Ocean Mixing and Antarctic Bottom Water

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Ocean Mixing and Antarctic Bottom Water

• Karen J. Heywood
• School of Environmental Sciences
• University of East Anglia
• k.heywood_at_uea.ac.uk
• GEFD
• Summer School
• 2005

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Antarctic Bottom Water

• Where does it form?
• Does the formation of Antarctic Bottom Water
  matter for the stability of our climate?
• How much of it is formed? 
• Where does it escape from polar regions into the
  world ocean?
• What happens to this water? How and where is it
  mixed upwards?
• What are the implications for the Meridional
  Overturning Circulation?

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Schmitz (1996)
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Section A23
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Elements of the Southern Ocean overturning
circulation
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How does Antarctic Bottom Water form?
Baines Condie (1998)
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Bottom potential temperature in the Southern Ocean
1.2
-0.4
0.4
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Antarctic Bottom Water

• Forms in the Weddell Sea, Ross Sea and Adelie
  coast.
• Important processes are sea ice formation, ice
  shelves, downslope flow, amongst others..
• Does the formation of Antarctic Bottom Water
  matter for the stability of our climate?

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The day after tomorrow?Change in surface air
temperature 30 years after a freshwater pulse in
the North Atlantic (Vellinga Wood, 2002)
What if a similar freshwater pulse were added to
the Southern Ocean?
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HadCM3 coupled ocean-ice-atmosphere model Ocean
resolution 1.25 x 1.25, 20 levels Atmosphere
resolution 3.75 x 2.5, 19 levels Atlantic
salinity section at 35W in control run of HadCM3
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Perturbation Experiment
Add freshwater for one timestep only in upper 500
m by reducing salinity by 1 Equivalent to
1.7x1014 m3 of freshwater cf. 6x1014 m3 by
Vellinga Wood (2002) Run for 10
years Ensembles of 5 perturbed and 5 control
runs performed on same UEA computer
Initial surface salinity difference (perturbed
control)
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Time series of temperature, ice and transport
anomaliesenvelope of control runs perturbed
runs
Southern hemisphere surface air temperature
Southern ocean sea surface temperature
Drake Passage volume transport
Southern Hemisphere ice extent
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Sea Surface Temperature differences (perturbed
control), averaged over years 6-10 (C).
Significance (95) outlined in black.
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Anomalies in Antarctic sea ice thickness (m),
averaged over years 6-10. Dashed contour shows
maximum extent in control run.
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12 month running mean of Southern Ocean Sea
Surface Temperature anomalies over 100 year run
(control, 1xFW run)
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AABW overturning strength over 100 year run
(control, 1xFW run)
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• Summary of impacts of freshwater pulse
• Addition of surface freshwater around Antarctica
  has a significant impact on global climate within
  5 years.
• Stratification in ocean near Antarctica
  increases, trapping warm deep water that would
  otherwise warm upper ocean
• Sea ice cover around Antarctica increases,
  preventing heat flux from ocean to atmosphere
• Southern Hemisphere air temperature and sea
  surface temperature decrease by 0.5 1 C
• North Atlantic SST and SLP patterns appear to
  shift to North Atlantic Oscillation (NAO)
  negative phase
• Southern Ocean cooling begins to weaken after
  10-15 years, and climate returns to normal within
  50 years.
• Richardson et al., Short-term climate response to
  a freshwater pulse in the Southern Ocean,
  Geophysical Research Letters, 32, February 2005.

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Antarctic Bottom Water

• Yes, the formation of Antarctic Bottom Water
  appears to matter for climate stability on both
  short (years) and long (decades to centuries)
  time scales.
• How much of it is currently formed? 
• Where does it escape from polar regions into the
  world ocean?

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Transport of AABW (Sverdrups) in the Weddell and
Scotia Seas
Upper WSDW 28.26 lt ?n lt 28. 31
Lower WSDW 28.31 lt ?n lt 28.40
WSBW ?n gt 28.40
1 Sverdrup 1 x 106 m3 s-1
Naveira Garabato, McDonagh, Stevens, Heywood
Sanders, Deep Sea Research II (2002)
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Antarctic Bottom Water

• Weddell Sea Bottom Water is too dense to escape
  from the Weddell Basin.
• About 10 Sv of Weddell Sea Deep Water exits from
  the Weddell Sea, half of it northward through
  gaps in the South Scotia Ridge and half to the
  east. 
• Unknowns include the amount entering the Weddell
  Sea from the east.
• What happens to this AABW? How and where is it
  mixed upwards?

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The missing mixing problem
TURBULENT MIXING
DEEP WATER FORMATION
North Atlantic
Neutral Density
Southern Ocean
Turbulent mixing maintains the stratification of
the deep ocean against the upwelling of dense
waters ? globally averaged K? 10-4 m2 s-1 (Munk
Wunsch 1998).But most observations of mixing
are an order of magnitude less than this.
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AIR-SEA
EDDY STIRRING
North Atlantic
Southern Ocean
Neutral Density
Some of this diapyncal transport is provided by
eddy-driven isopycnal upwelling in the Southern
Ocean (Webb Suginohara 2001).But there must
still be large mixing occurring somewhere where?
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Heat Budget for Enclosed Basin

• In steady state, advection of cold water ?
  diffusive heat flux ?.
• ? in situ density
• V volume transport into basin
• Cp specific heat capacity
• ?i , ?u potential temperature of inflowing,
  outflowing water
• A area of basin
• G geothermal heat flux
• Kr thermal diapycnal eddy diffusivity
• mean vertical gradient of potential
  temperature.

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Heat budget of the abyssal Scotia Sea
Extent of gn 28.31 kg m-3 surface
In steady state, advection of cold AABW denser
than 28.31 kg m-3 into the basin must be balanced
by a downward diffusive heat flux across that
density surface. Diapycnal across density
surfaces. Isopycnal along density surfaces.
AABW
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• Scotia Sea exhibits high mixing of Antarctic
  Bottom Water
• Diapycnal eddy diffusivity
• 39 (? 10) x 10-4 m2 s-1
• over 7 x 105 km2
• Why?
• probably rough topography of the Scotia Sea.
• interaction of internal tides (stratified ocean)
  or other internal waves with topography. e.g.
  internal wave reflection at sloping boundaries.
• internal lee waves generated by interaction of
  the currents with the rough topography.
• basin-trapped barotropic planetary wave modes.
• Heywood, Naveira Garabato Stevens, Nature, 415,
  1011-1014, 2002

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CTD / LADCP survey of SE Pacific and SW Atlantic
Naveira Garabato, Polzin, King, Heywood
Visbeck, Science, 2004
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Calculating turbulent dissipation e and diapycnal
diffusivity Kr from CTD / LADCP
profilesTurbulent mixing at scales of 1 cm is
related to energy level of internal waves
measured at scales gt 50 m. Nonlinear interaction
between internal waves drives an energy cascade
to smaller scales that results in turbulent
mixing. Measured internal wave energy level
quantifies how intense that interaction is.
GM76 model
Function of Rw, the ratio of (LADCP) shear to
(CTD) strain
CTD buoyancy frequency
N-normalised LADCP shear variance
where G 0.2 is the mixing efficiency
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Spatial distribution of Kr in the deep Southern
Ocean
log10 (Kr m2 s-1) along ALBATROSS cruise track
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Dependence of large-scale distribution of Kr on
stratification topography
m
m
Kr m2 s-1 N2 s-2 e W kg-1
ACC
m
m
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Polarisation of the internal wavefield
Southern Hemisphere
v water velocity k wavenumber vector Cg
group velocity
Infer direction of energy propagation from
polarisation of LADCP velocity profiles
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Energy sources direction of energy propagation
m
m
Down
Down
CCW Energy CW Energy
Up
ACC
m
m
Down
Up
Up
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So there is much mixing upwards of AABW in the
Southern Ocean. What causes this mixing? Is it
internal tides?
Garrett (2003)
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Upward internal lee wave momentum flux
Lee wave generation
Bell (1975) ?(k) ? k . p(k) / k2 .
(?2 - f2) . (N2 - ?2) 1/2
Spectrum of topography
Vertical lee wave momentum flux
Frequency ? k . Ubottom
Horizontal wavenumber
For ACC bottom speeds of 2-5 cm s-1 and a
characteristic topographic spectrum
?k ?(k) 0.04 - 0.16 N m-2 cf. wind stress
0.1 N m-2
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Upper ocean mixing is caused by air-sea
interaction processes (such as winds). Deep
mixing may be caused by interaction of mean
geostrophic flow, barotropic tides and/or eddies
with rough topography. Not all AABW is mixed
upward in the Southern Ocean. Much of it
penetrates northward into the Atlantic
Ocean. What are the implications of this large
Southern Ocean mixing for the oceans meridional
overturning circulation?
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Implications for the MOC
Other hotspots of enhanced diapycnal mixing in
the deep Southern Ocean?
ACC
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Implications for the MOC
TURBULENCE
Stronger coupling between the two MOC cells?
Significant modification to models of ACC eddy
dynamics?
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Antarctic Circumpolar Current dynamics an extra
term in the balance?
WIND STRESS
AIR-SEA-ICE
EDDY E-P FLUX
INTERNAL WAVE E-P FLUX
DEEP BOUNDARY CURRENT
TURBULENCE
AABW
TOPOGRAPHIC STRESS
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Antarctic Bottom Water
AABW forms in the Weddell and Ross Seas and
Adelie coast. Important processes are sea ice
formation, ice shelves, downslope flow, amongst
others.. The formation of Antarctic Bottom
Water appears to matter for climate stability on
both short (years) and long (decades to
centuries) time scales. Weddell Sea Bottom Water
is too dense to escape from the Weddell
Basin. About 10 Sv of Weddell Sea Deep Water
exits from the Weddell Sea, half of it northward
through gaps in the South Scotia Ridge and half
to the east.  This is the water mass we know as
AABW. Unknowns include the amount entering the
Weddell Sea from the east.

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Antarctic Bottom Water
In the Scotia Sea we find large diapycnal
diffusivity K? and e for mixing of deep and
bottom waters. High mixing rates are associated
with rough topography in the Antarctic
Circumpolar Current. Generation of internal
waves as flow interacts with topography (mean /
eddy flow, tides?) enhances K? over lower
background values (near-inertial
waves). Topographically enhanced mixing is
generally more vigorous and widespread in the
Southern Ocean than at lower latitudes. Turbulenc
e may represent an important modification to
current theories of ACC dynamics and the closure
of the Southern Ocean limb of the Meridional
Overturning Circulation.
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Still Unanswered Questions
What determines the amount of AABW formed? What
is the role of ice shelves? How much AABW enters
the Weddell Sea from the east? What are the
driving mechanisms of internal wave generation,
and hence mixing, as the ACC interacts with
topography? What is the impact of this mixing on
the meridional overturning circulation and the
ACC momentum balance?
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Future work - Turbulence in an ACC standing
meander
Naveira Garabato (NOC) Heywood Stevens (UEA)
Polzin (MIT)

• ACC standing meander around Kerguelen Plateau.
• CTD / LADCP / microstructure survey.
• Short-term and long-term moored array.
• Fieldwork 2007-8.