Back to 1725 with tree-ring data
Valerie Trouet and Matt Meko, both of the LTRR, and their colleague Flurin Babst, have recently published an article on reconstructing variations in the Northern Hemisphere polar jet stream. Unusual positions and strengths of the jet can drive some extreme weather patterns, such as floods, droughts and heatwaves in particular areas, so are of great practical interest. However the jet is difficult to observe directly: although there are long histories of temperature and rainfall observations in some places, reliable records of the jet only extend back to the mid-twentieth century (even with the help of other meteorological observations to fill in some missing information). These records are too short to evaluate whether the jet has changed its behavior in recent years (for example in response to recent warming trends in the Arctic). The article uses tree-ring data to reconstruct the North Atlantic Jet back to the year 1725 during the summer (the season when it most directly affected tree growth), and from this longer context the unusually frequent extreme weather events in Europe in recent years can be linked to an unusually variable jet. The reconstruction is based on tree-ring chronologies newly constructed for the purpose, using old data from many sites in Europe where there were X-ray density measurements of the wood formed near the end of the growing season in each ring (which are particularly useful for climatic reconstructions).