Post on 30-Apr-2020
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Overflows and Convectively-driven flows
Sonya Legg
Princeton University/GFDL
Aosta summer school 2010
Lecture 1: Introduction to overflows
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Dense Overflows: localized features with global importance
Nordic and Antarctic overflows are the source of most of the dense bottom waters.Other overflows, e.g. Mediterranean, are source of important intermediate waters.
Map due to Arnold Gordon, LDEO
Macrander et al, 2005, Denmark Straits
The Denmark Straits Overflow
Wide channel
Dense water banked against left of channel (looking upstream)
100km
Density at the sill (sigma-theta kg/m3)
Girton et al, 2001
Velocity field at the sill
The Denmark Straits Overflow
Flow is approximately independent of depth
Macrander et al, 2007 Girton et al, 2003
The Denmark Straits Overflow
Density along the overflow plume path
Downstream
Downstream
Density signal is diluted downstream
Girton et al, 2003
The Denmark Straits Overflow
Overflow path (center of mass)
Overflow path slowly crosses topographic contours
Girton et al 2003
The Denmark Straits Overflow
What controls the rate of descent?
Controlling forces: gravity, coriolis and frictional/interfacial stresses
ghddHb
'sinsin
ρτβα
ξ==
Girton et al, 2003.
The Denmark Straits Overflow
Transport of sigma-theta > 27.8 water
Transport increases downstream due to entrainment
Eddies
cross-section velocity contours
Kase et al, 2003
The Denmark Straits Overflow
Surface cyclonic eddies seen in SST over domes of dense water (Bruce 1995, Girton)
Flow becomes more baroclinic downstream
Denmark Strait overflow: summary
• Wide channel: importance of rotation
• Barotropic flow: barotropic instability generates eddies with surface signature
• Balance between coriolis and gravity broken by friction allowing flow to cross isobaths
• Entrainment dilutes density signal downstream, increases transport
Faroe Bank Channel overflow
Girton et al, 2006Topography and the overflow path
Very narrow deep channel near sill
Broader slope
Bank ~ 200m deep, only constrains densest water
Temperature signal is diluted downstream by entrainment. Plume broadens about 100km downstream of sill.
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Faroe Bank Channel overflowOverflow plume temperature (Mauritzen et al, 2005)
Sill section
Broad slope section
Mauritzen et al, 2005
Faroe Bank Channel overflow
Accelerated flow in channel and on slope
Velocities in dense overflow layer
Large velocities and thin layer -> Froude numbers close to 1
Girton et al, 2006
0, <∫∫∫∫ nnn VdzdxVdzdxVS
sill
DHFaroe Bank Channel overflow
Mauritzen et al 2005
Transport in different T,S classes
Sill sectionBroad slope section
Plume becomes warmer and saltier downstream. Total transport increases.Both are evidence of entrainment.
Fer et al, 2004
Faroe Bank Channel overflow
Where do dissipation and mixing occur in FBC overflow?
Downchannel V Stratification Dissipation
Dissipation in stratified interfacial shear layer
Dissipation in frictional boundary layer
Two different locations of dissipation/mixing
Faroe Bank Channel overflow
Eddies/oscillations in FBC overflow
Temperature near bottom for section across broad slope
Geyer et al, 2006
88 hour regular oscillations
Faroe Bank Channel Overflow summary
• Narrow channel followed by broad slope
• Flow accelerates in narrow channel and when slope steepens
• Rapid broadening of overflow may be transverse jump, transition from supercritical to subcritical flow
• Mixing is associated with interfacial shear layer, and frictional bottom layer
• Entrainment leads to warming, salinification and increase in transport
The Red Sea Overflow
Numerical simulation, Mehmet Ilicak
2 narrow channels ~5km wide Bab Al Mandab
Northern Channel
Southern channel
Tadjura Rift
Peters et al 2005
Southern channel
The Red Sea Overflowwinter summer
Northern channelSalinity
Northern channel velocity
Strong seasonal variations in overflow strength
Peters et al 2005b
The Red Sea Overflow
Vertical structure of overflow plume
Stratified interfacial shear layer
Homogenized frictional boundary layer
The Red Sea Overflow
Transport as a function of salinity class
Upstream Downstream
Entrainment causes an increase in total transport and reduction in salinity
Matt and Johns, 2007
Bower et al, 2005
The Red Sea Overflow
Salinity near Tadjura rift
Northern channel Southern channel
Overflow water has reached neutral buoyancy level at Tadjura Rift. Deeper salinity signature suggests overflow can be denser at times
The Red Sea Overflow: summary
• Very narrow channels and low latitude: limited influence of rotation
• Entire descent is within channel: most mixing occurs in interfacial layer, bottom most layer partly shielded from mixing
• Strong seasonality
Baringer and Price 1997
The Mediterranean Outflow
Atlantic water flows into Mediterranean at surface, dense salty water flows out below
Numerical simulations, Mehmet Ilicak
Overflow path influenced by topography in Gulf of Cadiz
The Mediterranean Outflow
Salinity at topography
The Mediterranean OutflowSchematic of flow dependence on tidal phase
Outflow: strong mixing downstream of sill
Inflow: dense layer arrested, weak mixing
Wesson and Gregg, 1994
The Mediterranean OutflowAcoustic image of Kelvin-Helmholtz billows
Wesson and Gregg, 1994
The Mediterranean Outflow
silldownstream
Transport as function of depth and density class
Baringer and Price, 1997
As dense plume descends, transport increases and salinity is diluted, due to entrainment.
The Mediterranean Outflow
Salinity sections in Gulf of Cadiz
Near sillWestern end of gulf
Borenas et al, 2002
Salinity plume spreads out at neutral buoyancy level
Double core
The Mediterranean Outflow
Eddies of salty water (Meddies) shed at intermediate depths
Serra and Ambar, 2002
Mediterranean Outflow: summary
• An exchange flow of surface inflowing Atlantic water and subsurface outflowing salty Mediterranean water at Gibraltar Strait.
• Flow, and associated mixing vary considerably on tidal cycles.
• As for other overflows, entrainment leads to increase in transport, dilution of salinity anomaly.
• Topographic variations in Gulf of Cadiz lead to a splitting of the plume.
• The outflow plume reaches a neutral buoyancy level and flows into the interior, often in the form of eddies.
Antarctic overflows
Weddell SeaFoldvik et al, 2004
Pathways of dense water
Dense water from Filchner depression is strongly influenced by topographic spur as it moves down continental slope
Potential temperature in Weddell sea overflow
Foldvik et al, 2004
Antarctic overflows
Antarctic overflows
Ross Sea topography
Padman et al, 2009
Antarctic overflows
Bottom velocity and salinity at outflow from Drygalski Trough
Gordon et al, 2004
Strong dense current flowing approximately along isobaths
Antarctic overflows
Section through Ross Sea outflow: cold salty current flowing along isobaths
Gordon et al, 2009
Antarctic overflows
Ross sea: Influence of tides Tidal velocities
Padman et al, 2009
Temperature and salinity at mooring W. of Drygalski Trough (Gordon et al, 2004)
days
Large amplitude tides cause intermittent cascades of cold salty water down slope
Visbeck and Thurnherr, 2009
Antarctic overflowsVertical structure and mixing in Ross Sea overflow
Regions of high backscatter associated with high shear and low Richardson number: interfacial layer, and frictional boundary layer
Antarctic overflows: summary
• Ross Sea and Weddell Sea overflows carry cold salty water from shelf down slope.
• Tides and small scale topography play important role in carrying dense water down slope.
• Like other overflows, mixing occurs in interfacial layer and frictional boundary layer.
Ferron et al, 1998
Overflows over sills in abyssal canyons
Potential temperatureThorpe scales
Romanche Fracture Zone
Mixing occurs downstream of large sills
St Laurent and Thurnherr, Nature 2007
Overflows over sills in abyssal canyons
Turbulence dissipation and potential temperature
Velocity and Froude number
Lucky Strike Canyon
Velocity increases downstream of ridge crest, increasing Froude number and leading to increased dissipation and mixing
Shear instability
Neutral buoyancy level
Geostrophic eddies
Downslope descent
Bottom friction
x
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y
Entrainment of ambient water
Upper ocean flow
Summary of overflow processes
Final properties, transport and depth of overflow product water depend on all these processes Hydraulic
control
Differences between overflows:• Topography: narrow canyons constrain flow, wide
slopes allow more lateral mixing and eddy formation, ridges and canyons can steer flow downslope.
• Tides: Tides can influence mixing at sills, and advect dense water directly downslope.
• Rotation: Overflows near equator are less influenced by rotation. Rotation steers flow along topography and leads to eddy formation.
• Overlying flow: Properties of overlying flow influence entrainment and eddy formation.