North Atlantic Current

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North Atlantic Current (red / orange)

The North Atlantic Current (English: North Atlantic Current, NAC) is a warm ocean current that the Gulf Stream north-east as far as Europe extended. It is powered by the thermohaline circulation , here called the Atlantic overturning circulation .

Due to its heat transport, the North Atlantic Current acts like a large heater, thanks to which large parts of Western and Northern Europe , such as Ireland , Great Britain and Scandinavia , have a warmer climate than would be expected due to their high geographical latitude .

course

The Gulf Stream meets the cold Labrador Current off the coast of Newfoundland and partially merges with it. It branches out and forms branches. This extension of the Gulf Stream to Europe is the North Atlantic Current, even if it is also often referred to as the Gulf Stream in some representations.

North of Ireland, part of the North Atlantic Current continues as the Norwegian Current to Spitsbergen , while another part drifts towards Iceland .

Thermohaline circulation

While the Gulf Stream is driven by winds and continental debris, the engine of the North Atlantic Current is the global conveyor belt or “thermohaline circulation”. On the way to the northern end of the North Atlantic Current, parts of the transported warm water evaporate ( evaporation ). This increases the salinity ( salinity ), which makes the water heavier and starts to sink. There the North Atlantic drift becomes part of the North Atlantic deep water , a southward ocean current. The system is also known as the North Atlantic Meridional Overturning Circulation (AMOC).

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Important ocean currents

effect

The North Atlantic Current has a significant influence on the European climate, which is why it is often referred to as "Europe's hot water heating". Its warm water also warms the air over the sea. The winds then transport the heat far into the European continent, which means that Europe has a much milder climate than other areas of the same latitude. North of the 50th parallel in Canada , for example, there is an extremely cold climate in which only mosses and lichens thrive, and only cold- resistant animals such as caribou live in this tundra . In contrast, the North Atlantic Current gives Central Europe deciduous forests and lush meadows, good conditions for arable farming and cattle breeding, and in the areas particularly close to the North Atlantic Current it creates very special opportunities:

In Cornwall, and especially on the Isles of Scilly , even plants grow that are normally only found in much warmer climates. Palm trees can survive the otherwise harsh northern winters there. The Logan Botanic Garden in Scotland, for example, benefits greatly from the North Atlantic Current: Individual specimens of the mammoth leaf ( Gunnera manicata ) have grown over 3 meters high.

Changes due to global warming

Index of the development of the Atlantic circulation since 900 based on anomalies of sea ​​surface temperatures in the subsidence area.

In connection with the current phenomenon of global warming , scientists have expressed concern that the subsidence mechanism described above under thermohaline circulation could become unbalanced in the next 20–100 years.

In the event of global warming, two processes can reduce the density of the seawater in the North Atlantic Current and thus slow down its sinking: increasing amounts of meltwater from the Greenland ice sheet bring in more fresh water, and lower heat losses from the near-surface water to the atmosphere mean that the volume of the water decreases less. The result would be a weakening and possibly a shift in the North Atlantic Current, which would result in a climate change in Northern Europe, with potentially significant consequences. Without the heat emission from the North Atlantic Current, there would be a maximum reduction in the average air temperature in Europe of five degrees Celsius, which would counteract the expected global warming in Europe caused by humans. Whether or from when the two effects would cancel each other out cannot be predicted; It is conceivable that temperatures in Europe would initially rise slightly and then fall permanently by up to 5 degrees below today's values.

Findings from sediment and ice cores suggest that similar events have happened several times in the past. These are known as the Heinrich Events .

The development of the North Atlantic Current over the past decades and centuries and its future development in the context of current climate change is the subject of intensive research. Work on a weakening of the North Atlantic Current over the last few decades did not produce any clear results: A study from 2005 concluded that there had been a 30% decrease in the North Atlantic Current between 1957 and 2004. This could not be confirmed in subsequent work because the North Atlantic current is subject to relatively strong short-term fluctuations; individual measurements were within this range of fluctuation and therefore did not allow any conclusions to be drawn about developments. Rather, long-term measurements in the Labrador Sea showed no signs of weakening until 2007. According to a study published in early 2008, the warming since the last ice age has increased the oceanic circulation; the study proposes that the further warming of the middle layers of the atmosphere in the course of global warming will lead to a further increase in ocean currents.

In 2011 a study on the Agulhas Current in the Indian Ocean was published. Accordingly, this is not completely reflected on the east coast of Africa, but also flows to a small extent into the Atlantic. This could have a greater effect on climate events in the northern hemisphere than previously assumed and thus the following model ideas on global warming, which were also published by the IPCC, turn out to be wrong: It is assumed that the freshwater input due to increased melting in the polar region of the northern hemisphere will weaken the North Atlantic current in the future and the reduced heat input would slow down the warming of the northern hemisphere. If the salt water input from the Agulhas Current continues to increase - as has been observed over the last few decades - this would also increase the North Atlantic Current and thus lead to additional warming instead of cooling.

The cold blob in the sinking area of ​​the North Atlantic current.

There have been various attempts to infer the evolution of Atlantic currents over several centuries from reconstructed anomalies of sea level or sea temperatures. Both parameters are influenced by the thermohaline current in the Atlantic and could serve as an indication of its strength in the past. A study from 2015 came to the conclusion that there had been an unusual slowdown in the past few decades. This would also explain the unusual " cold blob " in the sinking area of ​​the North Atlantic current, which, in contrast to the rest of the northern hemisphere, cools down instead of warms up. In February 2018, Stefan Rahmstorf and colleagues published a confirmation of a fifteen percent weakening of the Gulf Stream since the middle of the 20th century in the journal Nature . In addition to the large-scale cooling in the North Atlantic, they found a further indication of a temperature increase in the Gulf Stream off the northeast coast of the USA, which was also predicted by their climate models of global warming. This is due to the fact that the weakened Gulf Stream is less deflected by the Coriolis force and runs closer to the US coast. Observation data on sea temperature since 1870 were evaluated (Levke Caesar) and compared with a high-resolution model calculation, the climate model CM2.6, taking into account the increase in anthropogenic greenhouse gas emissions. This was done on the 11,000-processor high-performance computer at the Geophysical Fluid Dynamics Laboratory in Princeton for over half a year. In the same edition of Nature, a study by David Thornalley and colleagues was published, which is based on the analysis of marine drill cores, the deep circulation in terms of flow velocity (size of sediment grains) and temperature (isotope data from calcareous shells) in the Labrador Sea over the past 1600 years examined. In doing so, they found a weakening of the circulation in the last hundred years, another indication of the weakening of the Gulf Stream.

exploration

In the course of the Cold War, the two superpowers at the time (USA and USSR) spent a lot of money and effort on researching the sea as well as sea wind and sea weather. For this purpose, satellites were launched into geostationary orbits or lower orbits, for example:

Thermal imaging cameras on board provide images of the surface temperature of the sea ​​water . Satellites with radar can measure the height of the sea surface (which changes due to tides, storms, etc.) and also its ripple.

literature

  • Martha W. Buckley and John Marshall: Observations, inferences, and mechanisms of the Atlantic Meridional Overturning Circulation: A review . In: Reviews of Geophysics . 2016, doi : 10.1002 / 2015RG000493 ( HTML ).

Web links

Individual evidence

  1. ↑ a b Stefan Rahmstorf, Jason E. Box, Georg Feulner, Michael E. Mann, Alexander Robinson, Scott Rutherford and Erik J. Schaffernicht: Exceptional twentieth-century slowdown in Atlantic Ocean overturning circulation . In: Nature Climate Change . 2015, doi : 10.1038 / nclimate2554 .
  2. ↑ Kirsten Zickfeld u. a .: Expert judgments on the response of the Atlantic meridional overturning circulation to climate change . In: Climatic Change . tape 82 , no. June 3 , 2007, doi : 10.1007 / s10584-007-9246-3 .
  3. ^ A b Martha W. Buckley and John Marshall: Observations, inferences, and mechanisms of the Atlantic Meridional Overturning Circulation: A review . In: Reviews of Geophysics . 2016, doi : 10.1002 / 2015RG000493 ( HTML ).
  4. ↑ Stefan Rahmstorf: The world goes roller coaster (July 1999)
  5. ^ HL Bryden, HR Longworth, SA Cunningham: Slowing of the Atlantic meridional overturning circulation at 25 ° N. In: Nature. Volume 438 (2005), December 1
  6. ↑ Thomas Haine et al. a .: North Atlantic Deep Water Formation in the Labrador Sea, Recirculation through the Subpolar Gyre, and Discharge to the Subtropics . In: Arctic-Subarctic Ocean Fluxes, Defining the Role of the Northern Seas in Climate . 2008, ISBN 978-1-4020-6773-0 .
  7. ↑ Mojib Latif et al. a .: Is the Thermohaline Circulation Changing? In: Journal of Climate . 2006, doi : 10.1175 / JCLI3876.1 .
  8. ↑ Dr. Andreas Villwock: How is the Gulf Stream reacting to climate change? New findings from a 10-year study in the subpolar North Atlantic. IFM-GEOMAR , March 16, 2007, accessed on June 12, 2016 (press release).
  9. ↑ JR Toggweiler, Joellen Russell, Ocean circulation in a warming climate , Nature 451, 286-288, January 17, 2008, doi: 10.1038 / nature06590 .
  10. ↑ Threading the Climate Needle: The Agulhas Current System Increased Agulhas "leakage" significant player in global climate variability Press release of the National Science Foundation 4/2011.
  11. ^ L. Caesar, S. Rahmstorf, A. Robinson, G. Feulner, V. Saba: Observed fingerprint of a weakening Atlantic Ocean overturning circulation. Nature, Volume 556, 2018, pp. 191-196, doi : 10.1038 / s41586-018-0006-5
  12. ↑ Summer Praetorius, Northatlantic Circulation slows down , Nature April 11, 2018
  13. ↑ Rahmstorf, Stronger Evidence for a Weaker Gulf Stream System, Scilogs, April 11, 2018
  14. ↑ Thornalley et al. a., Anomalously weak Labrador Sea convection and Atlantic overturning during the past 150 years, Nature, Volume 556, 2018, pp. 227-230, abstract