Investigations into precipitation patterns in eastern Central Europe since the end of the last ice age, conducted by an international research team led by Dr Sophie Warken of Heidelberg University, have shown that dynamic processes in atmospheric circulation, such as the North Atlantic jet stream, influence regional changes in precipitation. The researchers analyzed dripstones from the Cloşani Cave in Romania, which act as a natural climate archive that allows conclusions to be drawn about precipitation variability over a period of approximately 20,000 years. According to Dr Warken, the new findings on the dynamics of the climate in Europe could contribute to improving current climate models and the ability to more accurately predict the likelihood of extreme weather events.
Of particular importance for the regional weather and precipitation patterns in Europe's mid-latitudes is the North Atlantic jet stream, an atmospheric air flow that crosses the North Atlantic in a southwest-to-northeast direction carrying precipitation to Europe. In the past, climatic changes did affect the strength and trajectory of the jet stream – that much is known. But as Dr Warken explains, our understanding of how climate-induced fluctuations in the jet stream influenced local and regional precipitation patterns in Europe is limited.
Natural climate archives, like the dripstones in the Cloşani Cave in Romania, can provide information on the climate dynamics of bygone ages. The lime deposits, also known as speleothems, form from precipitation that seeps into the cave's interior over several thousands of years. Geochemical investigations allow conclusions to be drawn about the chronology of the deposits and thus past environmental conditions and precipitation amounts. The current study focused on three stalagmites that contain information on the hydroclimatic conditions in eastern Central Europe over the past 20,000 years.
The results show how the trajectory of the North Atlantic jet stream changed due to the warming and melting of the ice sheets of the Northern hemisphere – a process that lasted until about 5,000 years ago. As a result, precipitation in the late last Ice Age about 20,000 years ago and the early to mid-Holocene – the current interglacial epoch that followed the last Ice Age about 7,000 years ago – was 20 to 30 percent higher than it is today. It turns out that precipitation variability in the region over comparatively short time periods of centuries or even just decades fluctuated irrespective of long-term temperature developments in the North Atlantic region.
"Our research shows that dynamic processes in particular, such as changes in wind patterns and atmospheric currents like the jet stream, influence the precipitation and weather patterns in Central Europe," stresses Dr Warken. This helps fill in a research gap, she explains, because former reconstructions mainly refer to thermodynamic processes, i.e., the warming of the atmosphere, and thereby directly connect a rise in temperature to an increase in precipitation. These reconstructions are often based on climate models still fraught with uncertainties in simulating local and regional precipitation patterns.
"Climate change is already leading to more frequent and intense precipitation events; based on current prognoses, the number of extreme weather events and heavy rainfall in several regions in Europe will continue to rise," states Dr Warken. A better understanding of the underlying dynamic processes is key to more accurately predict future precipitation patterns and the likelihood of extreme weather events. Against this backdrop, the current results from the Cloşani Cave can help improve the accuracy of climate models and prognoses, adds the geoscientist, who together with her research group at the Institutes of Earth Sciences and Environmental Physics of Heidelberg University is reconstructing the climate of past millennia.
The research was carried out in collaboration with scientists from the Universities of Mainz and Innsbruck (Austria). Also involved were other institutions in Germany and Romania. The results were published in the journal "Communications Earth & Environment".
