Extratropical cyclone statistics during the last millennium and the 21st century

Abstract

Abstract. Extratropical cyclones in winter and their characteristics are investigated in depth for the Atlantic European region, as they are responsible for a significant part of the rainfall and extreme wind and/or precipitation-induced hazards. The analysis is based on a seamless transient simulation with a state-of-the-art fully coupled Earth system model from 850 to 2100 CE. The Representative Concentration Pathway 8.5 (RCP8.5) scenario is used in the 21st century. During the Common Era, cyclone characteristics show pronounced variations on interannual and decadal timescales, but no external forcing imprint is found prior to 1850. Thus, variations of extratropical cyclone characteristics are mainly caused by internal variability of the coupled climate system. When anthropogenic forcing becomes dominant in the 20th century, a decrease of the cyclone occurrences mainly over the Mediterranean and a strong increase of extreme cyclone-related precipitation become detectable. The latter is due to thermodynamics as it follows the Clausius–Clapeyron relation. An important finding, though, is that the relation between temperature and extreme cyclone-related precipitation is not always controlled by the Clausius–Clapeyron relation, which suggests that dynamical processes can play an important role in generating extreme cyclone-related precipitation – for example, in the absence of anomalously warm background conditions. Thus, the importance of dynamical processes, even on decadal timescales, might explain the conundrum that proxy records suggest enhanced occurrence of precipitation extremes during rather cold periods in the past.

Monthly storminess over the Po River Basin during the past millennium (800–2018 CE)

“…On the one hand, this suggests that recurrences of storminess become less statistically predictable outside the LIA. This may demonstrate that dynamic atmospheric processes are present in generating cyclone-related precipitation extremes throughout the LIA (Raible et al 2018), departing from the Clausius-Clapeyron thermodynamic expectations of an increase in precipitation intensity associated with atmospheric warming (Pall et al 2007, Trenberth 2011, Trenberth et al 2014, Kirby 2016, Prein and Pendergrass 2019. Our study thus shows the same tendency as revealed by Ljungqvist et al (2019) for a negative low-frequency temperature-hydroclimatic coupling (i.e., warm and dry) in southern Europe.…”

Extratropical cyclones over the North Atlantic and western Europe during the Last Glacial Maximum and implications for proxy interpretation

“…In this section, we analyse in how far these cyclone characteristics can help to explain the climate conditions in western and central Europe at the LGM and the frequent occurrence of dust storms in that area. According to the available proxy records, western and central Europe were colder and largely drier under LGM conditions (Bartlein et al, 2011;Annan and Hargreaves, 2013;Cleator et al, 2019) and were to a large extent covered by open shrublands and grasslands (Ray and Adams, 2001). The colder and generally drier conditions are reproduced by the Figure 6.…”

“…The availability of studies addressing the characteristics of cyclone activity outside of the period extending from the mid-19th century to the end of the 21st century decreases sharply. Raible et al (2018) analysed variations of cyclone statistics in a very long simulation with a fully coupled Earth system model from 850 to 2100 CE. While they identified variations on multiple timescales, they found no evidence for an external forcing imprint before 1850.…”

“…One period of particular interest is the Last Glacial Maximum (LGM; Clark et al, 2009), when the European climate was characterised by colder and mostly drier conditions (Bartlein et al, 2011;Annan and Hargreaves, 2013;Újvári et al, 2017;Cleator et al, 2019). Large parts of northern Europe were covered by permanent ice sheets and surrounded by polar desert conditions (Ray and Adams, 2001). Western, central and eastern Europe were largely characterised by open shrublands and grasslands (steppe tundra), while in southern Europe steppe with embedded forest (forest steppe) dominated (Ray and Adams, 2001).…”

“…Large parts of northern Europe were covered by permanent ice sheets and surrounded by polar desert conditions (Ray and Adams, 2001). Western, central and eastern Europe were largely characterised by open shrublands and grasslands (steppe tundra), while in southern Europe steppe with embedded forest (forest steppe) dominated (Ray and Adams, 2001). Under these conditions, dust storms triggered by strong winds must have been common in Europe, as documented by the major loess deposits found primarily around 50 • N over western and central Europe and over large parts of eastern Europe (Antoine et al, 2009Sima et al, 2013;Újvári et al, 2017).…”