Quantifying the effect of recurrent Rossby wave patterns on extremely hot summers in present-day and end-of-the-century climate

Abstract

Extremely hot summers have devastating impacts on society and ecology. Recurrent Rossby wave patterns (RRWPs) are among the atmospheric features that have been linked to extreme weather events involving persistent surface weather. In this thesis, the link between RRWPs and the occurrence of extremely hot summers in the Northern Hemisphere is investigated and quantified. To this aim the synoptic evolution of RRWP events is studied in three case studies of extremely hot summers using ERA5 and CESM data. Next, the link between RRWPs and the occurrence of extremely hot summers is quantified in present-day (1990 - 1999) and end-of-the-century (2091 - 2100) climate using a grid point-wise logistic regression model. In order to differentiate between the effect of collocated and upstream RRWPs on the occurrence of extremely hot summers, two different model implementations are used. Finally, we compare the results of the logistic regression model between the present-day and end-of-the-century climate in order to assess potential changes in the link between RRWPs and extremely hot summers due to global warming. We find that the effect of RRWPs on the occurrence of extremely hot summers is significant in many regions in the Northern Hemisphere, e.g., in the western U.S. (odds’ increase of up to +16%), eastern Europe (up to +14%) and in the region of the Caspian Sea (up to +18%). However, in the end-of-the-century climate, this link is much weaker. Uncertainties about the causes of a weakening link between RRWPs and extremely hot summers remain, and are possibly linked to the increasing importance of thermodynamic processes or altered jet stream dynamics. To conclude, RRWPs have significant effects on the occurrence of extremely hot summers in the Northern Hemisphere and are therefore essential when studying seasonal heat extremes. Thus, RRWPs should be further investigated - especially in the context of global warming.