION/ROTATION
Burger number S = NH/fL
frequency
N stability
f Coriolis parameter
H water depth
L seamount diameter
forcing frequency
OBSERVATIONAL/MONITORING
REQUIREMENTS
observational array during the TOPO project (1991)
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current meter moorings: summit plain, upper flanks, far-field
reference
large number of XBTs on radial transetcs
duration: several months (background fields) plus 1-2 weeks intense
survey
FIEBERLING GUYOT
NUMERICAL MODEL STUDY
high resolution ocean circulation model
(dominating tidal forcing)
model domain
(Beckmann and Haidvogel, 1997)
topography, mooring locations
FIEBERLING GUYOT
MODELING RESULTS
excellent quantitative agreement
time-mean flow
week-long velocity time-series
SUMMARY AND CONCLUSIONS
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the physical environment at seamounts is quite different from the ambient
ocean
there is a strong dominance of a number of physical processes and phenomena
• strong horizontal flows due to large amplitude seamount trapped waves,
• substantial up- and downwelling over the upper flanks,
• radial density gradients, and corresponding closed circulation cells, and
• a significantly enhanced level of turbulent vertical mixing.
some consequences for marine ecosystems are known (nutrient supply, retention,
endemisms), though not fully understood in all cases
•
determination of the regime and subsequent monitoring requires limited
time observational arrays accompanied by numerical simulations
the
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the
relative contribution of each of the above processes depends strongly on
seamount geometry (height, diameter, steepness of slopes),
geographical latitude,
stratification of the water column, and
ambient ocean currents;
physical regime for each seamount has to be determined separately from an array
of density and current measurements at various locations both on top of the
seamount. These and additional observations from the surrounding deep ocean can
then be used in numerical experiments to obtain a more complete view of the
physical environment, its sensitivity and consequences