sensitivity of the atlantic thermohaline circulation and ...€¦ · increase north atlantic deep...

Sensitivity of the Atlantic Thermohaline Circulation and Its Stability to Basin-ScaleVariations in Vertical Mixing

WILLEM P. SIJP AND MATTHEW H. ENGLAND

Climate and Environmental Dynamics Laboratory, School of Mathematics and Statistics, University of New South Wales, Sydney,New South Wales, Australia

(Manuscript received 6 September 2005, in final form 2 February 2006)

ABSTRACT

This study shows that a reduction in vertical mixing applied inside the Atlantic basin can drasticallyincrease North Atlantic Deep Water (NADW) stability with respect to freshwater perturbations applied tothe North Atlantic. This is contrary to the notion that the stability of the ocean’s thermohaline circulationsimply scales with vertical mixing rates. An Antarctic Intermediate Water (AAIW) reverse cell, reliantupon upwelling of cold AAIW into the Atlantic thermocline, is found to be associated with stable stateswhere NADW is collapsed. Transitions between NADW “on” and “off” states are characterized by inter-hemispheric competition between this AAIW cell and the NADW cell. In contrast to the AAIW reversecell, NADW eventually upwells outside the Atlantic basin and is thus not as sensitive to changes in verticalmixing within the Atlantic. A reduction in vertical mixing in the Atlantic weakens the AAIW reverse cell,resulting in an enhanced stability of NADW formation. The results also suggest that the AAIW reverse cellis responsible for the stability of NADW collapsed states, and thereby plays a key role in maintainingmultiple equilibria in the climate system. A global increase of vertical mixing in the model results insignificantly enhanced NADW stability, as found in previous studies. However, an enhancement of verticalmixing applied only inside the Atlantic Ocean results in a reduction of NADW stability. It is concluded thatthe stability of NADW formation to freshwater perturbations depends critically on the basin-scale distri-bution of vertical mixing in the world’s oceans.

1. Introduction

Stommel (1961) first proposed the possibility that theocean’s thermohaline circulation could have two stableregimes of flow. Bryan (1986) demonstrated how theoperation of a positive salt feedback could bring abouttwo asymmetric overturning states under symmetricsurface flux conditions in a rectangular basin geometry.In his idealized model, there is competition for inter-hemispheric freshwater (FW) export between two over-turning cells involving the sinking of water originatingat the antipodean polar surface of the basin. The realocean, however, contains an obstruction to southwardgeostrophic flow across the latitudes of Drake Passage(DP), thus inhibiting the formation of the vigorousSouthern Hemisphere (SH) cell observed by Bryan(1986). Instead, studies employing a more realistic ge-

ometry (e.g., Manabe and Stouffer 1988, 1999, hereaf-ter MS99; Saenko et al. 2003; Gregory et al. 2003;Rahmstorf 1996; Sijp and England 2005) find that SHoverturning states exhibit a shallower Antarctic Inter-mediate Water (AAIW) reverse cell inside the Atlanticbasin. MS99 point out that this small reverse cell isassociated with a stable North Atlantic Deep Water(NADW) “off” state. In contrast to the SH cell ofBryan (1986) and other studies employing a Drake Pas-sage closed geometry (e.g. Mikolajewicz et al. 1993; Sijpand England 2004), this AAIW reverse cell originatesat the AAIW formation regions. Saenko et al. (2003)use a model to suggest that due to the existence of thiscell in an NADW off state, the density difference betweensurface waters at the formation regions for AAIW andNADW determines the strength and polarity of the me-ridional overturning circulation (MOC), at least in theirmodel. Sijp and England (2005) show that the nature ofthis relationship depends on the depth of the DrakePassage sill, with Antarctic Bottom Water (AABW)playing a reduced role as the DP deepens. Further-more, Saenko et al. (2003) and Gregory et al. (2003)

Corresponding author address: Willem P. Sijp, School of Math-ematics and Statistics, University of New South Wales, Sydney,NSW 2052, Australia.E-mail: [email protected]

VOLUME 19 J O U R N A L O F C L I M A T E 1 NOVEMBER 2006

© 2006 American Meteorological Society 5467

JCLI3909

suggest that this AAIW reverse cell can contribute tomaintaining a stable NADW off state by importing FWinto the Atlantic basin, and that it cannot coexist withthe NADW formation cell.

In ocean models the upwelling branches of theNADW cell and the AAIW reverse cell rely in part onvertical mixing rates at lower latitudes. This upwellinginto the thermocline consists of a balance betweendownward diffusion of heat from the surface and up-ward advection of colder water from below the ther-mocline (e.g., Munk 1966; Munk and Wunsch 1998), thebalance determining the global thermocline depth. En-hanced vertical mixing therefore increases transport inthe upwelling branches of these cells. Indeed, Bryan(1987) found that larger vertical mixing rates lead toincreased MOC in an idealized sector geometry model.There have been several other studies that investigatethe role of vertical mixing in setting the thermohalinecirculation (THC) strength in global ocean climatemodels. MS99 showed that the stability of their NADWoff state depends on the value of vertical mixing. Theyalso find enhanced NADW formation and greaterNADW stability with respect to FW perturbations un-der a global increase in vertical mixing. Studies employ-ing a zonally averaged model (Schmittner and Weaver2001) and a three-dimensional ocean model (Prange etal. 2003) use hysteresis experiments to show that thestability of the NADW off and “on” states depend onthe rate of vertical mixing. For instance, in agreementwith MS99, Schmittner and Weaver (2001) find en-hanced NADW stability in their hysteresis experimentswhen vertical mixing is increased globally. These stud-ies, however, employ horizontally uniform vertical mix-ing modifications. In the present study, we will assessthe stability of the North Atlantic MOC with respect tobasinwide changes in the vertical mixing coefficient(K�).

The AAIW reverse cell of the NADW off state relieson upwelling inside the Atlantic basin, whereas in theNADW on state, NADW outflow leaves the Atlanticbasin at 30°S to upwell predominantly outside this ba-sin. Broecker (1991) highlights the role of the Indianand Pacific Oceans in the removal of NADW by verti-cal mixing at low latitudes in the classical “global oceanconveyor belt” schematic. Later studies (e.g., Togg-weiler and Samuels 1995; Döös 1995) suggest insteadthat a significant portion of NADW resurfaces viawind-driven upwelling in the Southern Ocean, under-going subsequent buoyancy changes due to surfacefluxes. Model studies employing weak background ver-tical mixing also support this idea (e.g., Saenko andMerryfield 2005). Another example of how a reason-able overturning and a thermocline of realistic depth

can be achieved without the need for strong verticaldiffusivity is described in the theory of the ventilatedthermocline (Luyten et al. 1983).

As the upwelling branches of the NADW cell and theAAIW reverse cell occur at different locations, the spa-tial distribution of vertical mixing in the World Oceanwill likely affect the relative potential of these cells fordominance of deep ocean ventilation. In particular, theAtlantic AAIW reverse cell relies upon the removal ofintermediate water across the Atlantic thermocline viadiapycnal mixing. Hence, a reduction in vertical mixingat low latitudes of the Atlantic should reduce thestrength of the AAIW reverse cell and therefore itsability to compete with NADW. The NADW cell, onthe other hand, does not rely on Atlantic upwelling andmay therefore be less sensitive to a reduction of verticalmixing inside the Atlantic. In this situation, a greaterrobustness of NADW overturning with respect to FWperturbations may ensue. In this study, we use an in-termediate-complexity coupled climate model to exam-ine the stability of NADW formation with respect toFW perturbations under different spatial distributionsof vertical mixing.

The remainder of this paper is divided as follows.Section 2 covers a description of the model and experi-mental design. We will consider experiments where K�

is changed only inside the Atlantic Ocean to examinethe role of the AAIW reverse cell in suppressingNADW formation. We will also assess experimentswherein a reduced K� is applied only inside the Indianand Pacific Oceans. To compare our findings to previ-ous studies, we further present an experiment similar toMS99 where K� is increased globally, and an experi-ment where K� is increased by a similar magnitude, butonly inside the Atlantic. In section 3 we first discuss thesteady state fields under the different vertical mixingscenarios, and then describe their response to the ap-plication of external FW pulses of varying magnitude.Finally, section 4 covers a discussion and the conclu-sions.

2. Model and numerical experiments

We use the University of Victoria Intermediate Com-plexity Coupled Model described in detail in Weaver etal. (2001), which comprises a global ocean general cir-culation model [Geophysical Fluid Dynamics Labora-tory (GFDL) Modular Ocean Model (MOM) Version2.2; Pacanowski 1995] coupled to a simplified atmo-spheric model and a dynamic–thermodynamic sea icemodel. A global domain is used with horizontal reso-lution of 1.8° latitude by 3.6° longitude in each modelcomponent. While air–sea heat and freshwater fluxes

5468 J O U R N A L O F C L I M A T E VOLUME 19

evolve freely in the model, a noninteractive wind fieldis employed. FW fluxes between the ocean and the at-mosphere are determined by evaporation and precipi-tation, while river runoff and changes in sea ice volumealso affect oceanic salinity. Moisture transport in theatmosphere occurs by way of advection and diffusion.Precipitation occurs when relative humidity exceeds athreshold value. The wind forcing is taken from theNational Centers for Environmental Prediction–NationalCenter for Atmospheric Research (NCEP–NCAR) re-analysis fields (Kalnay et al. 1996), averaged over theperiod 1958–97 to form a seasonal cycle from themonthly fields. Brine rejection during sea ice formationis parameterized after Duffy and Caldeira (1997). Theatmospheric model advects and diffuses moisture, andit diffuses heat. No flux corrections are used (for furtherdetails, see Weaver et al. 2001). Vertical mixing in thecontrol experiment is achieved using a horizontally uni-form diffusivity that increases with depth, taking avalue of 0.6 cm2 s�1 at the surface and increasing to 1.6cm2 s�1 at the bottom. The effect of subgrid-scale ed-dies on horizontal tracer transport is modeled by glo-bally uniform horizontal diffusion. The diffusion coef-ficient in both horizontal directions is Ah � 2 � 107

cm2 s�1. The constant lateral mixing coefficient for mo-mentum is 2 � 109 cm2 s�1, and the constant verticalmixing coefficient for momentum is 10 cm2 s�1. Foreconomy of computation, with many multimillennial in-tegrations, no parameterization of along-isopycnal dif-fusion (Redi 1982) or eddy-induced advective tracertransport (Gent and McWilliams 1990, hereafter GM)is used in the standard set of experiments. Rather, ourfocus is on model THC sensitivity to vertical diffusivemixing. It is noted, however, that we have reevaluatedseveral of the experiments under GM and confirm thatour results are robust in both GM and non-GM experi-ments.

To examine the effect of reducing K� in differentocean basins, we have integrated the following configu-rations of the model to equilibrium: 1) a control experi-ment CNTRL, where no reduction in K� is applied; 2)an experiment denoted 2

3KVAtl, where K� takes a value

reduced to two-thirds of CNTRL inside the Atlanticbasin; 3) an experiment denoted 1

3KVAtl, where K� is

multiplied inside the Atlantic basin by a factor of 1/3;and 4) an experiment where K� is similarly reducedinside the Pacific and Indian basins, denoted 1

3KVIP.

Figure 1 shows the vertical profile of K� in these ex-periments. Figure 2a shows the two areas where reduc-tion of K� is applied in the respective experiments. Thereduction is only applied between 35°S and 48°N ineach ocean basin. The above reductions in K� are alsoonly applied between the surface and 1257-m depth,

with no change in K� at levels deeper than 1257 m (Fig.1). A version of 1

3KVAtl using GM has been run to verify

the robustness of our results with respect to the choiceof parameterization of subgrid-scale eddy mixing oftracer properties.

To further examine whether our model exhibits anincrease in NADW stability upon increasing K� globallyat a depth range containing the pycnocline, as found byMS99, we have integrated an experiment MS99KVglob

similar to their so-called large vertical diffusion (LVD)experiment, where K� takes an increased global valueof 1.3 cm2 s�1 down to 2500-m depth (see Fig. 1). Below2500-m depth K� follows the profile of CNTRL. TheMS99 LVD experiment employs constant K� � 1.3cm2 s�1; however, such a profile would yield a decreasein K� at depth compared to CNTRL. Hence, the profileshown in Fig. 1 is adopted for direct comparison withthe CNTRL experiment. To elucidate the role of ver-tical mixing inside the Atlantic in the context of theexperiments of MS99, we have also integrated an ex-periment MS99KVAtl, where K� is changed in a similarfashion to MS99KVglob, but only inside the Atlantic.The above experiments should yield similar results toexperiments using the actual MS99 profile with K� �1.3 cm2 s�1 throughout the water column, as diapycnalmixing inside the pycnocline, rather than abyssal mix-ing, is the main factor affecting the strength of NADWformation. To confirm this, we have run experimentsusing a value of 1.3 cm2 s�1 K� throughout the water

FIG. 1. Vertical mixing profile of CNTRL, 2

3KVAtl, (1/3)KVAtl

[equivalent to that used in 1

3KVIP (solid) only over a different

subdomain], and the MS99KV experiments.

1 NOVEMBER 2006 S I J P A N D E N G L A N D 5469

column, as in MS99, and found results that are verysimilar to those of MS99KVglob and MS99KVAtl.Hence, only the one set of experiments with increasedK� are reported here.

We further subject the obtained equilibria for CNTRL,23KVAtl,

13KVAtl,

13KVIP, MS99KVglob and MS99KVAtl to

FW pulses applied to the North Atlantic (NA) for theduration of 300 yr. We vary the maximum magnitude ofthis pulse. In short, we vary both K� and the magnitudeof the FW pulse in this study, and all other model pa-rameters remain fixed. The FW perturbations are ap-plied in the NA between the black lines shown in Fig.2a. Figure 2b shows the FW perturbation against time.A linear increase from 0 to a maximum value M occursover the first 150 yr, followed by a linear decline backto 0 over the following 150 yr. After year 300, no fur-ther FW perturbation is applied. Note that we applythis perturbation for several values of M. This proce-dure, similar to that of Sijp and England (2005), is de-signed to examine the existence of multiple equilibria inthe respective experiments.

The ocean exhibits substantial regional variations inK� (e.g., Ledwell et al. 2000), thought to be a functionof tidal currents, subsurface bathymetry, and localstratification. Several studies have examined the effects

of locally enhanced vertical diffusivity over roughbathymetry in ocean models (e.g., Hasumi and Sugino-hara 1999; Saenko and Merryfield 2005). Unlike theseprevious studies, we make no attempt to examine theeffects of contrasts in vertical mixing that may existbetween basins in the real ocean. Instead our experi-ments are specifically aimed to examine the role of thelocation of deepwater removal in determining the glob-al THC and its stability.

3. Results

a. Equilibrated states

In this section we first analyze the steady states of theexperiments. After discussing these equilibria we willpresent a series of perturbations to these steady stateswhere the maximum value of the perturbation varies(section 3b). Figure 3 shows the MOC in the Atlanticbasin for the NADW on states and NADW off states ofexperiments CNTRL and 2

3KVAtl. Also shown are the

steady NADW on state of 13KVAtl and the only steady

state found for 13KVIP. A summary of the MOC trans-

port rates for all experiments is given in Table 1. TheNADW on state in CNTRL exhibits 21.5 Sv (1 Sv �106 m3 s�1; throughout this paper all transport valuesare quoted to the nearest 0.1 Sv) of NADW formation.The CNTRL NADW off state was obtained from theNADW on state by applying a perturbation of maxi-mum value M � 0.11 Sv (0.38 m yr�1). This NADW offstate exhibits an AAIW reverse cell of 9.3 Sv. This celloverlies an AABW cell recirculating below 2000-mdepth with an AABW inflow into the Atlantic sector of4.5 Sv.

The experiment 23KVAtl admits an NADW on state,

as shown in Fig. 3c, although with a reduction ofNADW recirculation within the Atlantic and an in-crease in NADW outflow. We obtain the NADW offstate in 2

3KVAtl from the NADW on state in a similar

fashion to the CNTRL experiment. In the NADW offstate of experiment 2

3KVAtl the AAIW reverse cell is

reduced by 3 Sv, taking a value of 6.3 Sv, and is re-stricted to shallower depths (Fig. 3d). This reduction inthe strength and depth of the AAIW reverse cell resultsfrom a decrease in AAIW upwelling into the Atlanticthermocline due to lower vertical mixing rates there.This result illustrates the dependence of the AAIW re-verse cell on vertical mixing inside the Atlantic basin.In contrast, the AABW recirculation in 2

3KVAtl is simi-

lar (4.3 Sv) to that of the NADW off state in CNTRL(4.5 Sv). This could be because AABW inflow into theAtlantic returns south at depth and is not affected bychanges in vertical mixing applied in the upper 1257 m,and changes in NADW formation are only small. The

FIG. 2. (a) Model domain and areas of the Atlantic (Atl, darkgray) and Indian/Pacific Oceans (IP, light gray) where K� is re-duced. (b) FW perturbation vs time. The region of FW addition/extraction is indicated in (a) by the region enclosed by solid lines.A maximum value M is attained after 150 yr. The duration of allperturbations is 300 yr. No values are indicated along the verticalaxis as perturbations of different magnitudes are used, rangingfrom �0.48 to 0.76 Sv.


NADW on state in 13KVAtl exhibits a reduction of

NADW formation from 21.5 Sv in CNTRL to 19.8 Sv.It is interesting to note that despite a reduction inNADW formation, NADW outflow1 is increased from12.2 Sv in CNTRL to 14.8 Sv in 1

3KVAtl. This implies a

reduction of NADW recirculation inside the Atlantic2

from 9.3 Sv in the control experiment to 5.0 Sv whenvertical mixing is reduced in 1

3KVAtl. Thus, as expected,

the rate of NADW recirculation inside the Atlantic ba-sin depends on the rate of localized vertical mixing. Areduction of vertical mixing in the Atlantic therefore

reduces this recirculation, which is compensated in partby reduced NADW formation, and in part by an in-crease in NADW outflow.

NADW formation is absent in the steady-state MOCin 1

3KVIP, despite being set up identical to CNTRL

apart from a reduction of K� in the tropical Indian andPacific Oceans. This suggests that the maintenance ofan NADW overturning circulation depends in part onvertical mixing rates in the ocean basins outside theAtlantic. For 1

3KVIP, the Atlantic MOC is similar to that

of the NADW off state of CNTRL (cf. Figs. 3b,f), withan AAIW reverse cell of 10.2 Sv overlying an AABWcell recirculating below 2000-m depth. This suggeststhat the Atlantic circulation of the NADW off state inCNTRL is not significantly influenced by vertical mix-ing outside the Atlantic basin. The similarity between

1 NADW outflow is measured as the local maximum at inter-mediate depth of the meridional transport streamfunction at 30°S.

2 We calculate this recirculation by subtracting the NADW out-flow rate from the NADW formation rate.

FIG. 3. Atlantic meridional overturning streamfunction (annual mean) for (a) the steady NADW on state of the control experimentCNTRL, (b) the steady NADW off state of CNTRL, (c) the steady NADW on state of 2

3KVAtl, (d) the steady NADW off state of

2

3KVAtl, (e) the steady NADW on state of 1

3KVAtl, and (f) the steady state found in 1

3KVIP. Values are given in Sv. The states shown in

(e) and (f) were the only stable equilibria found for those experiments in this study.


the CNTRL NADW off state and the unperturbed13KVIP, experiment arises because the AAIW reverse

cell relies almost exclusively on vertical mixing insidethe Atlantic for its upwelling branch.

In contrast to experiments CNTRL and 23KVAtl, we

were unable to obtain stable NADW off states in13KVAtl. This was attempted with additions of FW per-

turbations of up to M � 0.58 Sv. This shows a signifi-cant increase in NADW robustness with respect to FWperturbations in the NA when K� is reduced in theAtlantic sector. This is because a strong reduction of K�

in the Atlantic weakens the AAIW reverse cell thatis otherwise required to prevent the eventual reestab-lishment of the NADW cell after the forcing is re-moved. As we will see later, the reverse cell appearstemporarily during our FW perturbation, but with re-duced vigor in 1

3KVAtl. Unlike 1

3KVAtl, the existence of

a stable NADW off state in the 23KVAtl experiment al-

lows us to examine the effect of vertical mixing in theAtlantic on the AAIW reverse cell.

To test the relationship between the AAIW reversecell and the value of K� inside the Atlantic, we have runseveral experiments similar to 1

3KVAtl, only using mul-

tiplicative factors of 0.75, 0.85, and 1.25. In all cases weobtain NADW off states using an FW perturbation.Figure 4 shows the strength of the AAIW reverse cell inthe NADW off states against the multiplicative factorapplied to K� for these experiments, along with 2

3KVAtl

and CNTRL. The solid line is a linear “best fit” usingthe method of least squares. The close fit of this line toour experimental results suggests that the strength ofthe AAIW reverse cell has a linear dependence on therate of vertical mixing inside the Atlantic, as would

arise if a simple advection–diffusion balance were inplace. From this analysis we can assume that verticalmixing inside the Atlantic constitutes a limiting factoron the strength of the AAIW reverse cell.

MS99 found an increased stability of NADW whenglobally increasing K� to 1.3 cm2 s�1 in their so-calledLVD experiment. This result appears to be in contra-diction to our finding that decreasing K� inside the At-lantic results in an increase in NADW stability. There-fore, we now examine the results of our experimentMS99KVglob, where we set K� to 1.3 cm2 s�1 in theupper 2500 m globally, similar in design to the LVDexperiment of MS99. A second experiment MS99KVAtl

is evaluated where we apply this increase of K� onlyinside the Atlantic. Note that our CNTRL experimentemploys a value of 0.6 cm2 s�1 at the top layer. Experi-ments MS99KVglob and MS99KVAtl therefore employlarger values within the global and Atlantic pycnocline,respectively, than our CNTRL experiment. Figures5a–c show the MOC in the Atlantic basin for theNADW on state of MS99KVAtl, the NADW off state ofMS99KVAtl, and the NADW on state of MS99KVglob,respectively. Increasing K� only inside the Atlantic(MS99KVAtl) results in a 2.2-Sv increase in NADWformation to 23.7 Sv and yet a significant reduction inNADW outflow (from 12.2 Sv down to 7.8 Sv) for theNADW on state (Table 1). Enhancing vertical mixinginside the Atlantic therefore results in a significant in-crease in NADW recirculation within the Atlantic atthe expense of NADW outflow. AABW inflow remainsat similar values to CNTRL. As expected, the NADW

TABLE 1. Summary of the main features of the meridional masstransport streamfunction in Sv for all model experiments. Shownare the NADW formation and outflow rate for the NADW onstates, and the AAIW reverse cell for the NADW off states. Wehave been unable to find a stable NADW off state for 1

3KVAtl, and

therefore no AAIW reverse cell strength is indicated. Similarly,no values associated with the NADW on state are shown for1

3KVIP. The final column shows whether we find an increased or

reduced stability of NADW formation with respect to FW per-turbations in each experiment relative to CNTRL.

ExperimentNADW

cellNADWoutflow

AAIWreverse cell

NADWstability

CNTRL 21.5 12.2 9.3 Control2

3KVAtl 20.6 13.6 6.3 More stable

1

3KVAtl 19.8 14.8 — More stable*

1

3KVIP — — 10.2 Unstable

MS99KVglob 28.0 13.3 — More stable*MS99KVAtl 23.7 7.8 15.1 Unstable

* Only one stable state has been obtained in these experiments.

FIG. 4. AAIW reverse cell in NADW off states vs the multipli-cative factor applied to K� in the Atlantic Ocean. As shown in Fig.1, the multiplicative factor is only applied to K� in the upper1257-m depth. The solid line is a linear best fit using the methodof least squares. Values are given in Sv.


off state for MS99KVAtl (Fig. 5b) shows a significantlyincreased AAIW reverse cell, attaining a strength of15.1 Sv. In agreement with the results shown in Fig. 4,this indicates that the strength of the AAIW reversecell is set by vertical mixing inside the Atlantic. En-hancing K� only inside the Atlantic results in a signifi-cant increase in strength of the AAIW reverse cell dueto the advective–diffusive balance that the upwellingbranch of this cell relies on. Strong overturning (28.0Sv) occurs in MS99KVglob (Fig. 5c), and the stream-function in this experiment is very similar to that shownin Fig. 9a of MS99. Finally, it is noted that in agreement

with MS99, we have been unable to obtain a stableNADW off state for MS99KVglob.

b. Freshwater perturbations

As stated in section 2 we have subjected the equilib-ria obtained for all experiments to 300-yr FW pulses ofdifferent magnitude. We also apply FW pulses to ex-periments with K� multiplied by factors of 1/4 and 1/2 inthe Atlantic (denoted 1

4KVAtl and 1

2KVAtl, respectively).

Figures 6–9 summarize the results. Figure 6 showsthe time series of NADW formation and the AAIWreverse cell for experiments 1

4KVAtl,

13KVAtl,

12KVAtl,

and 23KVAtl under an FW perturbation attaining a maxi-

mum value M of 0.29 Sv (1.02 m yr�1). In other words,in this set of experiments, the maximum value M of theFW pulse is fixed, and K� varies. The FW pulse is suf-ficient to shut down NADW formation in 2

3KVAtl,

whereas in 13KVAtl and 1

4KVAtl a recovery of NADW

formation occurs after the pulse has ceased. In 12KVAtl

we see a slower recovery of NADW formation and anassociated decline of the AAIW reverse cell after theFW pulse has ceased. This set of experiments showsthat decreasing K� in the Atlantic results in an in-creased robustness of NADW formation with respect toa given FW pulse.

FIG. 5. Atlantic meridional overturning streamfunction (annualmean) for (a) the steady NADW on state of MS99KVAtl, (b) thesteady NADW off state of MS99KVAtl, and (c) the steady NADWon state of MS99KVglob. Values are given in Sv. The overturningshown in (c) is the only stable equilibrium found in theMS99KVglob experiment.

FIG. 6. Time series of (a) NADW production rate and (b) theAAIW reverse cell for experiments 1

4KVAtl (solid), 1

3KVAtl

(dashed), 1

2KVAtl (dashed–dotted), and 2

3KVAtl (dotted) under an

FW perturbation attaining a maximum value of 0.29 Sv (1.02 myr�1) after 150 yr. Values are given in Sv.


Figure 7 shows time series of NADW formation andthe AAIW reverse cell in 1

3KVAtl in response to the

application of a series of 300-yr FW pulses of varyingmagnitude M. Thus, in this analysis, we have kept thereduction of K� fixed while varying the magnitude M ofthe FW flux. For comparison, we have included theresponse of CNTRL to a smaller FW pulse. A pertur-bation of maximum value M � 0.11 Sv (0.38 m yr�1)yields a stable NADW off state in the control experi-ment. In contrast, when FW pulses attaining highermaximum values of M up to 0.43 Sv (1.53 m yr�1) areapplied to 1

3KVAtl, NADW formation eventually recov-

ers (Fig. 7a). The NADW shutdown in CNTRL coin-cides with the emergence of the AAIW reverse cell(Fig. 7b), which eventually attains a value of 9.3 Sv.When K� is reduced in the Atlantic, however, the roleof the AAIW reverse cell appears to be diminished.Despite the initial suppression of the NADW cell inresponse to the FW perturbations applied to 1

3KVAtl,

and the temporary establishment of a higher maximumstrength AAIW reverse cell in one of the experiments,there is no permanent sustenance of this cell. After theperturbation has terminated, NADW formation recov-ers over a period of 1500–2500 yr. This indicates that in

the absence of an external FW flux, circulation changesalone cannot maintain the light surface conditions atthe NADW formation regions required to prevent sink-ing when K� is reduced by a factor of 1/3 in the Atlantic.Indeed, Gregory et al. (2003) suggest that the AAIWreverse cell is responsible for importing FW into theAtlantic, maintaining buoyant NA surface conditionsthat prevent a transition to an NADW on state. In con-trast to the AAIW reverse cell, NADW finds its up-welling areas outside the Atlantic basin and is thus notsubject to the same impediment as the AAIW reversecell when K� is reduced in 1

3KVAtl. This indicates that a

reduction of K� inside the Atlantic basin favors stabilityof the NADW cell by inhibiting the AAIW reverse cell.

The enhanced stability of the NADW cell in 13KVAtl

is not the result of changes in surface density or iso-therm depth. First, the surface conditions in 1

3KVAtl in-

dicate a cooling and freshening of the NADW forma-tion regions relative to CNTRL (figure not shown),with the net effect of this change a reduction of surfacedensity in the North Atlantic. This proves that changesin NA surface density in 1

3KVAtl are not responsible for

the increased stability of NADW. Second, as expected,we observe a shoaling of the isotherms in the upper2000 m of the Atlantic in 1

3KVAtl relative to CNTRL

(figure not shown). A deepening of Atlantic isothermsis known to enhance NADW formation (Gnanadesikan1999) and perhaps its stability, yet we find shoaling ofthe Atlantic isotherms in the upper 2000 m in 1

3KVAtl.

Moreover, the slightly reduced rate of NADW forma-tion in 1

3KVAtl (19.8 Sv) might simplistically suggest re-

duced NADW stability, the opposite to what we havefound. We have shown in contrast that a reduction invertical mixing inside the Atlantic leads to an increasedstability of NADW formation. This results from theweakening of the AAIW reverse cell caused by thereduced vertical mixing in the Atlantic sector.

A very different picture emerges when K� is reducedin the Indian and Pacific Oceans. We have tried toexcite a transition to an NADW on state in 1

3KVIP by

applying FW extractions from the NA without success.For example, Fig. 8 shows the time series for experi-ment 1

3KVIP under an FW extraction attaining a maxi-

mum of 1.7 m yr�1 after 150 yr. Time series are shownfor the NADW production rate and the AAIW reversecell clearly showing the inability of the 1

3KVIP case to

maintain stable NADW production despite a strongFW extraction. Apparently, in our model, the Indianand Pacific Oceans comprise an area of significantdeepwater removal by vertical mixing. This sensitivityof NADW to vertical mixing inside the Indian and Pa-cific Oceans indicates that in our model, alternativeremoval mechanisms, such as mechanically driven up-

FIG. 7. Time series of (a) NADW production rate and (b) theAAIW reverse cell for experiments CNTRL under a perturbationattaining a maximum value of 0.11 Sv (0.38 m yr�1, solid) andexperiment 1

3KVAtl under perturbations attaining a maximum of

0.14 (0.51 m yr�1, dashed), 0.29 (1.02 m yr�1, dashed–dotted), and0.43 Sv (1.53 m yr�1, dotted). Values are given in Sv.


welling due to wind stress in the Southern Ocean, arenot sufficient alone to maintain the NADW cell in theabsence of strong Indo-Pacific vertical mixing. A dyetracer experiment tagging NADW recirculation (figurenot shown) shows upwelling of NADW in the lowerlatitudes of the Indian and Pacific Oceans, as well as atsome locations in the high latitudes of the SouthernOcean. This confirms the importance of the Indian/Pacific and Southern Oceans in the removal of NADWfrom the deep oceans in this model. In this case, a re-duction of vertical mixing inside the Indian and PacificOceans shifts the competitive advantage of the AAIW/NADW cells in favor of the AAIW cell. We have alsorun an experiment reducing K� globally by a factor of1/3 and again find weak overturning in the Atlanticbasin. In this case, both the AAIW reverse cell and theNADW cell are suppressed.

Figure 9 shows a time series of the CNTRL experi-ment under an FW pulse attaining a maximum value Mof 0.1 Sv (0.34 m yr�1). A recovery of NADW forma-tion occurs under this perturbation. As in the experi-ments of Gregory et al. (2003), the recovery of theNADW cell is simultaneous with the disappearance ofthe AAIW reverse cell. This is in contrast to the ex-periments 1

4KVAtl and 1

3KVAtl shown in Fig. 6, where

NADW formation recovers some time after the AAIWreverse cell has disappeared. This suggests that reduc-ing K� inside the Atlantic also removes the synchrone-ity between the reestablishment of the NADW cell andthe disappearance of the AAIW reverse cell in oceanmodels.

c. Importance of location of change in K�

Figure 10 shows time series of NADW productionrate and the AAIW reverse cell for experimentMS99KVAtl under FW perturbations attaining a maxi-mum of 0.08 and 0.11 Sv and for experiment MS99KVglob

under FW perturbations attaining a maximum of 0.11and 0.76 Sv. In agreement with MS99 we find a signifi-cantly enhanced NADW stability in MS99KVglob,where the 0.76-Sv perturbation is not sufficient to per-manently shut down NADW formation. This should becompared to the much lower value of 0.11 Sv that issufficient to excite a transition to a stable NADW offstate in CNTRL (Fig. 7). The NADW collapse inMS99KVAtl under a perturbation of only 0.08 Sv (Fig.10), however, shows that increasing K� only inside theAtlantic results in a reduction of NADW stability com-pared to the enhanced stability found under a globalincrease in K�. The reduction in NADW stability in

FIG. 8. Time series for experiment 1

3KVIP under the FW per-

turbation shown in Fig. 2 attaining a minimum M of �0.48 Sv(�1.7 m yr�1) after 150 yr. Time series are shown for (a) NADWproduction rate and (b) the AAIW reverse cell. Values are givenin Sv.

FIG. 9. Time series of (a) NADW production rate and (b) theAAIW reverse cell for experiment CNTRL under a perturbationattaining a maximum value of 0.1 Sv (0.34 m yr�1). Values aregiven in Sv.


MS99KVAtl arises from the significant increase instrength of the AAIW reverse cell in response to in-creased vertical mixing inside the Atlantic. This is con-sistent with our previous findings in experiments 1

3KVAtl

and 23KVAtl. In contrast, we could not obtain a stable off

state for MS99KVglob. In summary, a markedly differ-ent response of the ocean’s MOC is obtained whenaltering K� globally as compared to K� changes limitedto the Atlantic Ocean.

4. Summary and conclusions

In this article we have demonstrated that regionalvariations in vertical mixing can fundamentally affectthe global meridional overturning circulation and itsstability to FW perturbations. An important reason forapplying such changes in K� in our model is to demon-strate the role of the AAIW reverse cell in maintainingthe stability of circulation states. Our results show thata reduction of K� inside the Atlantic basin can drasti-cally increase NADW stability with respect to FW per-turbations applied to the NA. Conversely, a reductionof K� inside the Indian and Pacific basins can inhibitNADW formation, enabling the establishment of anAAIW reverse cell driven by upwelling inside the At-lantic basin. Furthermore, the FW perturbations we ap-

ply are unable to excite transitions to a stable NADW“off” state for the cases where K� is multiplied by afactor of 1/2 or less in the Atlantic Ocean. Multiplica-tion of K� in the Atlantic by a factor of 2/3, however,allows the model to admit a transition to a stableNADW off state in response to the FW perturbations.The reduction in strength and depth of the AAIW re-verse cell in this NADW off state illustrates that theAAIW reverse cell is indeed inhibited by a reduction ofK� in the Atlantic Ocean. It should be noted that wehave rerun a version of 1

3KVAtl using the computation-

ally more intensive Gent and McWilliams (1990) eddyadvection scheme combined with along-isopycnal mix-ing and no horizontal diffusion and also found in-creased NADW stability in this experiment.

Because a reduction in vertical mixing in the Atlanticinhibits the AAIW reverse cell, it in turn enhances thestability of the NADW cell. Indeed, Saenko et al.(2003) and Gregory et al. (2003) suggest that theAAIW reverse cell is responsible for the stability of theNADW off state, which we have also demonstrated inthis study. After an initial suppression of NADW dueto freshening of the NADW formation regions by anFW pulse (Fig. 6), no stable AAIW reverse cell devel-ops when K� is reduced significantly in the Atlantic.The recovery of NADW formation in the absence ofthis cell in 1

3KVAtl supports the idea that the AAIW

reverse cell (associated with NADW off states in oceanmodels) is a contributing factor to the stability of col-lapsed NADW states. Furthermore, changes in surfaceconditions and pycnocline depth inside the Atlantic donot contribute to the enhanced NADW stability we findin 1

3KVAtl. Rather, it appears to be exclusively due to

the suppression of the AAIW reverse cell in the South-ern Hemisphere in this experiment.

Although detrimental to the AAIW reverse cell, re-duced K� inside the Atlantic does not inhibit theNADW cell, as its deepwater removal branch is locatedelsewhere, in the Indian, Pacific, and Southern Oceans.Our inability to obtain a stable NADW “on” state inexperiment 1

3KVIP shows that the Indian and Pacific

basins are important locations for the removal ofNADW in our model. Indeed, reduced vertical mixinginside the Indian and Pacific basins in 1

3KVIP inhibits the

eventual removal of NADW from the Atlantic basinand is therefore detrimental to NADW formation. Incontrast, the similarity between the Atlantic MOC ofthe NADW off state in CNTRL and 1

3KVIP shows that

the AAIW reverse cell is not affected by the reductionof vertical mixing in the Indian and Pacific Oceans. Thisis not surprising as its upwelling branch occurs insidethe Atlantic basin. Therefore, a reduction of K� insidethe Indian and Pacific Oceans shifts the competitive

FIG. 10. Time series of (a) NADW production rate and (b) theAAIW reverse cell strength for experiment MS99KVglob andMS99KVAtl under several FW perturbation scenarios (see legendfor details). Values are given in Sv.


advantage from the NADW cell in favor of the AAIWreverse cell. While not explored here, this may haveimplications for ancient states of the global THC, as thedistribution of seafloor “roughness” has evolved gradu-ally over geological time scales. In particular, changesin the Southern Ocean could play an important role.

A global increase of K� in our model results in sig-nificantly enhanced NADW stability. However, anidentical enhancement of K� only inside the AtlanticOcean results in a reduction in NADW stability withrespect to the control experiment CNTRL. Our resultsare thus not in contradiction with the enhanced NADWstability under globally increased K� found by MS99.However, we find a markedly different response whenK� is increased only within the Atlantic Ocean, as inthat case NADW stability is diminished. This suggeststhat the enhanced stability of the NADW on statefound by MS99 under globally increased K� is due tothe increase in vertical mixing outside the AtlanticOcean, where NADW resurfaces.

The polarity of the global MOC is determined byinterhemispheric competition between the AAIW re-verse cell and the NADW cell (in today’s climate theAABW cell plays a minor role due to the Drake Pas-sage effect; Sijp and England 2005). Unlike the ideal-ized symmetric competition between an SH cell and anNH cell described by Bryan (1986), the NADW andAAIW cells depend on upwelling at different locations.In contrast to the AAIW reverse cell, the NADW celldepends on removal of its deepwater in areas outsidethe Atlantic, including the Southern and low-latitudeIndian/Pacific Oceans. While vertical mixing plays animportant role in NADW removal in our results, wind-driven upwelling in the Southern Ocean is thought tobe another important mechanism to remove NADWfrom the deep ocean (Toggweiler and Samuels 1995).NADW is sensitive to vertical mixing in the Indian andPacific Oceans in our model as a substantial componentof NADW upwells into those sectors in the simulations(as in Gordon 1986). If wind-driven upwelling was amore prominent NADW removal mechanism in ourmodel, perhaps by employing vertical mixing schemeswith lower background diffusivity values, we might ex-pect a reduction in this sensitivity. For instance, Saenkoand Merryfield (2005) employ a parameterization oftidally driven deep mixing combining mixing nearrough topography with a low background diffusivity of10�1 cm2 s�1. As this tidally driven mixing over roughtopography mainly occurs below the pycnocline, verti-cal mixing inside the pycnocline is small. In this modelsetup, Saenko and Merryfield (2005) find a dominanceof Southern Ocean deepwater removal, over the clas-sical notion of uniform abyssal upwelling (Munk 1966;

Stommel and Arons 1960), and the conveyor pathwayschematic of Gordon (1986). We would nonetheless ex-pect the sensitivity of the AAIW reverse cell to verticalmixing in the Atlantic to be robust in experimentswherein wind-driven upwelling in the Southern Oceandominates the removal of NADW.

We have shown that the relative strength of theNADW/AAIW cells, and therefore NADW stability,depends not only on surface density at the NADW andAAIW formation regions, but also on the relativestrength of their removal mechanisms. This removaltakes place in different geographic locations, so that therelative strengths of the NADW/AAIW cells can bealtered by changing the vertical mixing coefficient inthese regions. In the real ocean, NADW recirculationdepends on vertical mixing across the thermocline andmechanical wind-driven upwelling in the SouthernOcean. Of these fundamentally different processes, ver-tical mixing appears to be regulating the removal ofdeepwater when the AAIW reverse cell operates in aNADW off state. This has implications for the under-standing of the global thermohaline circulation in pastand future climates.

Acknowledgments. We thank the University of Vic-toria staff for support in usage of their coupled climatemodel. We thank Jonathan Gregory, Ron Stouffer, andan anonymous reviewer for useful comments and sug-gestions that allowed us to greatly enhance the clarityand content of this study. This research was supportedby the Australian Research Council and the AustralianAntarctic Science Program.

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