Identifying Key Driving Processes of Major Recent Heat Waves

Abstract

Heat waves lead to major impacts on human health, food production, and ecosystems.To assess their predictability and how they are projected to change under global warming, it is crucial to improve our understanding of the underlying processes affecting their occurrence and intensity under present-day climate conditions. Beside greenhouse gas forcing, processes in the different components of the climate system-in particular the land surface, atmospheric circulation, and the oceans-may play a key role in changing the odds for a particular event. This study aims to identify the role of the individual drivers for five heat waves (and, in some cases, of concurrent droughts) in the recent decade. Simulations are performed with the Community Earth System Model using nudging of horizontal atmospheric circulation and prescription of soil moisture. The fully constrained model accurately reproduces how anomalous an event was. Factorial experiments, which force the model toward observations for one or several key components at a time, allow us to identify how much of the observed temperature anomaly of each event can be attributed to each driver. Considering all analyzed events, atmospheric circulation and soil moisture play similarly important roles, each contributing between 20% and 70% to the events' anomalies. This highlights that the role of thermodynamics can be just as important as that of the dynamics for temperature extremes, a possibly underestimated feature. In addition, recent climate change amplified the events and contributed between 10% and 40% of the events' anomalies.

Summary of a workshop on extreme weather events in a warming world organized by the Royal Swedish Academy of Sciences

“…These results stand in contrast to modelling studies e.g. from CMIP6 suggesting a 0 to 8% probability for heat waves following cold surface temperatures (Borchert et al, 2019) or single-event attributions which show a small or no surface temperature control (Wehrli et al, 2019).…”

A profile shape correction to reduce the vertical sensitivity of cosmic‐ray neutron sensing of soil moisture

“…Antecedent soil moisture states are used for flood and landslide forecasting (Brocca et al., 2012; Koster, Mahanama, Livneh, Lettenmaier, & Reichle, 2010) and to study the interactions in soil–atmosphere processes (Seneviratne et al., 2010). The soil moisture state is important for drought prediction (Pendergrass et al., 2020) and can be decisive for the occurrence of heat waves (Wehrli, Guillod, Hauser, Leclair, & Seneviratne, 2019). Irrigated agriculture can especially benefit from soil moisture monitoring data (Barker, Franz, Heeren, Neale, & Luck, 2017; Finkenbiner, Franz, Gibson, Heeren, & Luck, 2019).…”

The 3D Structure of Northern Hemisphere Blocking Events: Climatology, Role of Moisture, and Response to Climate Change

“…Blocking events are large-scale, quasi-stationary, highpressure (anticyclonic) anomalous systems that last beyond the synoptic time scales (sometimes for weeks) and block or divert the midlatitude westerlies (Rex 1950;Green 1977;Hoskins and James 2014;Woollings et al 2018). Due to their persistence and size, depending on the season and the region, blocking events can cause, or contribute to, various types of extreme events such as heat waves, cold spells, droughts, and heavy rainfall episodes (e.g., Barriopedro et al 2011;Dole et al 2011;Pfahl and Wernli 2012;Brunner et al , 2018Schaller et al 2018;Zschenderlein et al 2019;Röthlisberger and Martius 2019;Wehrli et al 2019;Lenggenhager et al 2019;Chan et al 2019). Despite much effort, the dynamical mechanisms responsible for the generation and maintenance of the blocking events are still not well understood (Hoskins and James 2014;Hassanzadeh and Kuang 2015;Nakamura and Huang 2018;Woollings et al 2018).…”