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WIREs Clim Change
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A review of past changes in extratropical cyclones in the northern hemisphere and what can be learned for the future

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Abstract Extratropical cyclones, a major phenomenon of the mid‐latitude atmospheric dynamics, show strong variability over a range of time scales. Future projections hint at an increase of cyclonic intensity and the associated precipitation, an important fact to be considered when developing future risk assessments. This review presents a first overview of studies which (a) puts the current variability and projected future climate changes of extratropical cyclone characteristics in a long‐term perspective, (b) shows connections to natural external forcings, and (c) deepens our understanding of cyclone intensification processes for past climate periods. We summarize the current state of knowledge for two periods in the past—the last millennium and the Last Glacial Maximum (LGM, 21,000 years ago). For these two periods, the sparse information from paleo proxy archives are compared to climate modeling results on global and regional scales. For example, strong changes of the climate mean state, induced by orbital forcing and associated feedbacks, show strong effects on different cyclone characteristics, for example, a southward shift of the storm tracks over the North Atlantic during the LGM. Other findings indicate that dynamic processes could play at least an equally important role as thermodynamic processes for the variations of cyclone‐induced precipitation. This is in contrast to the projected future changes in cyclone‐related precipitation, which are driven primarily by thermodynamic processes. The review demonstrates how a paleoclimatic view can foster an extended process understanding and be instrumental to better understand future changes in extratropical cyclones and associated characteristics. This article is categorized under: Paleoclimates and Current Trends > Modern Climate Change
(a) External forcing of a PMIP‐type simulation for the last millennium (red: sum of the radiative forcing (RF) based on the greenhouse gases CO2, CH4, and N2O as well as solar irradiance; blue: mass of volcanic aerosols). The inset in (b) shows the area in which the following indices are estimated for winter (December–February): (b) number of extratropical cyclone time steps, (c) extreme cyclone depth, (d) extreme cyclone‐related precipitation. Note that in (b)–(d) a 30‐year running average is applied. (e) Regression coefficients between the cyclone‐related temperature and the extreme cyclone‐related precipitation estimated in a 150‐year running window. The yellow shading in (e) indicates the dominance of thermodynamical processes (Clausius–Clapeyron equation) (O'Gorman & Schneider, 2009). the cyan shading highlights periods where dynamical processes are mainly responsible for the generation of cyclone‐related precipitation extremes (Reprinted with permission from Raible et al. (2018))
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Schematic of the storm tracks, atmospheric circulation relevant for extratropical cyclones, and hydrological implications of extratropical cyclones comparing present day (PD) with the LGM based on model evidence. The compilation is based on several modeling studies using different models (as mentioned in the text). We only include features where models agree on. The dashed arrow between Iceland and Scandinavia is only based on limited model evidence. Note that we focus on the winter, where extratropical cyclones are mostly pronounced, and, thus, most of the studies focus on
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Schematic of the storm track, atmospheric circulation relevant for extratropical cyclones, and hydrological implications of extratropical cyclones comparing present day (PD) conditions with the LGM reconstructed based on proxy evidence. A clear specification of the season is not possible, as some proxies such as pollen data (which illustrate the hydrological impact) are more related to the growing season and other such as loess deposits (illustrating wind) are not attributable to a specific season
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