GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 6, 10.1029/2001GL014140, 2002
Localization of abrupt change in the North Atlantic
thermohaline circulation
Helen L. Johnson and David P. Marshall
Department of Meteorology, University of Reading, Reading, UK
Received 26 September 2001; accepted 5 February 2002; published 21 March 2002.
[1] Recent climate model experiments, as well as paleoclimate
records, suggest that the meridional overturning circulation or
‘‘thermohaline circulation’’ in the Atlantic Ocean could change
abruptly as a result of global warming, and that this could have a
significant impact on European climate. We use a reduced-gravity
model to investigate the response of the Atlantic overturning
circulation to changes in forcing. We find that variability at decadal
and higher frequencies is confined to a single hemisphere. This
implies that (a) overturning variability resulting from high
frequency changes in buoyancy forcing in the Labrador and
Greenland Seas will be limited to the North Atlantic, and (b) any
observed decadal and higher frequency fluctuations in North
Atlantic overturning can only result from changes in the surface
fluxes within the North Atlantic basin itself. These results suggest
that Southern Ocean wind forcing is not important for North
Atlantic overturning on decadal and shorter timescales.
INDEX
TERMS: 4215 Oceanography: General: Climate and interannual
variability (3309); 9325 Information Related to Geographic
Region: Atlantic Ocean; 1635 Global Change: Oceans (4203);
4203 Oceanography: General: Analytical modeling
1.
Introduction
[2] The meridional overturning circulation in the Atlantic
Ocean transports approximately 1 Petawatt (1015 W) of heat
northwards [Macdonald and Wunsch, 1996 ], which warms western Europe by 5 – 10C [Rahmstorf and Ganopolski, 1999]. Variations in its strength could therefore have a significant impact on
European climate [Manabe and Stouffer, 1993; Wood et al., 1999 ].
Whether variability in the circulation is driven by buoyancy
forcing in the high-latitude North Atlantic [Broecker, 1991], or
by remote processes in the Southern Ocean [Toggweiler and
Samuels, 1995], is an open question. Is the overturning circulation
‘pushed’ by the formation of dense water in the Labrador and
Greenland Seas, or is it ‘pulled’ by the Ekman upwelling and
divergence which results from Southern Ocean winds (Figure 1)?
An understanding of where the meridional overturning circulation
is forced from is essential if we are to monitor it for early signs of
abrupt change.
[3] Work so far has largely focused on the steady state problem.
More relevant to the question of abrupt climate change, and the
focus of this paper, is how the circulation responds to variability, at
a range of frequencies, in North Atlantic deep water formation and
in Southern Ocean winds.
2.
Model
[4] We first consider the effect of variability in the high-latitude
North Atlantic. We use a reduced-gravity model in a realistic
Atlantic domain to represent the adjustment of the surface ocean
Copyright 2002 by the American Geophysical Union.
0094-8276/02/2001GL014140$05.00
to variability in deep water formation [Johnson and Marshall,
2002] - such models have a well-established pedigree, and have
been widely used in previous studies of wave dynamics [Kawase,
1987; Gill, 1980; Zebiak and Cane, 1994 ]. The model can be
interpreted as representing the upper, warm limb of the thermohaline circulation, overlying a sluggish abyssal ocean. (More generally, the ocean circulation can be decomposed vertically into
baroclinic modes [Pedlosky, 1987] and the model approximates the
gravest of these.) It is discretized on a C-grid at 0.25 resolution.
[5] We do not attempt to model the deep water formation
process itself, nor do we include any wind forcing. We simply
represent anomalies in deep water formation rate by imposing a
volume flux into/out of the model’s surface layer in the north-west
corner of the domain. This is equivalent to a change in stratification
on the western side of the Atlantic at high latitudes, as a result of a
convection event in the Greenland and/or Labrador Sea. So that we
can focus on the response at different timescales we apply a Fourier
decomposition to the volume flux and consider sinusoidal variations about zero, with an amplitude of 2 Sverdrups (1 Sverdrup
(Sv) = 106 m3 s1). On the southern boundary the model layer
thickness is relaxed to a uniform value over a ‘‘sponge’’ region
which represents upwelling in the rest of the global ocean. Further
details are given in Johnson and Marshall [2002] .
[6] Figure 2 shows the overturning circulation strength (defined
as the zonally integrated meridional volume transport) as a function
of latitude and time, for three different frequencies of forcing on
the northern boundary. In A the forcing has a period of 40 years.
The amplitude of the variability decays away approximately
linearly throughout the basin, but a significant proportion
(about 40%) is transmitted to the southern boundary of the domain.
There is no apparent difference in the ocean’s response to a positive
overturning anomaly (red) and a negative overturning anomaly
(blue). For a forcing period of 5 years (B) the amplitude of the
variability no longer decays linearly with latitude, but is heavily
damped by the tropics. There is almost no signal south of 20S.
Finally, for a forcing period of 1 year (C) the variability is almost
entirely confined to the northern hemisphere. Figure 2 shows the
equator acting as a low-pass filter which damps high frequency
variability, restricting it to the North Atlantic.
3.
Theory
[7] A forcing at high latitudes excites coastal Kelvin waves
which propagate along the western boundary to the equator, where
they are deflected across the basin and then propagate poleward in
both hemispheres along the eastern boundary [Kawase, 1987;
Cane, 1989; Yang, 1999]. The damping of overturning anomalies
results from a fundamental asymmetry between the dynamics of
these Kelvin waves on western and eastern boundaries. In the
hemisphere in which forcing is applied, Kelvin waves on the
western boundary efficiently communicate transport anomalies to
the equator. Conversely, in the opposite hemisphere, where an
immediate response on the western boundary is not possible,
eastern boundary Kelvin waves are inefficient at communicating
transport anomalies poleward, because they radiate Rossby waves
westward into the interior. Thus the hemisphere in which forcing is
7-1
7-2
JOHNSON AND MARSHALL: ABRUPT CHANGE IN THERMOHALINE CIRCULATION
Figure 1. Schematic meridional cross-section of the Atlantic
ocean. Warm surface water overlies colder North Atlantic Deep
Water. The arrows show the direction of the overturning
circulation. Is the strength of the circulation set by the rate of
deep water formation at high latitudes in the northern hemisphere
(associated with the rate at which heat is lost to the atmosphere), or
by the rate at which water is driven northwards across isopycnals in
the Southern Ocean (determined by the wind stress t)?
applied responds rapidly, whereas the opposite hemisphere
responds much more slowly, on a timescale controlled by Rossby
wave propagation [Johnson and Marshall, 2002 ]. High frequency
variability is confined to the hemisphere in which it is generated.
The dependence of Rossby wave speed on latitude means that the
lower the frequency, the further a signal penetrates into the other
hemisphere.
[8] The response of the overturning circulation to changes in
forcing can be predicted by a simple analytical theory which relates
anomalies in meridional overturning to the zonal propagation of
heat content anomalies by Rossby waves (Figure 3A and Johnson
and Marshall [2002]):
TN TS ¼
provides a sharp cut off. In this case the linear theory captures the
qualitative aspects of the damping but there are quantitative differences, particularly in the northern hemisphere. These arise because
the linear theory assumes a separation between the timescale of the
forcing and the Kelvin wave propagation time. At annual frequencies this assumption starts to break down. In general though, the
theoretical results compare well with those diagnosed from the full
numerical calculations, which suggests that the theory successfully
captures the response of the Atlantic overturning circulation to
variability in forcing. Figure 3D shows the theoretical prediction for
forcing periods from 1 to 100 years. In the steady state (at zero
frequency) the overturning circulation strength at all latitudes is
equal to the imposed forcing on the northern boundary, T = TN.
4.
Discussion
[10] The theory confirms that the spatial extent of variability in
Atlantic meridional overturning is highly dependent upon frequency. Variability at decadal and higher frequencies is confined
to a single hemisphere, rather than being global in extent. Specifically, this implies that (a) overturning variability resulting from
high frequency changes in buoyancy forcing in the Labrador and
Greenland Seas will be limited to the North Atlantic, and (b) any
observed decadal and higher frequency fluctuations in North
Atlantic overturning can only result from changes in the surface
Z L
he ðt Þ R c df
he t c
where f is the latitude, L(f) is the width of the Atlantic, c(f) is the
Rossby wave speed, R is the radius of the Earth and TN and TS are
boundary conditions on meridional transport at the north and south
of the domain. The integral is evaluated over the entire latitudinal
extent of the basin. The only unknown is he, the surface layer
thickness on the eastern boundary, at the present time, and hence
the evolution of the system is described in terms of this single
variable. Assuming periodic anomalies allows us to derive the
damping of the meridional transport signal as a function of latitude
and frequency:
T ðf; wÞ
¼
TN
g0 H
2
sin fS
g0 H
2
sin fS
iwL
R c 1 e c df
Rf
iwL
fSN R c 1 e c df
Rf
fS
where g0 is the reduced gravity (here 0.02 m s2), H is the surface
layer depth, is the Earth’s rotation rate, w is the frequency, and
fN and fS are the northern and southern extents of the domain
respectively.
[9] Panels B and C of Figure 3 show this damping for each of
the three frequencies presented in Figure 2. Plotted is the amplitude
of the variability in meridional transport (T ) at each latitude as a
fraction of the meridional transport prescribed on the northern
boundary (TN). In B the damping diagnosed from the numerical
calculation is plotted, whilst C shows the theoretical results. At low
frequencies the theory captures the details of the latitudinal response
well. For a 40 year forcing period the overturning circulation
throughout the entire basin is significantly affected by variability
in forcing on the northern boundary. For a 5 year forcing period the
equatorial damping is evident, with the signal in the South Atlantic
limited to the tropics. For a 1 year forcing period the equator
Figure 2. The equator as a low-pass filter. Overturning circulation strength as a function of latitude and time, for a forcing on the
northern boundary with a period of 40 years, 5 years and 1 year.
The contour interval is 0.4 Sv. Positive overturning anomalies
greater than 0.6 Sv are darkly shaded, whilst negative anomalies
appear white. Note the damping effect of the tropics in all but the
low frequency case.
JOHNSON AND MARSHALL: ABRUPT CHANGE IN THERMOHALINE CIRCULATION
7-3
Figure 4. Overturning circulation strength for the case in which
Southern Ocean winds vary sinusoidally with a period of 10 years.
The contour interval is 0.4 Sv. Positive overturning anomalies
greater than 0.6 Sv are darkly shaded, whilst negative anomalies
appear white. Note that overturning in the mid- and high-latitude
North Atlantic is unaffected.
fluxes within the North Atlantic basin itself. This also allows for
the possibility of feedback upon the atmosphere through changes in
the tropical ocean circulation [Yang, 1999; Hakkinen, 2000; Marshall et al., 2001].
[11] It follows from this that the effect of Southern Ocean winds
on the North Atlantic overturning circulation is limited. Figure 4
shows the meridional overturning strength as a function of time for
the case in which northern hemisphere deep water formation is
replaced by Southern Ocean wind forcing. The effect of Southern
Ocean winds is incorporated by imposing a zonally uniform flow
into the domain along its southern boundary, with a period of 10
years. This represents variability in the surface Ekman flow in the
Southern Ocean around Antarctica [Toggweiler and Samuels,
1995]. The equator again acts as a low-pass filter, here restricting
decadal variability in meridional transport to the southern hemisphere. There is almost no effect in the mid- and high-latitude
North Atlantic. This suggests that on decadal and shorter timescales, Southern Ocean wind variability, whilst important for the
southern hemisphere circulation, is not important for the overturning circulation in the North Atlantic, and is consequently
unlikely to influence European climate.
[12] Under the UK Natural Environment Research Council’s
Rapid Climate Change thematic programme plans are afoot to
establish a monitoring system which will detect abrupt changes in
North Atlantic meridional overturning. The localization of variability to a single hemisphere demonstrated here suggests that
monitoring efforts should be concentrated in the North Atlantic.
Figure 3. (A) Schematic illustrating the key elements of the
theory. Kelvin waves propagate from the forcing region in the
northwest corner of the domain, along the western boundary and
across the equator. These remove any pressure gradients on the
eastern boundary on a timescale of months. A small but uniform
anomaly in heat content is then carried westwards into the interior
by Rossby waves radiated from the eastern boundary (TE). A mass
budget of the boundary region (indicated by the dashed line) relates
the difference between this anomaly and that arriving at the western
boundary (TW) to the meridional divergence in volume transport
(TN – TS). This results in a simple linear theory which can be solved
analytically for the time evolution of the system and the damping of
the transport signal with latitude. (B) Damping of the transport
signal as diagnosed from the full numerical calculation, for each of
the three forcing frequencies illustrated in Figure 2. (C, D) Damping
of the transport signal as calculated from the linear theory.
[13] Acknowledgments. This work was supported by the UK
National Environmental Research Council. We are grateful to Rowan
Sutton and two anonymous reviewers for useful comments.
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H. L. Johnson and D. P. Marshall, Department of Meteorology,
University of Reading, P.O. Box 243, Reading, Berkshire, RG6 6BB,
UK. ([email protected]; [email protected])