Thermohaline Circulation

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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 ow 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 Hwere 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- 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. 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 lation’, because water density in the ocean is measurements. There are thus two distinct extent do Europe’s mild winters depend on determined by both temperature and salinity. forcing mechanisms, but not two separate the transport of heat by the Gulf Stream and 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 that is familiar to oceanography students as currents will also change. It is because of ther- over the northern Atlantic and adjacent land ‘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 ᑻC over Greenland and even — occurs only at the ocean’s surface, except for ified water they would extend to the bottom. exceeding 20 ᑻC 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. MULLER-KARGER/UNIV. S. FLORIDA/INST. MAR. REMOTE SENSING MAR. REMOTE S. FLORIDA/INST. MULLER-KARGER/UNIV. 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. ■ 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). Wave review: ocean currents sculpted these sand out thermohaline forcing, with the exception Vellinga, M. & Wood, R. A. Clim. Change 54, and seaweed formations, dozens of miles across. of the Antarctic Circumpolar Current. 251–267 (2002). Wind-driven and thermohaline circula- Wunsch, C. Nature 405, 743–744 (2000).