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An integrated weather-system perspective on the characteristics, dynamics and impacts of extreme seasons

Periodic Reporting for period 4 - INTEXseas (An integrated weather-system perspective on the characteristics, dynamics and impacts of extreme seasons)

Reporting period: 2023-05-01 to 2023-10-31

Single extreme weather events can be hazardous, but for certain socioeconomic sectors the seasonal aggregation of weather is particularly harmful. Extremes on timescales up to two weeks are typically related to specific weather systems, but no such link exists for extreme seasons. Therefore, they are very difficult to meteorologically understand, despite their utmost societal relevance. This project aims at filling this gap, providing a multi-faceted analysis of different types of extreme seasons in a changing climate. Very large ensembles of climate simulations serve to investigate the characteristics and dynamics of the, e.g. hottest and coldest, and wettest and driest, season in regions worldwide. The extreme season characteristics include their spatial scale and their extremeness given the entire distribution of seasonal values in this region. Their dynamics is related to the fundamental understanding of the sequence of weather events that makes a season extreme: is it a single, highly unusual weather event that renders a season the most extreme (e.g. an unprecedented heat wave) or rather an unusual frequency of well-known weather systems (e.g. a series of strongly precipitating cyclones). These paradigms, referred to as “something new” vs. “more of the same”, are particularly relevant when considering extreme seasons in a warming climate. This project combined state-of-the-art climate modelling, a unique set of weather-system diagnostics informed by profound dynamical understanding, and novel statistical approaches to investigate the occurrence of extreme seasons in the present and future climate. Our project has shown that different types of extreme seasons differ in terms of their spatial scale and relation to weather systems; for specific types of extreme seasons, future climate simulations indicate a marked increase of intensity; and studying the structure and dynamics of extreme seasons is an important research theme at the interface of weather and climate dynamics.
The project led to more than 25 peer-reviewed publications. The most relevant ones are related to the identification of extreme seasons (Flaounas et al. 2021; Röthlisberger et al. 2021; Boettcher et al. 2023), their sub-structure (Röthlisberger et al. 2020), their characteristics in the Arctic (Hartmuth et al. 2022, 2023), and their impact on European forests (Hermann et al. 2023). Also the project produced important novel results about the physical processes leading to temperature extremes (Hermann et al. 2020; Spensberger et al. 2020; Röthlisberger and Papritz 2023a, 2023b; Papritz and Röthlisberger 2023) and warm conveyor belts in a warming climate (Joos et al. 2023; Binder et al. 2023).

Binder, H., H. Joos, M. Sprenger, and H. Wernli, 2023. Warm conveyor belts in present-day and future climate simulations – Part 2: Role of potential vorticity production for cyclone intensification. Weather Clim. Dynam., 4, 19–37.
Boettcher, M., M. Röthlisberger, R. Attinger, J. Rieder, and H. Wernli, 2023. The ERA5 extreme seasons explorer as a basis for research at the weather and climate interface. Bull. Amer. Meteor. Soc., 104, E631-E644.
Flaounas, E., M. Röthlisberger, M. Boettcher, M. Sprenger, and H. Wernli, 2021. Extreme wet seasons – their definition and relationship with synoptic-scale weather systems. Weather Clim. Dynam., 2, 71–88.
Hartmuth, K., M. Boettcher, H. Wernli, and L. Papritz, 2022. Identification, characteristics and dynamics of Arctic extreme seasons. Weather Clim. Dynam., 3, 89–111.
Hartmuth, K., L. Papritz, M. Boettcher, and H. Wernli, 2023. Arctic seasonal variability and extremes, and the role of weather systems in a changing climate. Geophys. Res. Lett., 50, e2022GL102349.
Hermann, M., L. Papritz, and H. Wernli, 2020. A Lagrangian analysis of the dynamical and thermodynamic drivers of large-scale Greenland melt events during 1979–2017. Weather Clim. Dynam., 1, 497–518.
Hermann, M., M. Röthlisberger, A. Gessler, A. Rigling, C. Senf, T. Wohlgemuth, and H. Wernli, 2023. Meteorological history of low-forest-greenness events in Europe in 2002-2022. Biogeosciences, 20, 1155–1180.
Joos, H., M. Sprenger, H. Binder, U. Beyerle, and H. Wernli, 2023. Warm conveyor belts in present-day and future climate simulations – Part 1: Climatology and impacts. Weather Clim. Dynam., 4, 133–155.
Papritz, L., and M. Röthlisberger, 2023. A novel temperature anomaly source diagnostic: Method and application to the 2021 heatwave in the Pacific Northwest. Geophys. Res. Lett., 50, e2023GL105641.
Röthlisberger, M., M. Sprenger, E. Flaounas, U. Beyerle, and H. Wernli, 2020. The substructure of extremely hot summers in the Northern Hemisphere. Weather Clim. Dynam., 1, 45–62.
Röthlisberger, M., M. Hermann, C. Frei, F. Lehner, E. M. Fischer, R. Knutti, and H. Wernli, 2021. A new framework for identifying and investigating seasonal climate extremes. J. Climate, 34, 7761–7782.
Röthlisberger, M., and L. Papritz, 2023a. Quantifying the physical processes leading to atmospheric hot extremes at a global scale. Nature Geosci., 16, 210–216.
Röthlisberger, M., and L. Papritz, 2023b. A global quantification of the physical processes leading to near-surface cold extremes. Geophys. Res. Lett., 50, e2022GL101670.
Spensberger, C., E. Madonna, M. Boettcher, C. M. Grams, L. Papritz, J. F. Quinting, M. Röthlisberger, M. Sprenger, and P. Zschenderlein, 2020. Dynamics of concurrent and sequential Central European and Scandinavian heatwaves. Quart. J. Roy. Meteorol. Soc., 146, 2998–3013.
Steinfeld, D., M. Boettcher, R. Forbes, and S. Pfahl, 2020. The sensitivity of atmospheric blocking to upstream latent heating – numerical experiments. Weather Clim. Dynam., 1, 405–426.
During the ongoing project, we report about our main projects and achievements on the INTEXseas webpage: Extreme season objects, identified in the ERA5 dataset, are openly available from the INTEXseas extreme season explorer:
screenshot from our ERA5 extreme season explorer,