Work package 4: Drivers behind anomalous mid-latitude flow linked to extreme weather in boreal summer and winter (lead by D. Coumou and R. Donner)

Within this GOTHAM activity, we will look at possible drivers behind recent extreme weather events in the Northern Hemisphere mid-latitudes. One aspect will be to understand strongly meandering Jetstream situations which are often associated with extreme weather events. An illustrative example is the summer of 2010 which saw simultaneously an extreme heat wave over western Russia and devastating flooding of the Indus River. Previous work has shown that these events were linked via a strongly meandering jet possible triggered by so-called wave-resonances. Also the two-way interaction between the Indian summer monsoon and the circulation in the mid-latitudes is thought to be important. In this GOTHAM activity we will study such interactions in detail using novel network analyses tools in combination with state-of-the-art climate simulations.

Another possible driver of such wavy Jetstream situations, in particular in winter, is the persistent cold anomaly over the Northern Atlantic just below Greenland. Some research indicates that this cold blob might be linked to a slow-down of the Gulf Stream system (more accurately: the Atlantic Meridional Overturning Circulation or AMOC). This large ocean circulation is very important for weather conditions in the mid-latitudes, especially Europe. Using coupled ocean-atmosphere simulations and paleo data stretching back over a Millennium, we want to better understand the causes behind the recent cold anomaly in the North Atlantic and its effect on extreme weather in Europe and other regions.