Europe has experienced a significant increase in the frequency and intensity of summer heat waves, particularly since 2003, causing substantial economic and societal impacts. While previous research has explored the influence of large-scale atmospheric circulation patterns (like the North Atlantic Oscillation), ocean conditions (like ENSO), and land surface thermal conditions (like Eurasian soil moisture variations), the role of diminishing Arctic sea ice and Eurasian snow cover remains unclear. This study aims to address this gap by examining the potential links between the shrinking cryosphere and the intensification of European heat waves. The hypothesis is that the combined reduction in Arctic sea ice and Eurasian snow cover leads to changes in atmospheric circulation, favoring the formation of persistent blocking high-pressure systems over Europe, and hence more extreme heat events. The importance of understanding this connection lies in improving our capacity to predict and mitigate the impacts of future heat waves in the context of ongoing climate change.

Existing literature primarily attributes European heat waves to various factors. Some studies emphasize the role of large-scale atmospheric circulation patterns, including the North Atlantic Oscillation (NAO) and midlatitude planetary waves, highlighting the importance of blocking events in prolonging heat waves. Other studies point to the influence of ocean and land surface conditions, such as El Niño-Southern Oscillation (ENSO), North Atlantic warming, and Eurasian soil moisture variations. Anthropogenic global warming, and the associated Arctic temperature amplification, has also been implicated as a contributing factor. However, while the impacts of reduced Arctic sea ice and high-latitude snow cover have been considered individually, a comprehensive investigation of their combined effect on European heat waves is lacking. This study addresses this research gap by examining the synergistic impacts of reduced sea ice and snow cover on the large-scale atmospheric circulation and the ensuing European heat waves.

This study employs observational analyses and numerical experiments to investigate the relationship between reduced Arctic sea ice and Eurasian snow cover and the increase in European heat waves. Observational data include gridded daily maximum temperature data from the Hadley Center-Global Historical Climatological Network (HadGHCN) and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis. Arctic sea ice concentration (ASIC) data is sourced from the Hadley Centre, while Eurasian snow cover fraction (EASC) data is from NOAA's National Centers for Environmental Information (NCEI). Additional data includes soil moisture and surface heat flux from the ERA-Interim reanalysis dataset. A heat wave magnitude index (HWMId) is calculated to quantify heat wave intensity and duration. Composite analyses are used to compare atmospheric conditions during periods of high and low heat wave activity, examining sea ice concentration, snow cover, soil moisture, surface heat flux, 200-hPa geopotential height (Z200), mean zonal wind (U200), and synoptic-scale transient eddy activity (STEA). The study defines a combined index (ICE/SNOW) incorporating both ASIC and EASC. Numerical experiments were conducted using the NCAR CAM3.1 atmospheric general circulation model. Three pairs of comparative experiments were performed: low-ASIC (LICE) and high-ASIC (HICE), low-EASC (LSNOW) and high-EASC (HSNOW), and low-ASIC-EASC (LICESNOW) and high-ASIC-EASC (HICESNOW). These experiments prescribe observed monthly composites of ASIC and EASC to investigate the causal links between the cryosphere changes and the atmospheric responses. Finally, projections from 13 CMIP5 climate models were analyzed to assess future trends in ASIC, EASC, and heat wave frequency.

The study found a statistically significant increasing trend in European summer heat wave magnitude from 1980-2015, particularly pronounced after 1997. Composite analysis revealed that during periods with more frequent and intense heat waves, there was reduced ASIC and EASC, especially in spring and summer. A combined ICE/SNOW index, reflecting the combined effect of reduced sea ice and snow cover, showed a significant negative trend, strongly correlated with the HWMId. Analysis of atmospheric circulation patterns indicated that during high-HW periods, a classic "Omega" blocking pattern was observed, with enhanced meridional flow and reduced synoptic-scale transient eddy activity over Europe. Numerical experiments using the CAM3.1 model largely reproduced the observed atmospheric responses to reduced ASIC and EASC. The model simulations confirmed that both reduced ASIC and EASC contributed to the intensification of European heat waves through changes in large-scale circulation and eddy activities. The combined effect of both forcings was greater than the individual effects. Finally, projections from CMIP5 climate models suggest a continued decline in ASIC and EASC over the next century, with a concomitant increase in the frequency and intensity of European summer heat waves.

The findings strongly support the hypothesis that the decline in ASIC and EASC contributes significantly to the increase in European heat waves. The study's results highlight the importance of considering the combined effects of these two cryosphere-related factors, rather than focusing on individual forcings. The mechanisms identified – weakening of the midlatitude STEA, a "double-jet" regime, and a midlatitude wave train pattern – provide a robust physical explanation for the observed changes in atmospheric circulation. These results have significant implications for future climate projections and highlight the need for improved predictions of extreme weather events in a changing climate. While the study addresses potential limitations related to the impact of the changing mean climate and the relative contributions of advection and turbulent heat flux, further research is needed to completely address these.

This study presents compelling evidence for the link between shrinking Arctic sea ice and Eurasian snow cover and the increase in European heat waves. The combined effect of these two factors drives changes in large-scale atmospheric circulation, leading to more frequent and intense blocking events and consequently, more extreme heat. The CMIP5 model projections suggest that this trend will likely continue into the future, necessitating further research to refine our understanding of these complex interactions and improve our ability to predict and adapt to future heat waves. Future studies should focus on improving the representation of land-surface processes and the interactions between the cryosphere, atmospheric dynamics and heat waves.

The study acknowledges the limitations associated with distinguishing the effects of changing mean summer temperatures and increasing extreme events. Additionally, the relative contributions of advection and turbulent heat flux to surface temperature variations warrant further investigation. The use of only 13 CMIP5 models for future projections introduces uncertainty, and the use of a limited number of ensemble members in the model experiments could also impact the generalizability of the findings. Despite these limitations, the study's findings provide valuable insights into the complex relationship between Arctic sea ice, Eurasian snow cover, and European heat waves.