13.2 Concepts Cds

concepts
The current climate
Thermohaline
circulation
downward penetration of heat at low latitudes,
Stefan Rahmstorf
due to turbulent mixing, that provides this
Heat and freshwater fluxes at the
How are ocean currents caused, and how thermal engine. His speculation that salinity- ocean’s surface play a key role in
do they affect climate? These questions
driven convection causes the low-latitude
were hotly debated in the nineteenth
mixing was wrong, however — turbulent
forming ocean currents, which in
century. Some argued that water is simply
mixing is instead powered by winds and tides.
turn have a major effect on climate.
pushed along by the wind; others postulated
Thermohaline circulation is thus caused by the
‘convection currents’ caused either by heat-
joint effect of thermohaline forcing and turbu-
ing and cooling or by evaporation and pre-
lent mixing — it can be defined as currents
tions can thus be disentangled to some extent
cipitation. Even today, the driving forces and
driven by fluxes of heat and freshwater across
by models, which helps us understand what
climatic effects of ocean currents are still not
the sea surface and subsequent interior mixing
aspects of the circulation are linked to what
completely understood.
of heat and salt. Although winds and tides are
surface boundary conditions. This is useful in
In 1908, Johan Sandström laid the founda-
important in creating turbulence, this driving
analysing the effect of a change in forcing, such
tions of our modern understanding of ocean
mechanism is clearly distinct from wind-
as a freshwater influx, on currents — a typical
currents with a series of classic experiments
driven circulation: thermohaline circulation
problem in palaeoclimate studies, in which
carried out at Bornö oceanographic station in
requires thermohaline surface forcing, where-
sediment data suggest that freshwater influx
Sweden. He filled a small tank with water layers
as wind-driven circulation does not.
has caused major changes in currents in the
of different densities from an adjacent fjord
To complicate matters, the ocean’s density
past. It is also highly relevant when consider-
TE SENSING
and then blew air over the surface and/or heat-
distribution, which determines pressure gra-
ing the ocean’s response to global warming,
ed and cooled the fluid at different levels. He
dients and thus circulation, is itself affected
because evaporation, precipitation and runoff
thus elucidated the properties of ‘wind-driven’
by currents and mixing of any kind. Thermo-
are expected to increase in a warmer world.
. MAR. REMO
and ‘thermal’ circulation. The latter term was
haline and wind-driven currents cannot
How strongly might changes in thermo-
amended by the 1920s to ‘thermohaline circu-
therefore be separated by oceanographic
haline circulation affect climate? To what
A/INST
lation’, because water density in the ocean is
measurements. There are thus two distinct
extent do Europe’s mild winters depend on
ORID
determined by both temperature and salinity.
forcing mechanisms, but not two separate the transport of heat by the Gulf Stream and
. S. FL
Sandström found that thermal forcing can
circulations. Change the wind stress, and the
North Atlantic Current? Simulations in
give rise to a steady circulation only if heating
thermohaline circulation will change; alter
which the ocean’s heat transport is switched
occurs at a greater depth than cooling — a fact
thermohaline forcing, and the wind-driven
off consistently show a large winter cooling
GER/UNIV
AR
that is familiar to oceanography students as
currents will also change. It is because of ther-
over the northern Atlantic and adjacent land
-K
‘Sandström’s theorem’. But ‘thermohaline
mohaline forcing that wind-driven currents
areas, reaching several degrees in inland
forcing’ — that is, fluxes of heat and freshwater
are relegated to the upper ocean — in unstrat-
Europe, up to 10 C over Greenland and even
. MULLER
— occurs only at the ocean’s surface, except for
ified water they would extend to the bottom.
exceeding 20 C over the Nordic seas. This
a small contribution from geothermal heating.
But although thermohaline circulation is
heat transport warms the climate on both
So what is the deep heat source that drives
not measurable, the concept is still a useful
sides of the Atlantic, and is therefore not the
the ocean’s observed thermohaline circula-
one, and modern computer models of oceans
main reason that Europe is warmer than
S. ANDREFOUET/F
tion? Sandström recognized that it is the
can be used to carry out experiments (not
Newfoundland — this phenomenon is mainly
unlike those of Sandström with real water) to
due to the prevailing winds in the two
study the properties of these currents. In these
regions. But ocean currents do make the
models, different surface forcing fields can be
northern Atlantic much warmer than at
prescribed, and by designating the surface
comparable latitudes in the northern Pacific.
wind-stress as zero, a purely thermohaline Changes in these currents are our best expla-
circulation can be computed. The required
nation for the abrupt and marked climate
turbulent mixing can be varied independently
swings that occurred over the north Atlantic
from the surface wind stress, as they appear in
many times during the most recent glacial
different terms of the hydrodynamic equa-
period, as shown by Greenland’s ice cores and
tions. The resulting zonally integrated circu-
by deep-sea sediments. Circulation changes
lation is essentially similar to the circulation
might again be triggered by global warming.
obtained with wind-stress forcing, but lacks
Despite its known limitations, the concept
the wind-driven cells known as Ekman cells
of a thermohaline ocean circulation remains
(which consist of surface water that is pushed
well defined and useful. Understanding its
along by the wind and returns within the
past and future behaviour is crucial to our
upper few hundred metres of the ocean).
understanding of climate change.
s
In this sense, zonally integrated stream-
Stefan Rahmstorf is at the Potsdam Institute for
lines can be interpreted as a superposition of
Climate Impact Research, Box 601203,
wind-driven and thermohaline components,
14412 Potsdam, Germany.
much as Sandström interpreted his tank
experiments. On the other hand, when wind
FURTHER READING
stress remains constant, the vertically inte-
Rahmstorf, S. Nature 419, 207–214 (2002).
grated circulation looks similar with or with-
Seager, R. et al. Q. J. R. Meteorol. Soc. 128, 2563 (2002).
out thermohaline forcing, with the exception
Vellinga, M. & Wood, R. A. Clim. Change 54,
Wave review: ocean currents sculpted these sand
of the Antarctic Circumpolar Current.
251–267 (2002).
and seaweed formations, dozens of miles across.
Wind-driven and thermohaline circula-
Wunsch, C. Nature 405, 743–744 (2000).
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