Lagrangian analysis of tracked anticyclonic structures in reanalysis data 

Since heatwaves are among the most impactful natural hazards, a better understanding of the atmospheric processes that drive near-surface temperature extremes may help to mitigate the future outcomes of such events in a warming climate. One common ingredient for mid-latitude summer heatwaves are blocking anticyclones, quasi-stationary and persistent high pressure areas, whose stable and mostly cloud-free conditions can favor the buildup of heat. A strong link between blocking anticyclones and heatwaves has been demonstrated in previous studies, but the physical processes leading to both the near surface temperature extremes and the blocking conditions are still debated. Consequently, the question arises as to what distinguishes blocking high-pressure systems from non-blocking ones and why some lead to near-surface temperature extremes while others do not.To address this question, a threshold-based tracking algorithm is applied to mid-tropospheric geopotential height anomalies within a 40 year period in ERA5 reanalysis data. The tracking itself is restricted to the extended summer period (MJJAS) and the Northern Hemisphere and the resulting spatio-temporal structures are labeled and classified based on their overlap with regions over Europe marked as (un)affected by blocking or heatwaves. Thus, differences between anticyclones characterized by either atmospheric blocking or temperature extremes or neither / both of them can be subsequently explored.With the help of Lagrangian backwards trajectories, a detailed view of the origin of near surface air masses within and in the proximity tracked anticyclones is obtained along their path and lifetime. Using a recently published temperature anomaly decomposition method, the physical processes leading to heatwaves (advection, diabatic and adiabatic heating) are explored and and compared to air masses in tracked anticyclones not associated with heatwaves.