Is the stability of the Atlantic MOC changed by global warming?

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Why 35S is the best location to monitor (the
stability of) the THC?
Fig mov (fi) from Andrea
Courtesy Andrea Cimatoribus
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What determines THC stability?

• Instability results from salt feedback in ocean
• Assume freshwater (salinity) anomaly at high
  latitudes temporal decrease of THC and MOC
• If MOC exports salt Atlantic becomes saltier
  anomaly damped
• If MOC exports freshwater Atlantic becomes
  fresher anomaly amplified

De Vries and Weber (2005)
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In the subpolar gyre the salt feedback is
associated with oscillations

• Flat bottom modes
• ZOC-advection adjustment by Kelvin and westward
  prop. PV-waves 20-70 yrs
• Greatbatch Petterson (96) Colin de Verdiere
  Huck (99) Te Raa Dijkstra (02)
• NA modes resembling Deser Blackmon (93)
  Kushnir (94)
• Gyre spin-up by Jbar 40-80 yrs Delworth (93)
  Chen ea (04) dOrgueville Peltier (09)
• Gyre spin-up by NAO through winds 10-20 yrs
  Grötzner ea (98) Eden Willebrand (01) Dong
  Sutton (05) Dai ea (05) Danabasoglu (08)
• Gyre spin up by NAO through buoyancy 30-40 yrs
  Timmerman ea (98)
• Arctic freshwater exchange (sea-ice/storage)
  20-80 yrs Holland ea (01)
  Jungclaus ea (05) Hawkins Sutton (07)
• Basinwide Atlantic mode resembling AMO
• Cross-hemispheric salt transport ITCZ
  Arctic? 100-150 yrs Vellinga Wu (04)

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First three SST EOFs in ECEARTH
Courtesy Bert Wouters
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• In subpolar gyre damping salt feedback (NAO,
  ZOC, gyre) is regionally confined, but depends on
  advection of salt across subpolar boundaries
• Damping spg salt feedbacks become less effective
  if basin-scale MOC-salt feedback enhances local
  MOC-salt feedback
• Damping spg salt feedbacks become more effective
  if basin-scale MOC-salt feedback counteracts
  local MOC-salt feedback

• Mov gt 0
• strong spg feedbacks

• Mov lt 0
• weaker spg feedbacks

A.C.
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Basin-scale Salt Feedback
Movgt0
Movlt0
Weber and Drijfhout, 2007
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(No Transcript)
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• Damping subpolar gyre salt feedbacks involved in
  recovery from off-state to on-state
• (Same?) damping subpolar gyre feedbacks
  determine (inter)decadal oscillations in the MOC,
  together with amplifying MOC-salt feedback in
  subpolar gyre
• Basin-scale MOC-salt feedback determines whether
  off-state is stable or unstable
• Basin-scale MOC-salt feedback is associated with
  sign Mov at 35S

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Drijfhout et al. (2010)
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How to measure Mov?
Mov is independent of level of no motion!
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Weber et al. (2007)
Obs Weijer et al. (1999)
Obs Weijer et al. (1999)
Drijfhout et al. (2010)
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ORCA025 Hindcast
Mov -.025
Trend -.01/cnt
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Monitoring Mov?
Correlation 0.9
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Trend
Interannual variability
Drijfhout and Hazeleger (2007)
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Short-term variability at 40N and 35S is
uncorrelated!
Black is 40N blue is 35S, cor 0.1
A.C.
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Conclusions

• MOC-Salt feedbacks can be assessed from the sign
  of the latitudinal divergence of Mov
• Subpolar MOC-Salt feedbacks determine
  oscillations and involved in recovery MOC
• Basin-scale MOC-salt feedbacks determine
  stability off-state sign of Mov at 35S
• Mov can be estimated from CTD-sections. We need
  salinity and velocity relative to a level of no
  motion,
• plus direct boundary current velocity
  estimates. Most variability is related to Ekman
  flow.
• A much more robust MOC-trend estimate can be made
  by combining Rapid and 35S estimates the trend
  at the two locations is correlated, while the
  noise isnt.