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

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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

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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.