Agulhas Leakage The Neglected Player in the Variability

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

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

Influences from the Sub-arctic Atlantic deep-water formation areas North Atlantic Circulation [schematic by G. Holloway]

Effect of Atmospheric Forcing on MOC in the North Atlantic Labrador Sea Water formation events (high NAO) HEAT+SALT REF Time-mean Atlantic MOC WIND MOC anomalies at 1000 m in different experiments [Biastoch, Böning, Getzlaff, Molines, Madec; J. Clim, 2008]

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

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

Nested Agulhas Model DRAKKAR Hierarchy, based on NEMO v 2. 3 [Madec, 2006] • Tripolar grid, coupled configuration with 2 -way interaction between Ø Global coarse-resolution model (1/2°) Ø 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, 6 h/1 d-resolution, inter-annual variability (1958 -2004) ORCA 05 AG 01

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

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

Agulhas-Induced MOC Variability Difference in MOC and North Brazil Current at 6°S Interannually filtered MOC-difference at 1000 m: Exp. with minus without Agulhas nest Ø MOC shows decadal variability ± 1. 5 Sv rapid communication to the North Atlantic [Biastoch, Böning, Lutjeharms; Nature, 2008]

Signal Propagation Topographic Shelf Waves Rossby W aves / A gulhas R ings Large-scale horizontal circulation and eddy kinetic energy [Biastoch, Böning, Lutjeharms; Nature, 2008]

Agulhas Variability vs. Subpolar Deepwater Formation Complete Forcing Effect of heat flux variability Effect of Agulhas mesoscale variability Standard deviation of interannual MOC strength Ø Agulhas influence reaches into northern hemisphere [Biastoch, Böning, Lutjeharms; Nature, 2008]

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

Large-scale Circulation Changes South of Africa Time-mean horizontal streamfunction Zonal averages of streamfunction and wind stress in the 1970 s (dashed) and 2000 s (solid) Latitude of zero SSH in model and Aviso satellite altimetry Ø Super-gyre extended poleward due to the shift of the westerlies [Biastoch, Böning, Schwarzkopf, Lutjeharms; Nature, 2009]

Increase in Agulhas Leakage Good. Hope trend : 1. 2 Sv/decade Fractional Agulhas transport crossing Good. Hope section [Van Sebille et al; GRL, 2009] Ø The Agulhas leakage has increased due to the poleward shift of the westerlies [Biastoch, Böning, Schwarzkopf, Lutjeharms; Nature, 2009]

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

Nested Agulhas model Ø High-resolution nest realistically simulates the Agulhas Current system Ø Two-way nesting scheme feedback of the Agulhas region on the global circulation Wave process Ø Mesoscale Agulhas leakage dynamics introduces decadal MOC variations of ± 1. 5 Sv Ø … quickly reaches into northern hemisphere, with similar magnitude as sub-arctic deepwater formation events Advective process Ø Super-gyre has extended due to poleward shift of the westerlies Ø The Agulhas leakage has increased Ø … leading to a 25% increase of the salt export towards the North Atlantic

PO 33 E. 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: Agulhas leakage dynamics affects decadal variability in Atlantic overturning circulation, 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 perturbations control inter-ocean exchange south of Africa, Geophys. Res. Lett. 35, L 20602. • Biastoch, Böning, Getzlaff, Molines & Madec, 2008: Causes of interannual – decadal variability in the meridional overturning circulation of the mid-latitude North Atlantic Ocean, J. Clim. , 21, 6599– 6615.