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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. Motivation: The importance of the Southern Ocean for AMOC. The Southern Ocean plays a key
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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
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?
“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)
“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
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
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)
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)
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)