What contributions have the authors mentioned in the paper "On the driving processes of the atlantic meridional overturning circulation" ?

Here the authors review both observational data and model results concerning the two main candidates: vertical mixing processes in the ocean ’ s interior and windinduced Ekman upwelling in the Southern Ocean. In distinction to the energy source the authors also discuss the role of surface heat and freshwater fluxes, which influence the volume transport of the meridional overturning circulation and shape its spatial circulation pattern without actually supplying energy to the overturning itself in steady state.

What future works have the authors mentioned in the paper "On the driving processes of the atlantic meridional overturning circulation" ?

For the large-scale circulation the downward mixing of heat can be considered as the deep heat source that Sandström ’ s theorem calls for, and Sandström ’ s insight has inspired much further work to identify this deep heat ( or buoyancy ) source. In conclusion, the authors find that further research about both the driving processes and the deepwater formation processes is necessary to understand the AMOC and to represent it adequately in ocean and climate models. Beyond the scientific challenge itself these research efforts are indispensable to reliably assess how the ocean circulation will change in the future and what consequences this may have for the global climate. [ 134 ] ACKNOWLEDGMENTS. He was incorrect, however, in suggesting that salinity differences could drive this mixing.

(Open Access) On the driving processes of the Atlantic meridional overturning circulation (2007) | Till Kuhlbrodt

ON THE DRIVING PROCESSES OF THE ATLANTIC

MERIDIONAL OVERTURNING CIRCULATION

T. Kuhlbrodt,

A. Griesel,

M. Montoya,

A. Levermann,

M. Hofmann,

and S. Rahmstorf

Received 3 December 2004; revised 10 August 2006; accepted 26 October 2006; published 24 April 2007.

[1] Because of its relevance for the global climate the

Atlantic meridional overturning circulation (AMOC) has

been a major research focus for many years. Yet the

question of which physical mechanisms ultimately drive the

AMOC, in the sense of providing its energy supply, remains

a matter of controversy. Here we review both observational

data and model results concerning the two main candidates:

vertical mixing processes in the ocean’s interior and wind-

induced Ekman upwelling in the Southern Ocean. In

distinction to the energy source we also discuss the role

of surface heat and freshwater fluxes, which influence the

volume transport of the meridional overturning circulation

and shape its spatial circulation pattern without actually

supplying energy to the overturning itself in steady state.

We conclude that both wind-driven upwelling and vertical

mixing are likely contributing to driving the observed

circulation. To quantify their respective contributions, future

research needs to address some open questions, which we

outline.

Citation: Kuhlbrodt, T., A. Griesel, M. Montoya, A. Levermann, M. Hofmann, and S. Rahmstorf (2007), On t he driving processes of

the Atlantic meridional overturning circulation, Rev. Geophys., 45, RG2001, doi:10.1029/2004RG000166.

1. INTRODUCTION

[

2] The deep Atlantic meridional overturning circulation

(AMOC) consists of four main branches: upwelling pro-

cesses that transport volume from depth to near the ocean

surface, surface currents that transport relatively light water

toward high latitudes, deepwater formation regions where

waters become denser and sink, and deep currents closing

the loop. These four branches span the entire Atlantic on

both hemispheres, forming a circulation system that consists

of two overturning cells, a deep one with North Atlantic

Deep Water (NADW) and an abyssal one with Antarctic

Bottom Water (AABW). A highly simplified, illustrative

sketch of this circulation is given in Figure 1. The AMOC

exerts a strong control on the stratification and distribution

of water masses, the amount of heat that is transported by

the ocean, and the cycling and storage of chemical species

such as carbon dioxide in the deep sea. Thus the AMOC is a

key player in the Earth’s climate. In the North Atlantic its

maximum northward heat transport is about 1 PW (10

[Hall and Bryden, 1982; Ganachaud and Wunsch, 2000;

Trenberth and Caron, 2001], contributing to the mild

climate predominant in northwestern Europe. A reduction

in AMOC is likely to have strong implications for the

El Nin˜o–Southern Oscillation phenomenon [Timmermann

et al., 2005], the position of the Intertropical Convergence

Zone [Vellinga and Wood, 2002], the marine ecosystem in

the Atlantic [Schmittner, 2005], and the sea level in the

North Atlantic [Levermann et al., 2005]. Evidence from the

geological past [Bond et al., 1992; McManus et al., 2004]

suggests that reorganizations of the AMOC were involved

in climatic temperature changes of several degrees in a few

decades (see also the reviews by Clark et al. [2002] and

Rahmstorf [2002]). In the future, there is a risk that

substantial changes in ocean circulation could occur as a

result of global warming [Manabe and Stouffer, 1994;

Rahmstorf and Ganopolski, 1999; Wood et al., 1999;

Schaeffer et al., 2002; Zickfeld et al., 2007].

[

3] Ultimately, the influence of Sun and Moon is respon-

sible for oceanic and atmospheric circulations on Earth. The

surface fluxes of heat, fresh water, and momentum as well

as gravity and tides set the ocean waters in motion, either

directly or via intermediate processes such as waves. The

main aim of this paper is to discuss the physical mecha-

nisms that drive the AMOC in the sense that they provide an

energy input into the ocean that is capable of sustaining a

steady state deep overturning circulation.

[

4] Presently, two distinct mechanisms for driving the

meridional overturning circulation (MOC) are discussed.

The first one is the traditional thermohaline driving mech-

anism proposed by Sandstro¨m [1916] and Jeffreys [1925].

In this view the driver is mixing that transports heat from

the surface to the deepwater masses, downward across

Click

Here

for

Full

rticl

Potsdam Institute for Climate Impact Research, Potsdam, Germany.

Scripps Institution of Oceanography, La Jolla, California, USA.

Departamento Astrofisica y Ciencias de la Atmo´sfera, Facultad de

Ciencias Fı´sicas, Universidad Complutense de Madrid, Madrid, Spain.

8755-1209/07/2004RG000166$15.00

Reviews of Geophysics, 45, RG2001 / 2004

1of32

Paper number 2004RG000166

RG2001

surfaces of equal density (diapycnal mixing). Munk and

Wunsch [1998] described this mechanism in detail. The

action of winds and tides generates internal waves in the

oceans. These waves dissipate into small-scale motion that

causes turbulent mixing. This mixing of heat lightens water

masses in the deep ocean and causes them to rise in low

latitudes. Resulting surface and intermediate waters are then

advected poleward into the North Atlantic where they are

transformed into dense waters by atmospheric cooling and

salt rejection during sea ice growth. These waters sink to

depth and spread, setting up the deepwater mass of the

ocean. Thereby a meridional density gradient between high

and low latitudes is established. A sketch of the involved

processes and their locations is given in Figure 2.

[

5] The second candidate is wind-driven upwelling, as

put forward by Tog gweiler and Samuels [1993b, 1995,

1998]. On the basis of observational radiocarbon constraints

they concluded that the actual amount of upwelling of

abyssal water caused by diapycnal mixing is insufficient

to sustain an estimated overturning of about 15 Sv (1 Sv = 1

Sverdrup = 10

1

) in the Atlantic Ocean. As an

alternative they suggested that most of the oceanic upwell-

ing is wind-driven and occurs in the Southern Ocean. The

strong westerly circumpolar winds induce a vigorous north-

ward transport of waters, called Ekman transport, near the

ocean surface. Since there is a horizontal divergence of the

Ekman transport, an upwelling from depth is induced that is

associated with the so-called Drake Passage effect (se e

Figure 2). In this view it is the strength of Southern Ocean

winds rather than the oceanic diapycnal mixing that governs

the strength of the AMOC. Note that in this theory the

winds induce large-scale motion of the water masses in

the Southern Ocean, which enter the Atlantic and flow to

the northern deepwater formation sites. Wind-driven mix-

ing, i.e., small-scale turbulent motion that is induced by

surface wind stress, is part of the mixing processes and is

not considered as a direct wind-driven upwelling.

[

6] Determining which of these two processes is the main

driving mechanism of the MOC is of great interest, even

beyond the mere aim of physical understanding. The two

could imply different sensitivities to variations in external

forcing [Schmittner and Weaver, 2001; Prange et al., 2003]

and thus a different evolution of the MOC under continued

global climate change. In the present paper we review work

on theory, modeling, and observations that argue for either

or both of the possible driving mechanisms.

[

7] We wish to emphasize that the driving processes do

not fully determine the AMOC’s spatial extent and strength.

The amount of water that actually sinks in the North

Atlantic is controlled by a variety of processes including

the horizontal gyre circulation, atmospheric cooling, pre-

cipitation, evaporation, and ice melting. These processes

Figure 1. Strongly simplified sketch of the global overturning circulation system. In the Atlantic, warm

and saline waters flow northward all the way from the Southern Ocean into the Labrador and Nordic

Seas. By contrast, there is no deepwater formation in the North Pacific, and its surface waters are fresher.

Deep waters formed in the Southern Ocean become denser and thus spread in deeper levels than those

from the North Atlantic. Note the small, localized deepwater formation areas in comparison with the

widespread zones of mixing-driven upwelling. Wind-driven upwelling occurs along the Antarctic

Circumpolar Current (ACC). After Rahmstorf [2002].

RG2001 Kuhlbrodt et al.: DRIVERS OF THE AMOC

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RG2001

can change the AMOC’s spatial pattern drastically, and they

can temporarily reduce or increase the amount of deep water

formed, with a strong impact on climate. However, our

focus here is on the AMOC as a large-scale coherent

circulation system and on longer timescales, that is, on

which mechanism provides the ocean with the energy

necessary to sustain a steady state deep overturning circu-

lation.

[

8] The terms ‘‘meridional overturning circulation’’ and

‘‘thermohaline circulation’’ (THC) have sometimes been

used almost like synonyms, but they have very different

meanings. ‘‘MOC’’ is merely a descriptive, geographic

term: It is simply a circulation in the meridional-vertical

plane, as depicted by an overturning stream function as in

Figure 3. The term ‘‘MOC’’ thus does not refer to any

particular driving mechanism.

[

9] The term ‘‘THC,’’ by contrast, is a definition of flow

by driving mechanism. There are three qualitatively differ-

ent physical mechanisms to drive oceanic flows: (1) direct

momentum transfer by surface winds, (2) acceleration of

water by tidal forces, and (3) thermohaline forcing. This

classification has been found in oceanography textbooks

since the early 20th century [e.g., Defant, 1929; Neumann

and Pierson, 1966]. A simple, archetypal example of the

latter would be the regional thermohaline (or, in this case,

thermal) circulation caused by ‘‘hot spots’’ of geothermal

heating at the ocean bottom near mid- ocean ridges [Joyce

and Speer, 1987; Thompson and Johnson, 1996]. Another

example is the flow driven by strong surface cooling of a

previously warmer body of wate r, as occurs, for example,

when a polynya opens up in sea ice [Buffoni et al., 2002]. In

these examples, thermohaline fluxes at the ocean boundary

(surface or bottom) cause density changes that drive a flow

by setting up pressure gradients.

[

10] A complication arises when considering the large-

scale thermohaline circulation in steady state, as this steady

Figure 2. Idealized meridional section representing a zonally averaged picture of the Atlantic Ocean.

Straight arrows sketch the MOC. The color shading depicts a zonally averaged density profile derived

from observational data [Levitus, 1982]. The thermocline, the region where the temperature gradient is

large, separates the light and warm upper waters from the denser and cooler deep waters. The two main

upwelling mechanisms, wind-driven and mixing-driven, are displayed. Wind-driven upwelling is a

consequence of a northward flow of the surface waters in the Southern Ocean, the Ekman transport, that

is driven by strong westerly winds (see section 4). Since the Ekman transport is divergent, waters upwell

from depth. Mixing along the density gradient, called diapycnal mixing, causes mixing-driven upwelling;

this is partly due to internal waves triggered at the ocean’s boundaries (see section 3). Deepwater

formation (DWF) occurs in the high northern and southern latitudes, creating North Atlantic Deep Water

(NADW) and Antarctic Bottom Water (AABW), respectively. The locations of DWF are tightly linked

with the distribution of surface fluxes of heat and fresh water; since these influence the buoyancy of the

water, they are subsumed as buoyancy fluxes. The freshly formed NADW has to flow over the shallow

sill between Greenland, Iceland, and Scotland. Close to the zone of wind-driven upwelling in the

Southern Ocean is the Deacon cell recirculation, visible in the zonally integrated meridional velocity in

ocean models. Its relevance is discussed in section 4. Note that in the real ocean the ratio of the

meridional extent to the typical depth is about 5000 to 1.

RG2001 Kuhlbrodt et al.: DRIVERS OF THE AMOC

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RG2001

state ca nnot be maintained by su rface buoyancy fluxes

alone. As discussed in detail in section 2, a mechanical

energy input is required to sustain t he turbulence necessary

to mix down heat in order to maintain the pressure gradients

in addition to the surface fluxes. To account for this fact, the

large-scale thermoha line circulation has been defined as

‘‘currents driven by fluxes of heat and fresh water across

the sea surface and subsequent interior mixing of heat and

salt’’ [Rahmstorf, 2002, p. 208, 2003]. The same fact is

addressed by the definition of Huang [2004, p. 497]: [The

THC is] ‘‘an overtur ning flow in the ocean d riven by

mechanical stirring/mixing, which transports mass, heat,

fresh water, and other properties. In addition, the surface

heat and freshwater fluxes are necessary for setting up the

flow.’’ However, mechanical stirring is only necessary for

sustaining a steady state large-scale MOC not for the

examples of transient or regional thermohaline flows men-

tioned above. Being aware that the word ‘‘driver’’ is used

with different meanings in the literature, we use it for the

remainder of this paper with the meaning ‘‘the physical

process that provides the necessary energy input to sustain a

steady state deep MOC.’’

[

11] Since we discuss both a mechanism to drive the

large-scale MOC directly by winds (section 4) and the

traditional driving mechanism including turbulent mixing

and thermohaline forcing (section 3) in this paper, we will

generally use the driver-neutral terms ‘‘MOC’’ or

‘‘AMOC.’’ We restrict our discussion to the deep MOC,

excluding the shallow Ekman cells of the MOC that are

usually confined to the upper few hundred meters.

[

12] The paper is organized as follows. Section 2 deals

with ‘‘Sandstro¨m’s theorem.’’ On the basis of experiments

and theory it implies that the steady state MOC cannot be

driven by surface buoyancy fluxes alone. We present the

critical discussion that has followed the early statement of

this theorem. A focus is put on the energy budget of the

general circulation. Subsequently, the two possible driving

mechanisms are presented. Diapycnal mixing as a driver is

discussed in section 3, centere d around the budg et of

turbulent diapycnal mixing energy and its sources, which

are basically winds and tides. In addition, we treat the

fundamental difficulties of representing turbulent mixing

in ocean general circulation models. Surface wind forcing as

a driver is addressed in section 4. This includes tracer

evidence for wind-driven upwelling in t he Southern Ocean

and a specific dynamical constraint favoring this upwelling.

Section 4 ends with a revisit to the budget of turbulent

diapycnal mixing energy. We find that likely both mixing

and wind-driven upwelling drive the AMOC. Next, in

section 5 we study the role of the surface buoyancy fluxes

in setting the strength and the shape of the AMOC; today’s

AMOC is characterized by deepwater formation in the

northern North Atlantic and the Southern Ocean. A question

of strong interest is the stability of the AMOC, to which we

devote section 6. Specific issues are the AMOC’s instabil-

ities in past climates and various sources of bistability, like

freshwater fluxes or the different driving mechanisms. A

reason for concern in the near future are transient changes of

the AMOC along with their consequences. In section 7 we

Figure 3. Stream function of the zonally integrated meridional overturning circulation in the Atlan-

tic Ocean, as simulated by a coupled ocean-atmosphere model (for a full description of the model see

Montoya et al. [2005]). Contour interval is a fixed volume flux of 3 Sv = 3  10

1

. Solid (dashed)

lines indicate a clockwise (a counterclockwise) circulation. While the maximum overturning of the model

NADW (at about 40°N and 1000 m depth) is 15.5 Sv, the outflow at 30°S is only 9.8 Sv. The model

AABW enters the Atlantic with a volume flux of about 3 Sv.

RG2001 Kuhlbrodt et al.: DRIVERS OF THE AMOC

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RG2001

summarize the central results and outline the main open

questions.

2. ENERGY BUDGET AND SANDSTRO

M’S

THEOREM

[

13] As we will see in sections 3 and 4, the question of

which mechanisms drive the AMOC is intimately linked to

where and how the upwelling of deep water takes place. The

ocean’s mechanical energy is continuously dissipated

through friction. To maintain a steady state circulation, an

energy source is required to overcome friction. Identifying

such a process, its role in the energy budget of the ocean,

and the energy pathways has been the subject of substantial

research in the past decade [e.g., Munk and Wunsch, 1998;

Toggweiler and Samuels, 1998; Huang, 1999; Gade and

Gustafsson, 2004; Huang, 2004; Wunsch and Ferrari,

2004; Gnanadesikan et al., 2005]. Our intention herein is

to review the current understanding.

2.1. Sandstro¨m’s Theorem

[

14] The starting point of discussions on energetics of the

ocean and the drivers of the ocean’s overturning circulation

is often what is commonly known as Sandstro¨m’s theorem

[e.g., Defant, 1961; Dutton, 1986; Houghton, 1986; Colin

de Verdie`re, 1993; Munk and Wunsch, 1998; Huang, 1999;

Gade and Gustafsson, 2004; Huang, 2004; Wunsch and

Ferrari, 2004; Gnanadesikan et al., 2005; Hughes and

Griffiths, 2006]. Sandstro¨m [1908] performed a series of

tank experiments in which he analyzed under which con-

ditions buoyancy forcing alone, applied at different depths,

could lead to a deep overturning circulation in a water tank.

A heating and a cooling source were placed at opposite

extremes in the tank. In one of the experiments the heating

source was situated above the cooling source; in another

one it was below. Sa ndstro¨m [190 8] conclu ded that a

thermally driven, closed, steady circulation in the ocean

can only be established if the heating source is situated at a

lower level than the cooling source.

[

15] Sandstro¨m’s theorem was later put on a more theo-

retical foundation by Sand stro¨m [1916] and Bjerknes

[1916]. Neglecting the Earth’s rotation, the forces acti ng

upon a fluid parcel are pressure gradient forces, gravity, and

friction. The circulation equation along a closed streamline

S thus gives [Defant, 1961]:

u  dr ¼

 dr

¼

a dp þ

F  dr þ

g  dr; ð1Þ

where t is time, u is the velocity, a is the specific volume (a =

1

, where r is the density), p is the pressure, F is the friction

force per unit mass, and dr is a distance element along the

streamline. Because g = 

f, the last term of the right-hand

side vanishes. The two remaining terms on the right-hand

side represent the work done by pressure gradient forces and

the dissipation of energy through friction, respectively. In

steady state,

¼

a dp þ

F  dr ¼ 0; ð2Þ

that is, pressure gradient forces must do work against friction

in order to balance frictional energy dissipation. Any extra

positive work by pressure gradient forces will contribute to

accelerate the fluid.

[

16] Sa ndstro¨m [1916] (see also the discussions by

Defa nt [1961] and Huang [1999]) conside red a Carnot

cycle, consisting of two isobars (dp = 0) and two adiabatic

curves operating between a heating and a cooling source

(Figure 4). In the first case the heating source is located at a

lower pressure than the cooling source, and the cycle takes

place clockwise. The system is heated from 1 to 2 and

expands at constant pressure p

. At 2 the heating source is

removed, and the system is adiabatically compressed from 2

to 3. From 3 to 4 a cooling source is applied, and the system

Figure 4. Idealized Carnot cycle for the ocean, as proposed by Sandstro¨m [1916] [after Defant, 1961]:

(a) heating source at lower pressure (smaller depth) than cooling source and (b) heating source at higher

pressure (larger depth) than cooling source. Here p is pressure; a is specific volume.

RG2001 Kuhlbrodt et al.: DRIVERS OF THE AMOC

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RG2001

On the driving processes of the Atlantic meridional overturning circulation

Figures

Citations

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References

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Frequently Asked Questions (2)

Q1. What contributions have the authors mentioned in the paper "On the driving processes of the atlantic meridional overturning circulation" ?

Q2. What future works have the authors mentioned in the paper "On the driving processes of the atlantic meridional overturning circulation" ?