Role of the Southern Ocean in controlling the Atlantic meridional overturning circulation

1. Role of the Southern Ocean in controlling the Atlantic meridional overturning circulation Igor Kamenkovich RSMAS, University of Miami, Miami Timour Radko Naval Postgraduate School, Monterey

2. Motivation: The importance of the Southern Ocean for AMOC • The Southern Ocean plays a key role in the global ocean circulation • It is an origin and mixer for several important water masses • Its Antarctic Circumpolar Current (ACC) acts as a connector between oceanic basins • Several Southern Ocean processes are known to affect AMOC: • Winds (e.g. Toggweiler and Samuels 1995; McDermott 1996) • Mesoscale eddies (e.g. Gnanadesikan et al. 2003; Kamenkovich and Sarachik 2004) • Surface buoyancy fluxes (e.g. Hasumi and Suginohara 1999, Saenko et al. 2003 ) • All these processes determine the orientation of ACC isopycnals • To what degree AMOC is controlled by the ACC stratification?

3. “Pacific” “Atlantic” “ACC” Numerical Models. Global configuration • The numerical model (based on GFDL MOM, z-coordinate model): • Intermediate resolution (0.5-1 degree) • Highly idealized geometry (global and Atlantic-only) • Depth is 3km (no AABW) • Vertical diffusion is 10-5 m2sec-1 Maximum is 20.2 Sv AMOC in the global model: Streamfunction (Sv)

4. “Atlantic” Atlantic-only configuration. “Free run” • Next consider the Atlantic-only model • The surface forcing and all parameters in the Atlantic are the same as in the global model • The difference is in the lack of: • constraint of ACC on Atlantic isopycnals • volume and buoyancy exchanges between Atlantic and ACC Maximum is 7.6 Sv Atlantic-only models cannot reproduce AMOC of realistic strength unless unrealistically strong diapycnal diffusion is used AMOC in the Atlantic-only model

5. Control of AMOC by ACC isopycnals • Decouple Atlantic-ACC system: Fix the stratification and parameterized eddy buoyancy fluxes at the Atlantic southern boundary to the values from the global run • Two experiments differ by the prescribed fields: • Experiment 1: Full 2D density (and geostrophic velocities) and fluxes • Experiment 2: Zonally averaged density and no fluxes 20.6 Sv 19 Sv Experiment 2: Zonal-mean density is prescribed Experiment 1: 2D density + fluxes are prescribed

6. Effects on density • Atlantic stratification is very similar between the global simulation and “uncoupled” Experiments 1 and 2 • Isopycnals are significantly shallower in the “free” Atlantic-only run Water is circulating along approximately the same isopycnals in all 3 simulations Shallower isopycnals correspond to weaker pressure gradients and thinner AMOC cell Atlantic isopycnals in the global run (black), Experiment 2 (red) and the “free” Atlantic-only run (blue)

7. Response to a buoyancy anomaly • Experiment GW1: Adding a buoyancy anomaly in the North Atlantic slows AMOC down to 8.4 Sv in the global model • Experiment GW2: If, in the Atlantic-only model, the ACC density is prescribed to its values in the standard simulation, AMOC becomes even weaker (3.9 Sv) Overturning takes place at lighter isopycnals Experiment GW1: Isopycnals deepen as the surface density decreases (blue contours) Experiment GW2: isopycnals (red) are shallower than in the global run => AMOC is even weaker Atlantic isopycnals in the control run (black), Experiment GW1 (blue) and Experiment GW2 (red)

8. Summary and Conclusions • Stratification in ACC has significant control of AMOC • The depth of the Atlantic isopycnals that outcrop in ACC is to a large degree determined at the southern boundary of the Atlantic • The effects of the Ekman transport into the Atlantic and volume/buoyancy exchanges between the Atlantic and ACC on AMOC are indirect • Decrease in AMOC caused by a buoyancy anomaly in the North Atlantic is greater if the ACC density is held constant => • Delayed response of the Southern Ocean to global warming can amplify the resulting AMOC weakening We acknowledge the support by the National Science Foundation (OCE)