The Southern Ocean and Climate: What did we learn during WOCE?

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The Southern Ocean and ClimateWhat did we learn
during WOCE?

• Steve Rintoul
• CSIRO Marine Research and Antarctic CRC
• Australia

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Pre-WOCE view of the ACC/SO

• 2 circumpolar fronts
• wind-driven, in (flat-bottom) Sverdrup balance
• bottom form stress balances wind?
• Drake Passage transport 13413 Sv
• transport variability is barotropic
• no net meridional flow through Drake Passage gap
• poleward eddy heat flux in Drake Passage, SE NZ
• zonal circulation independent of meridional
  circulation
• water masses exported to lower latitudes, but
  rates and mechanisms unknown

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Progress in the WOCE era

• remote sensing (SST, SSH)
• new instruments (e.g. ALACE floats)
• observations outside of Drake Passage
• improved model realism/resolution/diagnostics
• air-sea flux estimates from reanalyses
• advances in dynamical understanding

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10,000 stations south of 25S since 1990
Orsi, 2002
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Oxygen on 27.4
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4-year mean SST gradient from ATSR reveals
multiple filaments and branches, which merge and
split.
Rintoul, Hughes and Olbers 2001
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Tracking ACC fronts using satellite altimetry
Careful comparison of hydrography and absolute
sea surface height maps shows each frontal branch
corresponds to a particular SSH contour. We can
use altimetry to track fronts, every 10 days
since 1992.
Sokolov and Rintoul, JMS, 2002
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• SAF 3 branches, merge near 140E, eddy-rich
  downstream of change in orientation of SEIR.
• PF 2 branches, separated by gt500 km at SR3,
  merge after crossing ridge crest.
• PF, SACCF strong equatorward deflection over
  ridge.
• Narrow meander envelopes near ridge.

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ACC Transport
Repeat sections show heat transport south
of Australia varies by 0.6 x 1015 W (relative
to 0?C). Variability is large (e.g. relative to
north-south heat flux in Indian
and Pacific.) Climate impact?
Rintoul and Sokolov, JGR, 2001
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Drake Passage transport 136?8.5 Sv
Cunningham et al., JGR, 2002
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ACC transport ? 500 billion Lone Stars/sec
www.mylifeisbeer.com
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ACC transport in neutral density layers Australia
(SR3) color Drake Passage (SR1) black
Rintoul and Sokolov, 2001 Cunningham et al.,
JGR, 2002
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The tight relationship between temperature at 650
m and the baroclinic transport streamfunction
can be used to determine transport (above 2500
m) from temperature msmts. alone.
Rintoul, Sokolov and Church, JGR, 2002
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Net baroclinic transport time series from XBT
data (squares) and CTD data (diamonds)
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Net baroclinic transport south of Australia
(1993-2000)
Empirical relationship between surface
height and transport fn used to estimate
transport. Continuous record from altimeter
shows XBT time series is aliased.
Transport estimated from altimeter (thin line),
low-passed (thick blue line).
Rintoul, Sokolov, Church, 2002
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Streamwise average of absolute velocity of
Subantarctic Front Total transport 116 Sv
barotropic 16 Sv.
Phillips and Rintoul, JPO, 2002
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Eddy heat flux
Poleward eddy heat flux across SAF south of
Australia is larger than previously measured
elsewhere in the Southern Ocean.
Phillips and Rintoul, JPO, 2000
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Is the ACC in Sverdup balance?
ß?x ?pb ? ?H ??? ??F
Bottom pressure torque (color) barotropic
streamfn (black)
Rintoul, Hughes and Olbers 2001
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Steady, zonally-integrated momentum balance
-fV1 - ?'1p'1x ?o - R1
?1
Surface (includes Ekman)
-fV2 ?'1p'1x - ?'2p'2x - R2
?2
unblocked layer
?3
-fV3 ?'2p'2x - hpbx - R3
blocked layer
V net meridional volume flux ?o wind stress ?
layer thickness p pressure R Reynolds
stress divergence pb bottom pressure
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No interfacial form stress
V1 - ?o/f
Ekman transport in surface layer
V2 0
No transport in unblocked layer
V3 hpbx /f ?o/f
Deep geostrophic flow balances Ekman
? Overall balance of zonal momentum is between
wind stress and bottom form stress.
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Interfacial form stress ? 0
Adding the three equations and using fact
that mass is conserved (?(Vi) 0)
?o hpbx
? Again, overall balance of zonal momentum is
between wind stress and bottom form stress.
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Adiabatic flow (Vi 0)
?o ?'ip'ix hpbx
? Wind stress interfacial form stress
bottom form stress Note that
both standing and transient eddies contribute to
interfacial form stress.
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Diabatic flow (Vi ? 0)
Mixing and surface buoyancy fluxes drive mass
exchange between layers, so Vi net diapycnal
exchange ? 0.
???z(?'ip'ix) ? 0
? Divergence of interfacial form stress drives
meridional flow in the unblocked layer. ?
Buoyancy forcing, eddy stresses, and meridional
flow are intimately linked to the zonal
momentum balance.
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What controls the transport of the ACC?

• Observations and a variety of models suggest ACC
  transport is a function of
• ?n (n 0-1?)
• ?? x ?
• buoyancy flux
• topographic interactions
• baroclinic instability / eddy fluxes
• (Gent, Tansley, D. Marshall, J. Marshall,
  Karsten, Olbers, Rintoul, Sokolov, Gille,
  Gnanadesikan, Hallberg, )

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Schmitz (1996)
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Orsi et al., 1999
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CFC inventory 8 Sv AABW 21 Sv total input to
deep ocean
Orsi et al., JGR, 2002
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SO Overturning

• By including the water mass transformations
  driven by air-sea fluxes, we can quantify the
  overturning circulation for the first time.
• vigorous deep cell
• weak upwelling through the thermocline
• NADW global cell closed by DW ? IW conversion in
  SO

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4
?2
eddy mass flux
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Speer et al., 2000 Sloyan and Rintoul, JPO, 2001
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Models also suggest the NADW overturning cell is
closed by upwelling and water mass transformation
in the SO.
Döös and Coward (1997)
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Formation, circulation and consumption of
intermediate and thermocline waters.
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10
4
25
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8
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Sloyan and Rintoul (2001)
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Upper branch of the global OTC
cold 6.5 Sv
warm 5.3 Sv
cool 3.1 Sv
Speich et al., GRL, 2001
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Intermediate depth waters in both hemispheres
have become fresher in recent decades.
Wong et al., 1999
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Climate models show similar response suggest
strongest ocean climate change signal in SO.
Banks et al., GRL, 2000
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Observations south of Australia show large
variability in mode water properties from
year-to-year, driven by changes in cross-frontal
Ekman transport (not air-sea fluxes).
Circles show T-S properties of SAMW south of
Tasmania size of dot is proportional to strength
of mode. Triangles and squares are data from 1968
and 1978.
Rintoul and England, JPO, 2002
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Warming of the Southern Ocean
Gille, Science, 2002
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Warming of Weddell Sea Warm Deep Water
Warm Deep Water flowing into and out of the
Weddell Sea has warmed by about 0.3C since the
mid-1970s. (Robertson et al., 2002)
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Climate models suggest SO overturning will slow
down as a result of global warming.
Warming and freshening increases the high
latitude stratification, shutting down
AABW formation. Is this result realistic? Can we
observe the change in stratification?
Hirst (1999)
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The Southern Ocean is the largest
zonally-integrated sink of anthropogenic CO2.
Sabine et al., 2002
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Massom et al., 2001
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Southern Annular Mode/Antarctic Oscillation
Thompson and Solomon, Science, 2002
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Antarctic Circumpolar Wave
White and Peterson, 1996
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Air temperature
Sea ice extent
Antarctic Dipole Subtracting May composites for
El Nino and La Nina events reveals the impact of
ENSO on the Southern Ocean. Response consists of
a dipole with centres in the Atlantic and Pacific
sectors, driven by the PSA teleconnection. (Yuan,
2001).
SLP El Nino
SLP La Nina
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• Modes of variability
• local or remote forcing?
• ocean response?
• feedback?
• coupled?
• regional climate impact?

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New view of the ACC/SO

• multiple filaments, which split and merge
• bottom pressure torque important (i.e. not in
  flat-bottom Sverdrup balance)
• transport f (?, ?x?, buoyancy forcing,
  topography)
• zonal and meridional circulations intimately
  linked
• eddies carry mass and heat poleward across Drake
  Passage gap
• quantified rate and mechanisms of water mass
  formation
• water mass transformation in SO closes
  overturning cells
• observed change at all depths
• identified modes of variability

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Science questions

• Strength, variability and sensitivity of SO
  overturning?
• Dynamics and climate impact of SH atmosphere,
  ocean, ice variability?
• How much mixing takes place in the Southern
  Ocean?
• Does the SO gain or lose heat and freshwater?
• Impact of SO variability (low latitudes, regional
  climate, global overturning)?

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Conclusions

• We have made remarkable progress in understanding
  the Southern Ocean during the WOCE era.
• The Southern Ocean strongly influences regional
  and global climate, and is sensitive to change.
• The prospects for further progress are good. We
  can now identify specific hypotheses and design
  observing systems and models to test them.

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A similar relationship can be used to determine
transport for satellite measurements of sea
surface height.
Relationship between surface dynamic height and
transport function, determined from the 6 CTD
sections.
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A test of how well baroclinic transport can be
estimated from altimeter data. Residuals are
typically small (less than 5 Sv). Demonstrates
most of altimeter signal is due to changes in
baroclinic structure above 2500 m.