What is the thermohaline circulation
As opposed to wind-driven currents and tides (which
are due to the gravity of moon and sun), the thermohaline circulation
(Fig. 1) is that part of the ocean circulation which is driven
by density differences. Sea water density depends on temperature
and salinity, hence the name thermo-haline. The salinity and temperature
differences arise from heating/cooling at the sea surface and
from the surface freshwater fluxes (evaporation and sea ice
formation enhance salinity; precipitation, runoff and ice-melt decrease
salinity). Heat sources at the ocean bottom play a minor role.
Figure 1. Schematic
representation of the global thermohaline circulation.
Surface currents are shown
in red, deep waters in light blue and bottom waters in dark
blue. The main deep water formation sites are shown in orange.
(After , modified by S.R.)
In contrast to the wind-driven currents, the THC
is not confined to surface waters but can be regarded as a big overturning
of the world ocean, from top to bottom. The thermohaline circulation
- Deep water formation: the sinking of water
masses, closely associated with (but not to be confused with)
convection, which is a vertical mixing process, ). Deep water
formation takes place in a few localised areas: the Greenland-Norwegian
Sea, the Labrador Sea, the Mediteranean Sea, the Wedell Sea, the
- Spreading of deep waters (e.g., North
Atlantic Deep Water, NADW, and Antarctic Bottom Water,
AABW), mainly as deep western boundary currents (DWBC).
- Upwelling of deep waters: this is not
as localised and difficult to observe. It is thought to take place
mainly in the Antarctic Circumpolar Current region, possibly aided
by the wind (Ekman divergence).
- Near-surface currents: these are required
to close the flow. In the Atlantic, the surface currents compensating
the outflow of NADW range from the Benguela Current off South
Africa via Gulf Stream and North Atlantic Current
into the Nordic Seas off Scandinavia (Fig. 2). (Note that the
Gulf Stream is primarily a wind-driven current, as part of the
subtropical gyre circulation. The thermohaline circulation
contributes only roughly 20% to the Gulf Stream flow.)
|Figure 2. Thermohaline circulation of the
This highly simplified cartoon of Atlantic currents
shows warmer surface currents (red) and cold north Atlantic
Deep Water (NADW, blue). The thermohaline circulation heats
the North Atlantic and Northern Europe. It extends right up
to the Greenland and Norwegian Seas, pushing back the winter
sea ice margin. (From .)
Some observational data
The volume transport of the overturning
circulation at 24 N has been estimated from hydrographic section data
() as 17 Sv (1 Sv = 106 m3/s), its heat transport
as 1.2 PW (1 PW = 1015 W). More recently, an inverse model
by  yielded 15+-2 Sv NADW overturning in the high latitudes. (Note:
when comparing these numbers with models care needs to be taken what
exactly is compared - in models, the most common measure of NADW overturning
is the maximum of the zonally integrated transport stream function
in the North Atlantic, sometimes also the outflow value at 30 S.)
The short answer would be: high-latitude
cooling. In cold regions the highest surface water densities are
reached, this causes convective mixing and sinking of deep water,
which drives the circulation.
What drives the THC?
Reality is more complex. Pressure
gradients at depth, resulting from density gradients in the overlying
waters, are the driving force in the equations of motion. As the density
forcing occurs at the surface (see above), a subtle question is why
the density differences and the circulation affect the whole ocean
depth and are not confined to a near-surface layer.  showed that
a deep circulation only arises when heating (buoyancy source) is at
depth and cooling at the surface. The reason that there is a deep
circulation after all is turbulent mixing, which brings down
the heat on a time scale of ~1000 years. It has been shown that in
the long-term equilibrium the strength of the thermohaline circulation
in models depends on the turbulent mixing coefficient , and that
the energy required for this turbulent mixing comes to a large extent
from the moon via tidal currents ().
This discussion can be labelled:
is the THC pushed or pulled ()? I.e., pushed by formation
of cold deep water, or pulled by downward diffusion of heat through
the thermocline? The answer is a question of time scale: ultimately,
in the long run, it is pulled. But on shorter time scales, up to centuries,
it can be considered pushed in the sense that it is density changes
in the deep water formation regions which affect the circulation strength.
If this density drops too much so that deep water formation is not
possible, the circulation stops. Ultimately, on the long time scale
of turbulent mixing, the deep ocean density will drop as well until
new deep water formation can start.
As mentioned above, highest surface
densities in the world ocean are reached where water is very cold,
while lower densities are found in the saltier but warmer tropical
and subtropical areas. In this sense the THC is thermally driven.
Nevertheless, the influence of salinity is important and is what causes
the non-linearity of the system. This was first described in a classic
paper by  with the help of a simple box model. Salinity is involved
in a positive feedback: higher salinity in the deep water formation
area enhances the circulation, and the circulation in turn transports
higher salinity waters into the deep water formation regions (which
tend to be regions of net precipitation, i.e., freshwater would accumulate
and surface salinity would drop if the circulation stopped). Put simply,
in Stommel's model the high-latitude salinity increases linearly with
the flow, and the flow increases linearly with high-latitude salinity,
which combined gives a quadratic (i.e., non-linear) equation. This
leads to two possible equilibrium states, the system is bistable
in a certain parameter range. This becomes more than an academic point
as complex circulation models behave in the same way, and as the present
North Atlantic in many models is in the bistable regime (). The
first coupled climate model to show these two equilibria (discovered
quite by accident) is the one by .
Non-linear behaviour of the
The situation can be described
with a simple stability diagram showing strength of the THC as a function
of the freshwater input into the North Atlantic. This shows the bistable
regime and a saddle-node bifurcation point where the circulation
breaks down. It is discussed in more detail (but for the non-specialist)
An important point is that the
salt transport feedback is not the only feedback rendering the system
non-linear. The convective mixing process is itself a highly
non-linear, self-sustaining process. In models this can lead to multiple
stable convection patterns ([14, 15]), which on one hand can cause
artefacts related to the coarse model grid. On the other hand this
may be part of a real mechanism for shifts in convection location,
as have apparently occured during glacial times.
The bottom line is: salinity leads
to non-linearity which causes the existence of multiple equilibria
and thresholds in the THC.
A related question is: why is
no deep water formed in the North Pacific? Salinity there is too low,
but why? A body of literature exists on this topic; it is discussed
e.g. in . My opinion is: for geographical reasons so much freshwater
enters the North Pacific that it is far in the monostable regime where
no deep water formation is possible.
The climatic effect of the THC
is still to some extent under discussion, and is due to the heat
transport of ~1 PW of this circulation. Back-of-the-envelope calculations
suggest that this amount of heat transported into the northern North
Atlantic (north of 24 N) should warm this region by ~5K. This is indeed
roughly the difference between sea surface temperature (SST) in the
North Atlantic as compared to the North Pacific at similar latitudes.
A look at sea ice margins suggest that they are pushed back by the
warm surface currents in the Atlantic sector as compared to the North
Pacific (Fig. 1), this in turn leads to reduced reflection of sunlight
and thus warming (albedo feedback). A look at global surface
air temperatures is also quite suggestive: over the three main deep
water formation regions of the world ocean, air temperatures are warmer
by up to ~10K compared to the latitudinal mean.
The effect on climate
These observations are, however,
no quantitative proof of the climatic effect of the THC, and other
explanations can be invoked, such as planetary waves in the atmosphere,
locked in place by the geography (Rocky mountains).
One way to estimate the effect
of the THC is to switch it off in coupled climate models (by adding
a lot of freshwater to the northern Atlantic), and compare the surface
climate before and after switching it off. Roughly, this leads to
a cooling with a maximum of ~10K over the Nordic Seas (e.g., [12,
17]). The maximum tends to occur near the sea ice margin due to the
ice albedo effect. Unfortunately, the details of this cooling are
model-dependent: one model shows cooling up to 22K in annual
mean and 33K in winter (). Models also differ in how widespread
the cooling is: most tend to affect temperatures over land in
northwestern Europe (Scandinavia, Britain) by several degrees, others
show strong cooling further west affecting Canada ().
|Figure 3. Deviation of surface air temperature
from zonal mean.
Deviations are shown in degree C. Based on NCAR
surface air temperature climatology, reproduced from .
Sediment data document that the THC has undergone major
changes in the history of climate (e.g., [21, 22]). Three major circulation
modes were indentified: a warm mode similar to the present-day
Atlantic, a cold mode with NADW forming south of Iceland in the Irminger
Sea, and a switched-off mode (). The latter appears to have occurred
after major input of freshwater, either from surging glacial ice sheets
(Heinrich events) or in form of meltwater floods (e.g., Younger
Dryas event). The most dramatic climate events recorded in Greenland,
the Dansgaard-Oeschger (D/O) events, were probably associated
with north-south shifts in convection location, i.e. transitions between
warm and cold modes of the Atlantic THC. Recent simulations of such
shifts show encouraging agreement with paleoclimatic data ().
History of the THC
Global warming can affect the
THC in two ways: surface warming and surface freshening,
both reducing the density of high-latitude surface waters and thus
inhibiting deep water formation.  was the first to warn that this
could lead to a breakdown of the THC and to abrupt climate change.
Subsequently, [26, 27] showed that this could indeed occur for strong
global warming (i.e., for a quadrupling, but not for a doubling of
CO2). In these scenarios there was no surface cooling,
as the high CO2 levels more than compensated for the reduced
ocean heat transport. The possibility of a real cooling (both a relative
cooling, i.e. a drop back to roughly pre-industrial temperatures after
an initial warming phase, and in the longer run an absolute
cooling below preindustrial values) as a result of anthropogenic warming
was first demonstrated in a sensitivity study by . Significant
absolute cooling can arise after CO2 levels decline, but
the THC remains switched off after its collapse is triggered in a
rapid warming phase.
The THC in anthropogenic
A THC collapse is now widely
discussed as one of a number of "low probability - high impact"
risks associated with global warming. More likely than a breakdown
of the THC, which only occurs in very pessimistic scenarios, is a
weakening of the THC by 20-50%, as simulated by many coupled
climate models ().
open questions include:
- What changes in freshwater
input to the North Atlantic will result from global warming? (Uncertainty
e.g. due to uncertain estimates of Greenland meltwater runoff,
ignored so far in most models, and due to possible changes in
- What is the risk of exceeding
a threshold for THC collapse for a given warming?
- What other thresholds exist?
(E.g., a local shutdown of convection in the Labrador Sea as simulated
by , rather than a full THC collapse.)
- What consequences would
result for marine ecosystems?
- How would temperatures
over land be affected by a collapse scenario? (Just a reduced
warming, or a warming followed by abrupt cooling?)
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