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

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

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

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

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

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 ( ) ORCA05 AG01 Nested Agulhas Model

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

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

Signal Propagation Topographic Shelf Waves Rossby Waves / Agulhas Rings Large-scale horizontal circulation and eddy kinetic energy [Biastoch, Böning, Lutjeharms; Nature, 2008]

Agulhas Variability vs. Subpolar Deepwater Formation Complete Forcing Effect of Agulhas mesoscale variability Effect of heat flux variability Standard deviation of interannual MOC strength  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 Latitude of zero SSH in model and Aviso satellite altimetry Time-mean horizontal streamfunction Zonal averages of streamfunction and wind stress in the 1970s (dashed) and 2000s (solid)  Super-gyre extended poleward due to the shift of the westerlies [Biastoch, Böning, Schwarzkopf, Lutjeharms; Nature, 2009]

Increase in Agulhas Leakage Fractional Agulhas transport crossing GoodHope section trend : 1.2 Sv/decade GoodHope  The Agulhas leakage has increased due to the poleward shift of the westerlies [Biastoch, Böning, Schwarzkopf, Lutjeharms; Nature, 2009] [Van Sebille et al; GRL, 2009]

Pathways of Agulhas Leakage Example trajectories of virtual floats released along the GoodHope section Analysis of historic profiles in NBC core off South America  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  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

References Biastoch, Böning, Schwarzkopf & Lutjeharms, 2009: Increase in Agulhas leakage due poleward shift in the southern hemisphere westerlies, Nature, 462, Biastoch, Böning & Lutjeharms, 2008: Agulhas leakage dynamics affects decadal variability in Atlantic overturning circulation, Nature, 456, Biastoch, Beal, Lutjeharms & Casal, 2009: Variability and coherence of the Agulhas Undercurrent in a High-resolution Ocean General Circulation Model, J. Phys. Oceanogr., 39, Biastoch, Lutjeharms, Böning & Scheinert, 2008: Mesoscale perturbations control inter-ocean exchange south of Africa, Geophys. Res. Lett. 35, L 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. 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