Natural fluctuations in the ocean temperature in the North Atlantic have a significant impact on the climate in the northern hemisphere. These fluctuations are the result of a complex dance between the forces of nature, but researchers at Aarhus University can now show that solar activity and the impact of volcanic eruptions have led this dance during the last two centuries.
Imagine a ballroom in which two dancers apparently keep in time to their own individual rhythm. The two partners suddenly find themselves moving to the same rhythm and, after a closer look, it is clear to see which one is leading.
It was an image like this that researchers at Aarhus University were able to see when they compared studies of solar energy release and volcanic activity during the last 450 years, with reconstructions of ocean temperature fluctuations during the same period.
The results actually showed that during the last approximately 250 years — since the period known as the Little Ice Age — a clear correlation can be seen where the external forces, i.e. the Sun’s energy cycle and the impact of volcanic eruptions, are accompanied by a corresponding temperature fluctuation with a time lag of about five years.
In the previous two centuries, i.e. during the Little Ice Age, the link was not as strong, and the temperature of the Atlantic Ocean appears to have followed its own rhythm to a greater extent.
The results were recently published in the scientific journal Nature Communications.
In addition to filling in yet another piece of the puzzle associated with understanding the complex interaction of the natural forces that control the climate, the Danish researchers paved the way for linking the two competing interpretations of the origin of the oscillation phenomenon.
Temperature fluctuations discovered around the turn of the millennium
Although the temperature fluctuations are small — less than 1°C — there is a general consensus among climate researchers that the AMO phenomenon has had a major impact on the climate in the area around the North Atlantic for thousands of years, but until now there has been doubt about what could cause this slow rhythm in the temperature of the Atlantic Ocean. One model explains the phenomenon as internal variability in the ocean circulation — somewhat like a bathtub sloshing water around in its own rhythm. Another model explains the AMO as being driven by fluctuations in the amount of solar energy received by the Earth, and as being affected by small changes in the energy radiated by the Sun itself and the after-effects of volcanic eruptions. Both these factors are also known as ‘external forces’ that have an impact on the Earth’s radiation balance.
However, there has been considerable scepticism towards the idea that a phenomenon such as an AMO could be driven by external forces at all — a scepticism that the Aarhus researchers now demonstrate as unfounded
“Our new investigations clearly show that, since the Little Ice Age, there has been a correlation between the known external forces and the temperature fluctuations in the ocean that help control our climate. At the same time, however, the results also show that this can’t be the only driving force behind the AMO, and the explanation must therefore be found in a complex interaction between a number of mechanisms. It should also be pointed out that these fluctuations occur on the basis of evenly increasing ocean temperatures during the last approximately fifty years — an increase connected with global warming,” says Associate Professor Mads Faurschou Knudsen, Department of Geoscience, Aarhus University, who is the main author of the article.
Convincing data from the Earth’s own archives
“We’ve only got direct measurements of the Atlantic Ocean temperature for the last 140 years, where it was measured by ships. But how do you measure the water temperature further back in time? Studies of growth rings in trees from the entire North Atlantic region come into the picture here, where ‘good’ and ‘bad’ growth conditions are calibrated to the actual measurements, and the growth rings from trees along the coasts further back in time can therefore act as reserve thermometers,” explains Associate Professor Knudsen.
The results provide a new and very important perspective on the AMO phenomenon because they are based on data and not computer models, which are inherently incomplete. The problem is that the models do not completely describe all the physical correlations and feedbacks in the system, partly because these are not fully understood. And when the models are thus unable to reproduce the actual AMO signal, it is hard to know whether they have captured the essence of the AMO phenomenon.
Impact of the sun and volcanoes
“An interesting new theory among solar researchers and meteorologists is that the Sun can control climate variations via the very large variations in UV radiation, which are partly seen in connection with changes in sunspot activity during the Sun’s eleven-year cycle. UV radiation heats the stratosphere in particular via increased production of ozone, which can have an impact on wind systems and thereby indirectly on the global ocean currents as well,” says Associate Professor Knudsen. However, he emphasises that researchers have not yet completely understood how a development in the stratosphere can affect the ocean currents on Earth.
Towards a better understanding of the climate
“During the last century, the AMO has had a strong bearing on significant weather phenomena such as hurricane frequency and droughts — with considerable economic and human consequences. A better understanding of this phenomenon is therefore an important step for efforts to deal with and mitigate the impact of climate variations,” Associate Professor Knudsen concludes.
Note : The above story is based on materials provided by Aarhus University. The original article was written by Christina Troelsen.