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Motivation

Agulhas Leakage: The Neglected Player in the Variability of the Atlantic Meridional Overturning Circulation Arne Biastoch In collaboration with Claus Böning and Johann Lutjeharms (Univ. Cape Town)

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Motivation

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  1. Agulhas Leakage: The Neglected Player in the Variability of the Atlantic Meridional Overturning Circulation Arne Biastoch In collaboration with Claus Böning and Johann Lutjeharms (Univ. Cape Town) Thanks to: Lisa Beal (RSMAS), Erik van Sebille (RSMAS), Franziska Schwarzkopf, Markus Scheinert, Gerold Siedler, Erik Behrens, DRAKKAR, NEMO, AGRIF and ARIANE System Teams

  2. Motivation Meridional Overturning Circulation (MOC)as an important measure of poleward transport, e.g. of heat in the subtropical North Atlantic North AtlanticCirculation [schematicby G. Holloway]

  3. Influencesfromthe Sub-arcticAtlantic deep-waterformationareas North AtlanticCirculation [schematicby G. Holloway]

  4. EffectofAtmosphericForcing on MOC in the North Atlantic Labrador SeaWaterformationevents(high NAO) HEAT+SALT REF Time-meanAtlantic MOC WIND MOC anomaliesat 1000m in different experiments [Biastoch, Böning, Getzlaff, Molines, Madec; J. Clim, 2008]

  5. Northern vs. Southern Influences deep-waterformationareas UpperBranch North AtlanticCirculation [schematicby G. Holloway]

  6. The Agulhas System as a Key Region of the Global Oceanic Circulation Warm water return path of the thermohaline circulation  key role in global climate (change) Gordon (2003)

  7. NestedAgulhas Model • DRAKKAR Hierarchy, based on NEMO v2.3 [Madec, 2006] • Tripolar grid, coupled configuration with 2-way interaction between • Global coarse-resolution model (1/2°) • Regional high-resolution model (1/10° ) • 46 vertical levels, partial cell topography • State-of-the-art physics / parameterizations • Thermodynamic-dynamic sea-ice model • 20-yr spinup with global model • O(50-yr) atmospheric forcing: NCEP/NCAR-derived (“CORE”), applied via Bulk formulae, 6h/1d-resolution, inter-annual variability (1958-2004) AG01 ORCA05

  8. I. Dynamical effect of the Agulhas mesoscale on the MOC: Wave Process [Biastoch, Böning, Lutjeharms; Nature, 2008]

  9. Goal: to isolate the net effect of the mesoscale Agulhas variability on the large-scale circulation  need two-way nesting Global Model withtwo-way Nest Global Model without Nest

  10. Agulhas-Induced MOC Variability Difference in MOC and North Brazil Current at 6°S Interannually filtered MOC-difference at 1000m: Exp. with minus without Agulhas nest • MOC shows decadal variability ±1.5 Sv rapid communication to the North Atlantic [Biastoch, Böning, Lutjeharms; Nature, 2008]

  11. Signal Propagation TopographicShelfWaves RossbyWaves / Agulhas Rings Large-scale horizontal circulationandeddykineticenergy [Biastoch, Böning, Lutjeharms; Nature, 2008]

  12. AgulhasVariability vs. Subpolar Deepwater Formation CompleteForcing Effectofheatfluxvariability EffectofAgulhas mesoscale variability Standard deviationofinterannual MOC strength • Agulhas influence reaches into northern hemisphere [Biastoch, Böning, Lutjeharms; Nature, 2008]

  13. II. Changes in Agulhas leakage and consequences for the MOC: Advective Process

  14. Large-scale Circulation Changes South of Africa Time-mean horizontal streamfunction Zonal averagesofstreamfunctionand wind stress in the 1970s (dashed) and 2000s (solid) Latitudeofzero SSH in model and Aviso satellitealtimetry • Super-gyre extended poleward due to the shift of the westerlies [Biastoch, Böning, Schwarzkopf, Lutjeharms; Nature, 2009]

  15. Increase in AgulhasLeakage GoodHope trend : 1.2 Sv/decade FractionalAgulhastransportcrossing GoodHope section [Van Sebille et al; GRL, 2009] • The Agulhas leakage has increased due to the poleward shift of the westerlies [Biastoch, Böning, Schwarzkopf, Lutjeharms; Nature, 2009]

  16. Pathways of Agulhas Leakage Analysis of historic profiles in NBC core off South America Example trajectories of virtual floats released along the GoodHope section • Increased Agulhas leakage leads to a 25% increase of the salt export towards the North Atlantic [Biastoch, Böning, Schwarzkopf, Lutjeharms; Nature, 2009]

  17. Nested Agulhas model • High-resolution nest realistically simulates the Agulhas Current system • Two-way nesting scheme  feedback of the Agulhas region on the global circulation • Wave process • Mesoscale Agulhas leakage dynamics introduces decadal MOC variations of ±1.5 Sv • … quickly reaches into northern hemisphere, with similar magnitude as sub-arctic deepwater formation events • Advective process • Super-gyre has extended due to poleward shift of the westerlies • The Agulhas leakage has increased • … leading to a 25% increase of the salt export towards the North Atlantic

  18. PO33E. The Greater Agulhas Current System and Its Key Role in Past, Present, and Future Climate  1-3 p.m. today The Climatic Importance of the Greater Agulhas System (SCOR WG 136)  Townhall meeting tomorrow 11:45-12:45 • References • Biastoch, Böning, Schwarzkopf & Lutjeharms, 2009: Increase in Agulhas leakage due poleward shift in the southern hemisphere westerlies, Nature, 462, 495-498. • Biastoch, Böning & Lutjeharms, 2008: Agulhasleakagedynamicsaffectsdecadalvariability in Atlanticoverturningcirculation, Nature , 456, 489-492. • Biastoch, Beal, Lutjeharms & Casal, 2009: Variability and coherence of the Agulhas Undercurrent in a High-resolution Ocean General Circulation Model, J. Phys. Oceanogr., 39, 2417-2435. • Biastoch, Lutjeharms, Böning & Scheinert, 2008: Mesoscale perturbationscontrol inter-oceanexchangesouthofAfrica, Geophys. Res. Lett. 35, L20602. • Biastoch, Böning, Getzlaff, Molines & Madec, 2008: Causesofinterannual – decadalvariability in the meridional overturningcirculationofthemid-latitude North Atlantic Ocean, J. Clim., 21, 6599–6615.

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