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Causes of MOC variability: what have we learned from previous ocean

Northern and southern influences on the MOC Claus Böning (IFM-GEOMAR, Kiel) with Arne Biastoch, Markus Scheinert, Erik Behrens. Causes of MOC variability: what have we learned from previous ocean modelling studies?. 26°N. Obs. 1/12°-mod. 1/3°-mod.

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Causes of MOC variability: what have we learned from previous ocean

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  1. Northern and southern influences on the MOCClaus Böning(IFM-GEOMAR, Kiel)withArne Biastoch, Markus Scheinert, Erik Behrens Causes of MOC variability: what have we learned from previous ocean modelling studies? 26°N Obs 1/12°-mod. 1/3°-mod.

  2. Relation between MOC and heat transport? Biastoch, Böning, Getzlaff, et al. (J. Climate, 2006) • Role of Nordic Seas (Overflows) vs. Labrador Sea (Convection)? Schweckendiek and Willebrand (J. Climate, 2005) Latif, Böning, Willebrand, et al. (J. Climate, 2006) • Relation convection - MOC? Böning, Scheinert, Dengg, et al. (GRL, 2006) • Changes in deep water formation vs. wind stress? Biastoch, Böning, Getzlaff, et al. (J. Climate, 2006) ----------------------- • Effects of Agulhas Leakage (I): wave mechanism Biastoch, Böning, Lutjeharms (Nature, 27 Nov 2008) • Effects of Agulhas Leakage (II): advection Biastoch, Böning, Schwarzkopf, Lutjeharms (Nature, 26 Nov 2009)

  3. MOC and heat transport Model simulations: Close correspondence between MOC and heat transport in the subtropical North Atlantic 1.6 Heat transport (PW) 0.6 26°N 20 MOC (Sv) 10

  4. MOC and heat transport … but caution: Correlation 1 r 0 1.6 Heat transport (PW) 0.6 MOC (in depth-coordinates) is a useful index only south of ~43°N 20°S 0° 20° 40° 60°N based on monthly and2-yr-filtered time series time series

  5. Changes in deep water formation: effect on MOC? (I) Labrador Sea: deep winter mixing (II) Outflow from the Nordic Seas:

  6. Role of changes in overflow vs. convection OGCM study by Schweckendiek & Willebrand (J. Clim., 2005) North Atlantic model, northern boundary: 70°N forced by anomalies from IPCC climate runs MOC (40°N) Greenland- Scotland sill OGCM Climate Model (GFDL)  south 66° 70°N 1960 2000 2040 2080

  7. Role of changes in overflow vs. convection OGCM study by Schweckendiek & Willebrand (J. Clim., 2005) North Atlantic model: northern boundary: 70°N forced by anomalies from IPCC climate runs MOC (40°N) Greenland- Scotland sill OGCM Climate Model (GFDL)  south 66° 70°N 1960 2000 2040 2080 (1) The combination of surface forcing and overflow density changes (prescribed in sponge layer 66°-70°N) gives an almost perfect reconstruction of the coupled experiments

  8. Exps. (2) and (3) show: trend related to changing overflow density! 2 3

  9.  relation between overflow density and MOC … as shown by a host of modelcases with prescribed changes in the overflow density: Latif, Böning, Willebrand, et al. (J. Climate, 2006)

  10. Labrador Sea deep winter mixing Snapshot of mixed layer depth in March (1/12°-model) 1800 m 1000 200 Greenland important factors: - Surface heat flux (destabilising) - Eddy-flux of fresh water from West Greenland Current (stabilising)

  11. Case study: Permanent shut-down of deep convection Evolution of MOC anomalies (Hovmoeller diagram) Hüttl and Böning, 2006 (1/3°-model)

  12. Hindcast simulation with NCEP-heat flux MOC (43°N) follows the LSW formation rate with a lag of ~ 2 years (r = 0.71) MOC LSW formation 6 – 8 Sv change in LSW formation gives ~2 Sv change in MOC Böning et al., GRL 2006

  13. MOC response to NCEP heat flux variability Years of intense LSW production MOC anomalies (in Sv) in exp. „Heat“ 1 -1 Range of decadal MOC variability: 10-15% of mean transport C.I. = 0.2 Sv

  14. but these decadal MOC changes are masked… … by high-frequency wind-driven variability and eddy effects MOC anomalies related to LSW changes in isolation …and superimposed by wind-driven variability Model case including wind stress variability Biastoch et al. (J. Clim., 2008)

  15. Northern vs. Southern Influences deep-water formation areas Upper Branch North Atlantic Circulation [schematic by G. Holloway]

  16. specifically: Role of the Agulhas System? Approach: two-way nesting of high- resolution regional model (1/10°) in global model domain (1/2°) ORCA025: 25 km grid resolution AG01: 10 km grid resolution ORCA05: 50 km grid resolution Mozambique eddies Africa Agulhas rings Agulhas Current Temperature and velocity at 450m depth

  17. (I) Decadal variability signal induced in Agulhas regime:propagation by waves Boundary Waves Rossby Waves / Agulhas Rings Colour: eddy kinetic energy [Biastoch, Böning, Lutjeharms; Nature, 2008]

  18. Comparison of model runs with and without nest: MOC changes manifested in the NBC at 6°S Hovmoeller-plot: MOC-difference Agulhas-induced MOC-variability of +/- 1.5 Sv, rapidly propagating to the North Atlantic Biastoch, Böning, Lutjeharms (Nature, 2008)

  19. Agulhas-induced MOC variability vs. effect of LSW formation: • Agulhas influence reaches into the North Atlantic • … in tropics comparable to effect of subarctic deepwater formation Standard deviation of interannual MOC strength Complete Forcing Effect of Agulhas variability Effect of LSW [Biastoch, Böning, Lutjeharms; Nature, 2008]

  20. (II) Changes in Agulhas leakage: advective effects

  21. Large-scale Circulation Changes South of Africa Trends 1970s – 2000s: streamfunction wind stress curl …also seen in SSH Latitude of zero SSH: model and Aviso satellite altimetry • Gyre extended poleward due to the shift of the westerlies Biastoch, Böning, Schwarzkopf, Lutjeharms: Nature, 26 Nov 2009

  22.  Increase in Agulhas Leakage i.e., Lagrangian transport fraction across the Good Hope section: …during the recent decades (1.2 Sv/decade) GoodHope REF CLIM Biastoch, Böning, Schwarzkopf, Lutjeharms: Nature,26 Nov 2009

  23. Pathways of Agulhas Leakage Example trajectories of virtual floats released along the GoodHope section (T≥10°C) Pathway to the North Atlantic (upper limb of the MOC): - No change in volume transport - but: 25% decrease in freshwater flux [Biastoch et al., Nature, 26 Nov 2009]

  24. Observed salinities in the North Brazil Current … invasion of salty Indian Ocean waters  increasing salinity in the NBC near the equator Analysis of historic profiles in NBC core off South America Biastoch, Böning, Schwarzkopf, Lutjeharms: Nature, 26 Nov 2009

  25. Thermocline Changes in the Southern Hemisphere 2000-2004 minus 1968-1972 temperatures Upper ocean (0-200m) Zonally averaged over SW Indian Ocean (30°-50°E, isolines show mean temperatures) Biastoch, Böning, Schwarzkopf, Lutjeharms: Nature, 26 Nov 2009

  26. Nested Agulhas model • High-resolution nest simulates all salient features of the Agulhas Current system • The two-way nesting scheme allows to study the feedback of the Agulhas region on the global circulation • Agulhas leakage dynamics affects decadal variability in Atlantic MOC (wave process) • Mesoscale Agulhas variability has no effect on mean MOC • … but decadal MOC variations of ±1.5 Sv • … quickly reaches into northern hemisphere, with similar magnitude as sub-arctic deepwater formation events • Agulhas leakage change affects Atlantic THC (advective process) • Super-gyre has extended due to poleward shift of the westerlies • The Agulhas leakage has increased • … with no effect on time-mean MOC • … but a 25% increase of the salt export towards the North Atlantic

  27. Conclusions • Aaaa • Bbbb

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