Draft Fact-sheet Thermohaline Circulation

Christof Appenzeller, christof@climate.unibe.ch    http://www.climate.unibe.ch/~christof/div/factthc.html 

1.What is the Thermohaline Circulation

The thermohaline circulation is a global ocean circulation. It is driven by differences in the density of the sea water which is controlled by temperature (thermal) and salinity (haline). In the North Atlantic it transports warm and salty water to the North. There the water is cooled and sinks into the deep ocean. This newly formed deep water is subsequently exported southward. This slow (~0.1 m/s), but giant circulation has a flow equal to about 100 Amazon Rivers. Together with the Gulfstream it contributes (2/3 and 1/3) to the comparatively warm sea surface temperature along the coast of western Europe and to the relative mild European winters. Once the water are in the deep, they remain from the atmosphere for up to 1000 years.

Broecker, W., Chaotic Climate, Scientific American, November, 62-68, 1995.
 

2.Has the thermohaline circulation changed in the past ?

There is evidence for rapid climate change events lasting 1000 years or so during the last glacial. It is believed that the North European winter temperature was lowered by as much as 10 degrees during such climatic transitions. The last such cold event is known as the Younger Dryas. It occurred during the transition from the last glacial into the present holocene (~11000 years ago). The idea is that the melt water of dying continental ice masses was released into the North Atlantic where it substantially reduced the density of the ocean surface water and thereby shut down the deep water formation. This scenario is supported by both paleoclimatological evidence as well as model studies.

Rahmstorf, S., Bifurcation of the Atlantic thermohaline circulation in response to changes in the hydrological cycle, Nature, 378, 145-149, 1995.
Stocker, T. F., and D. G. Wright, Rapid transitions of the ocean's deep circulation induced by changes in surface water fluxes , Nature, 351, 729-732, 1991, (Abstract).
 

3.Can the thermohaline circulation change due to global warming ?

Whether or not the thermohaline circulation will be affected by human induced global warming is strongly dependent on the future temperature distribution and fresh water supply over the North Atlantic region. Most models predict an increase in precipitation in high latitudes and a region of minimum warming over the North Atlantic using a scenario of doubling CO2 within the next 70 years. Most models also predict a decrease in the strength of the thermohaline circulation. However, the exact reduction varies from 30% to only 10%. The details and the long-term effects (more then 100 years) of these changes have so far only been explored by very few studies. One of these studies was done at the University of Bern using a zonally averaged climate model (Stocker and Schmittner, 1997). It shows that the thermohaline circulation not only reduces, but may shut-down completely under “strong“ global warming with a fourfold increase of CO2 concentration within the next 140 years. This illustrates that global warming can affect the climate system in a very non-linear fashion.

Manabe, S., and R. Stouffer, Century-scale effects of increased atmospheric CO2 on the ocean-atmosphere system, Nature, 364, 215-218, 1993.
Stocker, T. F., and A. Schmittner, Rate of Global Warming Determines the Stability of the Ocean-Atmosphere System, Nature, 388, 862-865, 1997, (Abstract).
 

3.1 Details of the thermohaline circulation shut-down experiment

Figure 1 , Figure 2 and Figure 3 illustrate the difference between reduced and collapsed thermohaline circulation as seen by global warming experiments using a zonally averaged climate model (Stocker and Schmittner, 1997). For the IPCC scenario of doubling CO2 within the next 70 years (red line) the thermohaline circulation was only temporarily reduced (by a factor of 30% in the first 100 years). After the CO2 level was maintained constant the circulation did recover, although the recovering process was much slower (~500 year). For the CO2 quadrupling experiment within 140 years (blue curve) the response is substantially different. After about 200 years the thermohaline circulation completely shut down and remained so, even after the CO2 levels were held constant. For these scenarios the climate sensitivity was comparatively high and the global mean temperature was increasing ~7 degree in contrast to 3.5 degree as for the doubling CO2 experiment. But three-dimensional coarse resolution coupled model suggest, that the regional temperature increase might vary strongly. The shut-down of the thermohaline circulation in such a model leads to substantial less warming over the North Atlantic (plus 4-5 degrees for 2xCO2, but only plus 5-6 degrees for 4xCO2 instead of the expected 8-10 degrees) and the surrounding land. The effect over central Europe was again a reduced warming (plus 5 degree for 2xCO2, plus 7-8 degree for 4xCO2 instead of 10 degrees). It should be kept in mind that these results are very much model and parameter sensitive, but they illustrate the range of possible future climate changes.

IPCC, Climate Change 1995, The Science of Climate Change, Intergovernmental Panel on Climate Change, Cambridge University Press, Cambridge GB, 572pp., 1996.
Manabe, S., and R. Stouffer, Century-scale effects of increased atmospheric CO2 on the ocean-atmosphere system, Nature, 364, 215-218, 1993.
Manabe, S., and R. Stouffer, Multiple-Century Response of a Coupled Ocean-Atmosphere Model to an Increase of Carbon Dioxide, J. Climate, 7, 5-23, 1994.
Stocker, T. F., and A. Schmittner, Rate of Global Warming Determines the Stability of the Ocean-Atmosphere System, Nature, 388, 862-865, 1997, (Abstract).
 

Is there a link between interdecadal temperature variations and the thermohaline circulation?

Temperature records going back to the beginning of the century show a range of interannual and interdecadal variations. It is believed that these are linked to natural ‘oscillations’ of the climate system such as the ENSO (El-Nino- Southern Oscillation) and the NAO (North Atlantic Oscillation) phenomena. For example the predominant phase of the NAO in the last decade has substantially contributed to the recent winter time warming over Europe. Since the ocean and the atmosphere are a coupled system, it is reasonable to assume that variations in the ocean circulation or in the atmosphere would interact with each other. Idealized model studies show indeed, that regular and irregular oscillatory behavior in the thermohaline circulation can exist with time scales of approximately 20 to 50 years. However their existence in the real climate system is still unclear. It is the aim of one of the projects of CLEAR 2 to contribute to the understanding of these processes.

Delworth, T., S. Manabe, and R. J. Stouffer, Interdecadal Variations of the Thermohaline Circulation in a Coupled Ocean-Atmosphere Model, J. Climate, 6, 1993-2011, 1993.
Hurrell, J. W., Decadal Trends in the North Atlantic Oscillation Regional Temperatures and Precipitation, Science, 269, 676-679, 1995.

 

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