German scientists say they've confirmed something long suspected: that the weather in some regions of Earth is influenced by the sun's 11-year cycle of activity.
While records of average, seasonal temperatures don't go back far enough to confirm any patterns, the team's used the freezing of Germany's largest river, the Rhine, as a proxy for particularly cold winters in Central Europe.
"The advantage with studying the Rhine is because it's a very simple measurement," says Frank Sirocko of Johannes Gutenberg University in Mainz.
"Freezing is special in that it's like an on-off mode. Either there is ice or there is no ice."
Using cargo transport data, the team was able to determine the number of freezing episodes since 1780, finding that between then and 1963, the Rhine froze in multiple places fourteen different times.
Mapping these freezing episodes against the 11-year solar cycle, they found that ten of the fourteen freezes occurred during years when the sun had minimal sunspots. Using statistical methods, the scientists calculated that there is a 99 percent chance that extremely cold Central European winters and low solar activity are inherently linked.
"We provide, for the first time, statistically robust evidence that the succession of cold winters during the last 230 years in Central Europe has a common cause," says Sirocko.
When sunspot numbers are down, the sun's emitting less ultraviolet radiation. This means less heating of Earth's atmosphere, changing the circulation patterns of the troposphere and stratosphere.
This affects the North Atlantic Oscillation, a pattern of atmospheric pressure variations that influences wind patterns in the North Atlantic and weather behavior in regions in and around Europe.
"Due to this indirect effect, the solar cycle does not impact hemispherically averaged temperatures, but only leads to regional temperature anomalies," says Stephan Pfahl of the Institute for Atmospheric and Climate Science in Zurich.
The authors show that this change in atmospheric circulation leads to cooling in parts of Central Europe but warming in other European countries, such as Iceland.