Assessment of the relationship of atmospheric blocking and heatwaves over the Mediterranean region

The increasing trends of extreme heat and record-high temperatures in Europe are linked to large-scale circulation. The aim of this work is to investigate the role of atmospheric blocking on heatwaves over the Mediterranean region and to assess the drivers of such anomalies. The study is focused on the summer period and on the omega block upper-level weather pattern (i.e., a low-high-low pattern that remains stationary and distorts the progression of weather systems). The identification of the omega block patterns is based on the 500 hPa geopotential anomalies from 1981 to 2020 for the European region, using the state-of-the-art ERA5 reanalysis product at a 0.25° x 0.25° horizontal resolution. A duration threshold was employed (5 days) and 75 omega blocks are identified in total. Their persistence was typically between 5 and 10 days, and in a few cases, the pattern was observed for more than 20 days. An omega block classification was developed based on the blocking intensity using the 500 hPa geopotential gradient, and the events are characterized as weak, moderate, or strong. The majority of the events were classified as omega blocks with moderate intensity and only 3 cases as strong. In addition, the effect of the North Atlantic Oscillation (NAO)was examined, and a positive correlation between the omega events and the NAO negative phase was found and the majority of these events (approximately 70%) were located in the western European region. A significant finding based on the omega block characteristics is that during the last decade, an increase is found in their duration and intensity. The identification of the heatwave events over the Mediterranean region was based on the Excess Heat Factor (EHF) which is used to further categorize heatwaves by their severity. The effect of omega blocks on the EHF distribution is illustrated over the examined sector and specific case studies are also discussed. The results of the study illustrate the impact of atmospheric blocking on the predictability of extreme heat.