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
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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?
Numerical Models. Global configuration
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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
“Pacific”
“Atlantic”
“ACC”
AMOC in the global model:
Streamfunction (Sv)
Maximum is
20.2 Sv
Atlantic-only configuration. “Free run”
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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
“Atlantic”
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
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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 1: 2D density +
fluxes are prescribed
Experiment 2: Zonal-mean
density is prescribed
Effects on density
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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
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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)
Atlantic isopycnals in the control run (black),
Experiment GW1 (blue) and Experiment GW2 (red)
Experiment GW2:
isopycnals (red) are
shallower than in the
global run => AMOC is
even weaker
Summary and Conclusions
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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)