In fact, an anomalously wet African monsoon season may have a detrimental impact on public health on continental scales (Polcher et al., 2011). Another example of seasonal-scale environmental risk is the direct relationship between the seasonal rainfall over the Sahel and the epidemics of meningitis (Sultan et al., 2005). The 2017 hurricane season did not include any record-breaking intense hurricanes, although it included several hurricanes producing extreme precipitation.
For instance, the costliest, hyperactive North Atlantic hurricane season of 2017 had a significant impact on the coastal population of the US due to an anomalous sequence of tropical cyclones that made landfall, causing damages of the order of USD 370 billion and loss of human life (Halverson, 2018 Taillie et al., 2020). Limited to the occurrence of single, outstanding extreme precipitation events, but they are also potentially related to accumulated precipitation on longer timescales. However, socio-economic impacts related to precipitation are not Finally, another important strand of research addressed the future evolution of extreme precipitation events in a changing climate, using a plethora of simulation ensembles, reanalysis datasets and observations (e.g., Easterling et al., 2000 Shongwe et al., 2011 Pfahl et al., 2017).
Wernli, 2012), fronts (Catto and Pfahl, 2013) and warm conveyor belts (Pfahl et al., 2014). In addition,Ĭlimatological studies quantified the relationship of extreme precipitationĮvents with specific synoptic-scale flow systems like cyclones (Pfahl and Indeed, many studies investigated single extreme precipitation events to identify the key dynamical and physical processes involved (e.g., Doswell et al., 1998 Massacand et al., 1998 Delrieu et al., 2005 Holloway et al., 2012 Moore et al., 2012 Winschall et al., 2012 Flaounas et al., 2016). The analysis of extreme precipitation events on timescales of hours to a few days has long been a centerpiece of weather and climate research due to their relevance for a variety of socio-economic aspects including damages to infrastructures and loss of life. Of single weather systems like thunderstorms or cyclones, but on the seasonal timescale. This study focuses on extreme precipitation, albeit not on short timescales Three illustrative examples provide insight into the synergetic effects of the four identified weather systems on the formation of extreme wet seasons in the midlatitudes, the Arctic and the (sub)tropics. But interlatitudinal influences are also shown to be important: tropical moisture exports, i.e., the poleward transport of tropical moisture, can contribute to extreme wet seasons in the midlatitudes, while breaking Rossby waves, i.e., the equatorward intrusion of stratospheric air, may decisively contribute to the formation of extreme wet seasons in the tropics. Warm conveyor belts contribute particularly strongly to extreme wet seasons Known to be associated with intense precipitation: cyclones, warm conveyorīelts, tropical moisture exports and breaking Rossby waves. To four objectively identified synoptic-scale weather systems, which are In the subtropics and more precisely within the transitional zones between arid areas and regions of frequent precipitation, both an anomalously high occurrence of daily extremes and of wet days are related to the formation of extreme wet seasons.Ī novel method is introduced to define the spatial extent of regionsĪffected by a particular extreme wet season and to relate extreme seasons This is in contrast to arid regions where wet seasons may occur only due to anomalously frequent wet days. Results show that daily extremes are decisive for the occurrence of extreme wet seasons in regions of frequent precipitation, e.g., in the tropics. All identified extreme seasons are statistically characterized in terms of climatological anomalies of the number of wet days and of daily extreme events. Nevertheless, colder periods might be also relevant, especially in coastal areas. In mostĬontinental regions, the extreme seasons occur during the warm months of the year, especially in the midlatitudes. Statistically characterize extreme wet seasons around the globe and second to elucidate their relationship with specific weather systems.Įxtreme wet seasons are defined independently at every grid point ofĮRA-Interim reanalyses as the consecutive 90 d period with the highestĪccumulated precipitation in the 40-year period of 1979–2018.
This study has a twofold aim: first to identify and To a so-called extreme wet season, can have substantial environmental and An extreme aggregation of precipitation on the seasonal timescale, leading
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