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

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

26°N 1.6 Heat transport (PW) MOC (Sv) 10 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 MOC (in depth-coordinates) is a useful index only south of ~43°N 20°S 0° 20° 40° 60°N 1r01r0 MOC and heat transport … but caution: Correlation based on monthly and 2-yr-filtered time series time series

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

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

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

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

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

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

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

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

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

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

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

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

(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]

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

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

(II) Changes in Agulhas leakage: advective effects

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

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

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]

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

Thermocline Changes in the Southern Hemisphere minus 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

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

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