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)