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| Credit: Illustration by Bo Holm Jacobsen, Aarhus Universitet |
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
The
climate is defined on the basis of data including mean temperature
values recorded over a period of thirty years. Northern Europe thus has a
warm and humid climate compared with other regions on the same
latitudes. This is due to the North Atlantic Drift (often referred to as
the Gulf Stream), an ocean current that transports relatively warm
water from the south-west part of the North Atlantic to the sea off the
coast of Northern Europe.
Around the turn of the millennium,
however, climate researchers became aware that the average temperature
of the Atlantic Ocean was not entirely stable, but actually fluctuated
at the same rate throughout the North Atlantic. This phenomenon is
called the Atlantic Multidecadal Oscillation (AMO), which consists of
relatively warm periods lasting thirty to forty years being replaced by
cool periods of the same duration. The researchers were able to read
small systematic variations in the water temperature in the North
Atlantic in measurements taken by ships during the last 140 years.
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
Researchers
have attempted to make computer simulations of the phenomenon ever
since the discovery of the AMO, partly to enable a better understanding
of the underlying mechanism. However, it is difficult for the computer
models to reproduce the actual AMO signal that can be read in the
temperature data from the last 140 years.
Associate Professor
Knudsen and his colleagues instead combined all available data from the
Earth's own archives, i.e. previous studies of items such as radioactive
isotopes and volcanic ash in ice cores. This provides information about
solar energy release and volcanic activity during the last 450 years,
and the researchers compared the data with reconstructions of the AMO's
temperature rhythm during the same period.
"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
attempt to simply explain how external forces such as the Sun and
volcanoes can control the climate could sound like this: a stronger Sun
heats up the ocean, while the ash from volcanic eruptions shields the
Sun and cools down the ocean. However, it is hardly as simple as that.
"Fluctuations
in ocean temperature have a time lag of about five years in relation to
the peaks we can read in the external forces. However, the direct
effect of major volcanic eruptions is clearly seen as early as the same
year in the mean global atmospheric temperature, i.e. a much shorter
delay. The effect we studied is more complex, and it takes time for this
effect to spread to the ocean currents," explains Associate Professor
Knudsen.
"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
"In
our previous study of the climate in the North Atlantic region during
the last 8,000 years, we were able to show that the temperature of the
Atlantic Ocean was presumably not controlled by the Sun's activity. Here
the temperature fluctuated in its own rhythm for long intervals, with
warm and cold periods lasting 25-35 years. The prevailing pattern was
that this climate fluctuation in the ocean was approximately 30-40%
faster than the fluctuation we'd previously observed in solar activity,
which lasted about ninety years. What we can now see is that the
Atlantic Ocean would like to -- or possibly even prefer to -- dance
alone. However, under certain circumstances, the external forces
interrupt the ocean's own rhythm and take over the lead, which has been
the case during the last 250 years," says Associate Professor Bo Holm
Jacobsen, Department of Geoscience, Aarhus University, who is the
co-author of the article.
"It'll be interesting to see how long
the Atlantic Ocean allows itself to be led in this dance. The scientific
challenge partly lies in understanding the overall conditions under
which the AMO phenomenon is sensitive to fluctuations in solar activity
and volcanic eruptions," he continues.
"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.
Story Source:
The above story is based on
materials provided by
Aarhus University. The original article was written by Christina Troelsen.
Note: Materials may be edited for content and length.
Journal Reference:
- Mads Faurschou Knudsen, Bo Holm Jacobsen, Marit-Solveig Seidenkrantz, Jesper Olsen. Evidence for external forcing of the Atlantic Multidecadal Oscillation since termination of the Little Ice Age. Nature Communications, 2014; 5 DOI: 10.1038/ncomms4323
