Lagrangian analysis of two flavours of Central European heatwaves: Formation under omega blocking versus initiation by subtropical ridges

Abstract

The formation of midlatitude heatwaves (HWs) is closely linked to quasi-stationary anticyclonic flow anomalies, which are often associated with so-called omega blocking. However, not all HWs require atmospheric blocking, especially over Southern and Central Europe, where they can also be enabled by poleward extensions of the subtropical high-pressure belt. We propose that HWs forming under omega blocking may differ in Lagrangian characteristics from those related to subtropical ridges, in terms of both air-mass origin and processes warming air parcels en route to Central Europe. With this aim, we select the 25 most archetypical cases of omega- and ridge-type Central European HWs, respectively, during the period 1950–2023. Using ERA5-based backward trajectories and a Lagrangian temperature decomposition, we assess both air-mass origin and the relative importance of the heat-generating processes: advection, adiabatic warming through subsidence, and diabatic heating. Our analysis shows that adiabatic warming and diabatic heating are the dominant near-surface heat-generating processes for both circulation types. Advection does not contribute to omega-type HWs, while ridge-type HWs show small but significant contributions by advection, as air masses originate from more southern, climatologically warmer regions. Adiabatic warming is more significant in ridge-type HWs, while omega-type HWs see a significantly stronger role of diabatic heating due to a combination of less diabatic cooling in the free troposphere and longer air-mass residence in the boundary layer. At higher levels ((Formula presented.) 800 hPa), high temperatures are only attributable to advection and adiabatic warming. Significant differences between both circulation types emerge only two days after HW onset, when advection becomes dominant and significantly more important in ridge-type HWs than in omega-type HWs. These results highlight the importance of the atmospheric circulation and thus the associated air masses to the development of HWs, with implications for both weather forecasting and climate-change perspectives.