Periodic Reporting for period 2 - HEATforecast (Dynamical constraints for the predictability of heat waves in current and future climates)
Reporting period: 2021-09-01 to 2023-02-28
Heatwaves are becoming increasingly frequent and extreme under a changing climate, with devastating effects on a wide range of sectors, including human health and ecosystems. The basic ingredients for heatwaves – and in particular, their interaction – are however not yet fully understood. These ingredients include the dynamics of the atmosphere, the land and ocean surface, atmospheric moisture, and land topography. Several of these ingredients experience drastic changes in a changing climate, and it is therefore crucial to understand their relative and combined contributions to heat extremes in present and future climates. This project aims at resolving this issue by building a process-based hierarchy of numerical models ranging from a dry dynamical core model to a prediction system using full physics. With this approach, the necessary ingredients for heatwaves can be evaluated and their relative and combined contribution to heatwaves can be understood. While solving a fundamental question in atmospheric fluid dynamics, the proposed research also aims to improve the predictability of heat extremes, thereby extending the warning horizon and minimizing the societal consequences for future heat waves.
During the first part of the project, the simplified part of the model hierarchy has been built and evaluated. In particular, the role of topography and atmospheric dynamics has been investigated. The findings show that different atmospheric drivers are responsible for heatwaves at different latitudes in the simplified model. In particular, atmospheric blocking is most important for high- and midlatitude heatwave, but less important for low-latitudes. Rossby-wave packets are the dominant drivers for midlatitude heatwaves, with horizontal advection being the main mechanism leading to heat extremes. Heatwaves exhibit a higher persistence and frequency near the pole and equator compared with midlatitudes, but a higher intensity in midlatitudes compared with higher and lower latitudes. Topography located along the midlatitude jet has the largest impact on the heatwave distribution around the planet, resulting in increased heatwave frequency upstream for moderate topographic forcing and a circumglobal increase for topographic elevations above 6 km. Amplified Rossby waves exhibit clear phase-locking behavior and a decrease in the zonal phase speed when a large-scale localized topographic forcing is imposed, leading to concurrent heat extremes at preferred longitudinal locations.
The main achievements so far are related to the atmospheric dynamics driving heatwaves. In particular, it was not resolved which processes can lead to phase-locking and hence to amplification of large-scale atmospheric waves. The finding that topography alone cannot only lead to phase-locking, but to circumglobal wavetrains is a major advancement, as so far it was not clear which processes drive such circumglobal waves that are associated with heat extremes. In addition, we are currently working on simplified models to characterize heat extremes and on improving the definition of heatwave intensity. For the remainder of the project, additional processes will be added to the model hierarchy to investigate their role in driving, modulating, and amplifying heatwaves. In particular, simplified atmospheric moisture, a simplified ocean, and a land surface will be added to investigate their contribution to heatwaves and their interaction with the dynamics of the atmosphere.