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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 - PowerPoint PPT Presentation
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Role of the Southern Ocean in Role of the Southern Ocean in controlling the Atlantic meridional controlling the Atlantic meridional
overturning circulationoverturning circulation
Igor KamenkovichIgor Kamenkovich RSMAS, University of Miami, MiamiRSMAS, University of Miami, Miami
Timour Radko Timour Radko Naval Postgraduate School, MontereyNaval Postgraduate School, Monterey
Motivation: The importance of the Southern Ocean for Motivation: The importance of the Southern Ocean for AMOCAMOC
The Southern Ocean plays a key The Southern Ocean plays a key role in the global ocean circulationrole in the global ocean circulation
It is an origin and mixer for severalIt is an origin and mixer for severalimportant water massesimportant water masses
Its Antarctic Circumpolar CurrentIts Antarctic Circumpolar Current(ACC) acts as a connector between(ACC) acts as a connector betweenoceanic basinsoceanic basins
Several Southern Ocean processes are known to affect AMOC:Several Southern Ocean processes are known to affect AMOC:• Winds (e.g. Toggweiler and Samuels 1995; McDermott 1996)Winds (e.g. Toggweiler and Samuels 1995; McDermott 1996)• Mesoscale eddies (e.g. Gnanadesikan et al. 2003; Kamenkovich and Mesoscale eddies (e.g. Gnanadesikan et al. 2003; Kamenkovich and
Sarachik 2004)Sarachik 2004)• Surface buoyancy fluxes (e.g. Hasumi and Suginohara 1999, Saenko et Surface buoyancy fluxes (e.g. Hasumi and Suginohara 1999, Saenko et
al. 2003 )al. 2003 )
All these processes determine the orientation of ACC isopycnalsAll these processes determine the orientation of ACC isopycnals To what degree AMOC is controlled by the ACC stratification?To what degree AMOC is controlled by the ACC stratification?
Numerical Models. Global configurationNumerical Models. Global configuration The numerical model (based on GFDL MOM, z-coordinate model): The numerical model (based on GFDL MOM, z-coordinate model):
• Intermediate resolution (0.5-1 degree)Intermediate resolution (0.5-1 degree)• Highly idealized geometry (global and Atlantic-only)Highly idealized geometry (global and Atlantic-only)• Depth is 3km (no AABW)Depth is 3km (no AABW)• Vertical diffusion is 10Vertical diffusion is 10-5-5 m m22secsec-1-1
AMOC in the global model: Streamfunction (Sv)
“Pacific” “Atlantic”
“ACC”
Maximum is 20.2 Sv
Atlantic-only configuration. Atlantic-only configuration. ““Free runFree run”” Next consider the Atlantic-only model Next consider the Atlantic-only model The surface forcing and all parameters in the Atlantic The surface forcing and all parameters in the Atlantic
are the are the samesame as in the global model as in the global model
The The differencedifference is in the lack of: is in the lack of:• constraint of ACC on Atlantic isopycnalsconstraint of ACC on Atlantic isopycnals• volume and buoyancy exchanges between volume and buoyancy exchanges between
Atlantic and ACCAtlantic and ACC
AMOC in the Atlantic-only model
“Atlantic”
Maximum is 7.6 Sv
Atlantic-only models cannot reproduce AMOC of realistic
strength unless unrealistically strong diapycnal diffusion is used
Control of AMOC by ACC isopycnalsControl of AMOC by ACC isopycnals DecoupleDecouple Atlantic-ACC system: Atlantic-ACC system:
Fix the stratification and parameterized eddy buoyancy fluxes at the Atlantic Fix the stratification and parameterized eddy buoyancy fluxes at the Atlantic southern boundary to the values from the southern boundary to the values from the global runglobal run
Two experiments differ by the prescribed fields:Two experiments differ by the prescribed fields:• Experiment 1Experiment 1: Full 2D density (and geostrophic velocities) and fluxes: Full 2D density (and geostrophic velocities) and fluxes• Experiment 2Experiment 2: Zonally averaged density and no fluxes: Zonally averaged density and no fluxes
Experiment 2: Zonal-mean density is prescribed
Experiment 1: 2D density + fluxes are prescribed
20.6 Sv
19 Sv
Effects on densityEffects on density Atlantic stratification is very similar between the global simulation and Atlantic stratification is very similar between the global simulation and
““uncoupleduncoupled”” Experiments 1 and 2 Experiments 1 and 2
Isopycnals are significantly shallower in the Isopycnals are significantly shallower in the ““freefree”” Atlantic-only run 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 anomalyResponse to a buoyancy anomaly Experiment GW1Experiment GW1: Adding a buoyancy anomaly in the North Atlantic slows : Adding a buoyancy anomaly in the North Atlantic slows
AMOC down to 8.4 Sv in the global modelAMOC down to 8.4 Sv in the global model
Experiment GW2Experiment GW2: If, in the Atlantic-only model, the ACC density is : If, in the Atlantic-only model, the ACC density is prescribed to its values in the standard simulation, AMOC becomes even prescribed to its values in the standard simulation, AMOC becomes even weaker (3.9 Sv)weaker (3.9 Sv)
Experiment GW2: Experiment GW2: isopycnals (isopycnals (redred) are ) are shallower than in the shallower than in the
global run => AMOC is global run => AMOC is even weakereven weaker
Experiment GW1: Experiment GW1: Isopycnals deepen as the Isopycnals deepen as the
surface density surface density decreases (decreases (blue contoursblue contours))
Overturning Overturning takes place at takes place at lighter isopycnalslighter isopycnals
Atlantic isopycnals in the control run (black), Experiment GW1 (blue) and Experiment GW2 (red)
Summary and ConclusionsSummary and Conclusions
Stratification in ACC has significant control of AMOCStratification in ACC has significant control of AMOC
The depth of the Atlantic isopycnals that outcrop in ACC is to a large degree The depth of the Atlantic isopycnals that outcrop in ACC is to a large degree determined at the southern boundary of the Atlanticdetermined at the southern boundary of the Atlantic
The effects of the Ekman transport into the Atlantic and volume/buoyancy The effects of the Ekman transport into the Atlantic and volume/buoyancy exchanges between the Atlantic and ACC on AMOC are indirectexchanges between the Atlantic and ACC on AMOC are indirect
Decrease in AMOC caused by a buoyancy anomaly in the North Atlantic is Decrease in AMOC caused by a buoyancy anomaly in the North Atlantic is greater if the ACC density is held constant =>greater if the ACC density is held constant =>
Delayed response of the Southern Ocean to global warming can amplify the Delayed response of the Southern Ocean to global warming can amplify the resulting AMOC weakeningresulting AMOC weakening
We acknowledge the support by theWe acknowledge the support by the National Science Foundation National Science Foundation (OCE) (OCE)