Ice ages are recurring phenomena in Earth’s recent geological history, and the resulting glacier advances significantly shaped the European landscape. An international research team led by Dr Stefanie Kaboth from the Institute of Earth Sciences at Heidelberg University has now shown that the extent of glaciation in Western Europe was largely driven by the advection of warm seawater towards Europe’s Atlantic coast, which raised precipitation on the continent. Their findings were published in the journal "Geophysical Research Letters".
The warm water advection is driven by sinking of salty and dense water masses that flow out of the Mediterranean Sea through the Strait of Gibraltar and into the Atlantic. This sinking motion thereby produces a suction effect that pumps warm surface waters from the subtropical Atlantic to the coast of Portugal. "During the ice ages, this raised the temperature contrast in relation to the cold continent of Europe. Westerly winds carried the evaporating ocean water to continental Europe, where it fell as precipitation and contributed to the advancing ice sheets," explains Dr Kaboth.
To better understand the role of the Mediterranean outflow in this process, the researchers reconstructed its dynamics over the past 250,000 years. Using geochemical methods, they studied drill cores from the Atlantic Ocean obtained through the international "Integrated Ocean Drilling Program". "For the first time, our data show that the Mediterranean outflow functioned as a pacemaker for the spread of continental ice sheets in Europe," continues Dr Kaboth.
The Heidelberg geoscientist further explains that the strength of the Mediterranean outflow depends mainly on the intensity of the African monsoon, as is evident from the origin of the water that flows into the Atlantic. Most of it originates from the eastern Mediterranean Sea, where hot and dry conditions lead to strong evaporation, thereby raising the salt content of the surface waters. In winter these water masses cool, become denser and flow west at greater depths, where they exit the Mediterranean Sea through the Strait of Gibraltar. If strong monsoons in Northeast Africa increase the flow of fresh water into the Mediterranean Sea, it inhibits the formation of these dense, salty waters and weakens the Mediterranean outflow. Conversely, the very dry conditions in the eastern Mediterranean that current climate models strongly predict will boost the salt content in the surface waters and thus strengthen the Mediterranean outflow.
"Our findings not only explain why the glaciers came to a halt at various geographical points on their advance towards the South, but also provide new insight into the transport of heat and humidity to Europe. If you apply our observations onto future climate development, a stronger Mediterranean outflow resulting from increased aridity in the Mediterranean region would bring more heat to the Iberian Peninsula. The added heat from the ocean would most likely raise the risk of severe rainstorms in the region," adds Dr Kaboth.
The Heidelberg geoscientist therefore suggests that more attention should be given to the role of the monsoon-driven outflow from the Mediterranean Sea and its impact on Atlantic oceanography at Europe’s doorstep in climate forecasts. Researchers from Germany, the Netherlands, Japan, France and Taiwan contributed to the study.