In Western Europe, several recent summers have been exceptionally warm, especially the summers of 2003 and 2015. Based on observations over the past century, meteorologists have come to see such major heat events as typically stemming from anticyclonic atmospheric circulation and a lack of spring rainfall in Southern Europe. But atmospheric conditions were anomalous in the recent hot summers. For example, a dry spring preceded the cool summer of 2011, and a wet spring came before a warm summer of 2013. The summer of 2015 was warm, yet exhibited persisting southerly atmospheric flows and little anticyclonic circulation. This pattern also contributed to the exceptional summer 2018 heatwave over northern Europe, an event that Faranda and colleagues are currently studying and will soon report on in another publication
In a recent paper, LML External Fellow Davide Faranda and colleagues challenge the traditional causal view of summer temperatures. They do so by investigating the earlier part of the 20th century using data from the 20th Century Reanalysis project, which offers a global atmospheric dataset of weather spanning 1836 to 2015 useful for placing current atmospheric circulation patterns into a historical perspective. To assess the robustness of the link between heat events and atmospheric circulation, they perform a statistical and dynamical analysis over the period 1851–2014, and conclude that the atmospheric circulation patterns leading to the most intense heat events have changed during the last century. The most extreme heat events of the second half of the 20th century occurred when the Scandinavian Blocking weather regime dominated the North-Atlantic region, causing increasing temperatures and more frequent and longer heat waves events.
Overall, the study finds significant changes in the dominating weather regimes associated with the warmest summers. If blocking events were dominant from the second part of 20th century, scarce occurrences of this weather regime are found before 1930, with the exception of a small period at late 19th century. These results reflect a change in the regime frequencies and dominance conducing to warm extremes in a multidecadal scale.
The paper is available at https://www.hindawi.com/journals/complexity/2018/2494509/