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

Extratropical cyclones are a dominant feature of the midlatitudes, as their passage is associated with strong winds, precipitation and temperature changes. The statistics and characteristics of extratropical cyclones over the North Atlantic region exhibit some fundamental differences between pre-industrial (PI) and Last Glacial Maximum (LGM) climate conditions. Here, the statistics are analysed based on results of a tracking algorithm applied to global PI and LGM climate simulations. During the LGM, both the number and the intensity of detected cyclones were higher compared to PI. In particular, increased cyclone track activity is detected close to the Laurentide ice sheet and over central Europe. To determine changes in cyclone characteristics, the top 30 extreme storm events for PI and LGM have been simulated with a regional climate model and high resolution (12.5 km grid spacing) over the eastern North Atlantic and western Europe. Results show that LGM extreme cyclones were characterised by weaker precipitation, enhanced frontal temperature gradients and stronger wind speeds than PI analogues. These results are in line with the view of a colder and drier Europe, characterised by little vegetation and affected by frequent dust storms, leading to reallocation and build-up of thick loess deposits in Europe

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

Extratropical cyclones are a dominant feature of the midlatitudes, as their passage is associated with strong winds, precipitation and temperature changes. The statistics and characteristics of extratropical cyclones over the North Atlantic region exhibit some fundamental differences between pre-industrial (PI) and Last Glacial Maximum (LGM) climate conditions. Here, the statistics are analysed based on results of a tracking algorithm applied to global PI and LGM climate simulations. During the LGM, both the number and the intensity of detected cyclones were higher compared to PI. In particular, increased cyclone track activity is detected close to the Laurentide ice sheet and over central Europe. To determine changes in cyclone characteristics, the top 30 extreme storm events for PI and LGM have been simulated with a regional climate model and high resolution (12.5 km grid spacing) over the eastern North Atlantic and western Europe. Results show that LGM extreme cyclones were characterised by weaker precipitation, enhanced frontal temperature gradients and stronger wind speeds than PI analogues. These results are in line with the view of a colder and drier Europe, characterised by little vegetation and affected by frequent dust storms, leading to reallocation and build-up of thick loess deposits in Europe

Extra-tropical cyclones in the present and future climate: a review

Based on the availability of hemispheric gridded data sets from observations, analysis and global climate models, objective cyclone identification methods were developed and applied to these data sets. Due to the large amount of investigation methods combined with the variety of different datasets, a multitude of results exist, not only for the recent climate period but also for the next century, assuming anthropogenic changed conditions. Different thresholds, different physical quantities, and considerations of different atmospheric vertical levels add to a picture that is difficult to combine into a common view of cyclones, their variability and trends, in the real world and in GCM studies. Thus, this paper will give a comprehensive review of the actual knowledge on climatologies of mid-latitude cyclones for the Northern and Southern Hemisphere for the present climate and for its possible changes under anthropogenic climate conditions

Cloud‐Radiative Impact on the Regional Responses of the Midlatitude Jet Streams and Storm Tracks to Global Warming

Previous work demonstrated the strong radiative coupling between clouds and the mid‐latitude circulation. Here, we investigate the impact of cloud‐radiative changes on the global warming response of the mid‐latitude jet streams and storm tracks in the North Atlantic, North Pacific and Southern Hemisphere. To this end, we use the ICON global atmosphere model in present‐day setup and with the cloud‐locking method. Sea surface temperatures (SST) are prescribed to isolate the circulation response to atmospheric cloud‐radiative heating. In the annual mean, cloud‐radiative changes contribute one‐ to two‐thirds to the poleward jet shift in all three ocean basins, and support the jet strengthening in the North Atlantic and Southern Hemisphere. Cloud‐radiative changes also impact the storm track, but the impact is more diverse across the three ocean basins. The cloud‐radiative impact on the North Atlantic and North Pacific jets varies little from season to season in absolute terms, whereas its relative importance changes over the course of the year. In the Southern Hemisphere, cloud‐radiative changes strengthen the jet in all seasons, whereas their impact on the jet shift is limited to austral summer and fall. The cloud‐radiative impact is largely zonally‐symmetric and independent of whether global warming is mimicked by a uniform 4 K or spatially‐varying SST increase. Our results emphasize the importance of cloud‐radiative changes for the response of the mid‐latitude circulation to global warming, indicating that clouds can contribute to uncertainty in model projections of future circulations

Tropical cloud-radiative changes contribute to robust climate change-induced jet exit strengthening over Europe during boreal winter

The North Atlantic jet stream is projected to extend eastward towards Europe in boreal winter in response to climate change. We show that this response is robust across a hierarchy of climate models and climate change scenarios. We further show that cloud-radiative changes contribute robustly to the eastward extension of the jet stream in three atmosphere models, but lead to model uncertainties in the jet stream response over the North Atlantic. The magnitude of the cloud contribution depends on the model, consistent with differences in the magnitude of changes in upper-tropospheric cloud-radiative heating. We further study the role of regional cloud changes in one of the three atmosphere models, i.e. the ICON model. Tropical cloud-radiative changes dominate the cloud impact on the eastward extension of the jet stream in ICON. Cloud-radiative changes over the Indian Ocean, western tropical Pacific, and eastern tropical Pacific contribute to this response, while tropical Atlantic cloud changes have a minor impact. Our results highlight the importance of upper-tropospheric tropical clouds for the regional circulation response to climate change over the North Atlantic-European region and uncertainty therein

The response of the regional longwave radiation balance and climate system in Europe to an idealized afforestation experiment

Afforestation is an important mitigation strategy to climate change due to its carbon sequestration potential. Besides this positive biogeochemical effect on global CO2 concentrations, afforestation also affects the regional climate by changing the biogeophysical land surface characteristics. In this study, we investigate the effects of an idealized global CO2 reduction to pre-industrial conditions by a Europe-wide afforestation experiment on the regional longwave radiation balance, starting in the year 1986 from a continent entirely covered with grassland. Results show that the impact of biogeophysical processes on the surface temperatures is much stronger than of biogechemical processes. Furthermore, biogeophysically induced changes of the surface temperatures, atmospheric temperatures and moisture concentrations are as important for the regional greenhouse effect as the global CO2 reduction. While the greenhouse effect is strengthened in winter, it is weakened in summer. On annual total, a Europe-wide afforestation has a regional warming effect, despite reduced CO2 concentrations. Thus, even for an idealized reduction of the global CO2 concentrations to pre-industrial levels, the European climate response to afforestation would still be dominated by its biogeophysical effects.</p

Cloud‐Radiative Impact on the Regional Responses of the Midlatitude Jet Streams and Storm Tracks to Global Warming

Previous work demonstrated the strong radiative coupling between clouds and the mid‐latitude circulation. Here, we investigate the impact of cloud‐radiative changes on the global warming response of the mid‐latitude jet streams and storm tracks in the North Atlantic, North Pacific and Southern Hemisphere. To this end, we use the ICON global atmosphere model in present‐day setup and with the cloud‐locking method. Sea surface temperatures (SST) are prescribed to isolate the circulation response to atmospheric cloud‐radiative heating. In the annual mean, cloud‐radiative changes contribute one‐ to two‐thirds to the poleward jet shift in all three ocean basins, and support the jet strengthening in the North Atlantic and Southern Hemisphere. Cloud‐radiative changes also impact the storm track, but the impact is more diverse across the three ocean basins. The cloud‐radiative impact on the North Atlantic and North Pacific jets varies little from season to season in absolute terms, whereas its relative importance changes over the course of the year. In the Southern Hemisphere, cloud‐radiative changes strengthen the jet in all seasons, whereas their impact on the jet shift is limited to austral summer and fall. The cloud‐radiative impact is largely zonally‐symmetric and independent of whether global warming is mimicked by a uniform 4 K or spatially‐varying SST increase. Our results emphasize the importance of cloud‐radiative changes for the response of the mid‐latitude circulation to global warming, indicating that clouds can contribute to uncertainty in model projections of future circulations

Future heat extremes and impacts in a convection permitting climate ensemble over Germany

Heat extremes and associated impacts are considered the most pressing issue for German regional governments with respect to climate adaptation. We explore the potential of an unique high-resolution convection permitting (2.8 km), multi-GCM ensemble with COSMO-CLM regional simulations (1971&ndash;2100) over Germany regarding heat extremes and related impacts. We find an improved mean temperature beyond the effect of a better representation of orography on the convection permitting scale, with reduced bias particularly during summer. The projected increase in temperature and its variance favors the development of longer and hotter heat waves, especially in late summer and early autumn. In a 2&deg; (3&deg;) warmer world, a 26 % (100 %) increase in the Heat Wave Magnitude Index is anticipated. Human heat stress (UTCI &gt; 32&deg;C) and local-specific parameters tailored to climate adaptation, revealed a dependency on the major landscapes, resulting in significant higher heat exposure in flat regions as the Rhine Valley, accompanied by the strongest absolute increase. A non-linear, exponential increase is anticipated for parameters characterizing strong heat stress (UTCI &gt; 32&deg;C, tropical nights, very hot days). Providing local-specific and tailored climate information, we demonstrate the potential of convection permitting simulations to facilitate improved impact studies and narrow the gap between climate modelling and stakeholder requirements for climate adaptation.</p