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(a) 2015 JJA sea surface temperature anomalies (over the ocean) and maximum 2 m air temperature anomalies (over land) in °C (shading). Contours delineate regions exceeding 2 and 2.5 standard deviations (contours). The left box indicates the area chosen to define the cold Atlantic SST anomaly; the right box indicates the area used to define the maximum 2 m air temperature anomaly. (b) Anomalies of geopotential height (in m) at 850 mb for JJA 2015. (c) Evolution of JJA SST anomalies (red, averaged over left box) and maximum 2 m air temperature anomalies (black, averaged over right box). The seasonal cycle for 1981–2010 has been removed. (d) Ocean heat content anomaly time series (in GJ m⁻²) up to Dec 2015 averaged over the box (45°–60°N, 40°–15°W) and extracted from the National Oceanographic Data Center (NODC) 0–700 m product (blue). Argo profiles represent the main source of data after 2003, as shown by an Argo-only index smoothed with a 12 month running window (red). Both time series are referenced to the period 2000–2015 and show a stronger reduction since 2013.
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The North Atlantic and Europe experienced two extreme climate events in 2015: exceptionally cold ocean surface temperatures and a summer heat wave ranked in the top ten over the past 65 years. Here, we show that the cold ocean temperatures were the most extreme in the modern record over much of the mid-high latitude North-East Atlantic. Further, by...
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Station-based observations and bias-corrected model simulations show that the frequency of short-term heat waves in central Europe has increased, albeit quantitative estimates of risk ratios differ considerably between methods.
Citations
... Several studies examined the CA region's physical drivers and consequences on weather (e.g., Henson, 2016;Josey et al., 2018;Robson et al., 2016), including occurrences prior to 2014-16 and as far back as the 1950s (e.g., Liang et al., 2017;Shi et al., 2023). The main mechanism behind the cold anomaly during 2015 is an intense surface heat loss due to variations in atmospheric circulation in the winter of 2014-15 coupled with an intense heat loss in preceding winter of 2013-14 that formed the re-emergence of a cold water anomaly (Duchez et al., 2016). About 70% of the re-emergence during the second half of 2014 was found due to vertical diffusion (Sanders et al., 2022), which constitutes another mechanism behind this re-emergence. ...
... The cold blob phenomenon has also been catching the press's attention with numerous newspaper articles as recent as late 2023 (e.g., Mooney, 2015;Ramirez Grand, 2023). Unlike the Pacific blob, which has been extensively studied as an MHW from physics (e.g., Di Lorenzo & Mantua, 2016) to biogeochemistry (Mogen et al., 2022) and ecosystem impacts (e.g., Suryan et al., 2021), the Atlantic cold blob has not been examined as a manifestation of MCS using the Hobday et al. (2016) framework (which objectively characterizes extreme temperature events), although it was studied as a region where unusually cool temperatures occurred (e.g., Duchez et al., 2016;Josey et al., 2018;Shi et al., 2023). The only exception is an application by Schlegel et al. (2021) that briefly looked at MCS 2013-2016 event over the CA region focusing only on a specific location (a focal point) rather than the wider CA region and without a complete assessment of MCS metrics (such as the frequency, total days and mean intensity). ...
Regional effects of marine cold spells (MCS, periods of anomalous cooling), their impact on ecosystem biogeochemistry, and link to salinity extremes remain underexplored. A case in point is North Atlantic's Cold Anomaly (CA) region (known as the “cold blob”), which hits record low temperatures during 2014–16 while most of the global ocean warmed. Using up to 42 years of observations, we characterize the CA as a manifestation of both MCS and Fresh Waves (FW, low salinity extremes) and analyze the surface biogeochemical response. We observe a quasiperiodic pattern of MCS from the 1980s and FW (at least) from the 1990s to early 2020s in the CA region with alternations from cool and freshwater to warm and saline conditions. Since 1990s, the CA region appears to be potentially undergoing MCS and FW compound events that are more frequent and prolonged but less intense than other North Atlantic areas. The 2014‐16 CA was among the most widespread and prolonged MCS and FW events associated with a deeper mixed layer and distinct biogeochemical signature, including elevated nutrients and oxygen, an overall increased chlorophyll‐a and intensified ocean acidification. These results suggest that MCS could mitigate certain climate change effects through cooling and enhanced productivity, while exacerbating others such as ocean acidification. We compare 2014–16 CA region effects with those of Pacific's warm blob, identifying contrasting behaviors from physical processes to biogeochemical impacts and discussing a common atmospheric driver. Our findings emphasize the need to further study ecological responses to MCS in the North Atlantic.
... There is existing evidence of a relationship between cold SST anomalies in the North Atlantic and the occurrence of heatwaves in Europe. For instance [32], analysed a cold SST event in the North Atlantic that occurred in 2015, finding that it preceded the development of a heatwave in Europe ranked in the top ten over the previous 65 years. This ocean anomaly was associated with a stationary jet stream, which contributed to sustained high surface temperatures over Central Europe. ...
In our changing climate we expect an increase in the frequency of heatwaves, which are extreme events that can have severe consequences for human health, ecosystems, and economies. In this study, we examine how the weakening of the atlantic meridional overturning circulation (AMOC) may influence the occurrence of extreme warm events in Europe. Our analysis is based on a series of numerical experiments conducted using the EC-Earth3 climate model, which includes three ensembles of 20 Atmospheric Model Intercomparison Project-like members. The large ensemble size makes these simulations well-suited to studying rare extreme events. Our findings reveal that during boreal summer, the Northern hemisphere (NH) experiences overall mean cooling, most pronounced in the North Atlantic region. Therefore, the extreme warm events in Europe identified by using a fixed threshold tend to be less frequent. However, the cooling pattern reduces the meridional gradient of near-surface air temperature at high latitudes of the NH. As a result, the speed of the summer jet stream decreases, while the frequency of Ural atmospheric blocking events increases. Atmospheric blocking in summer is closely linked to heatwaves, primarily through subsidence warming and enhanced downward shortwave radiation under clear sky conditions. Consequently, the weakening of the AMOC leads to an increased frequency of heatwaves in Eastern Europe, the only region that presents this opposite trend. This study highlights the significant role that three-dimensional ocean circulation plays in shaping weather patterns, including extreme events such as heatwaves in Europe. These findings have important implications for future climate projections, as the AMOC is expected to weaken by the end of this century. Thus, as the Earth continues to warm, we may face an increased risk of heatwaves due to the combined effects of global warming and a weakening AMOC.
... Previous research has established that changes in large-scale atmospheric circulation 12,13 , unusual jet stream configurations 14 , anomalous sea surface temperature (SST) patterns 15 , soil moisture deficits, and associated land-atmosphere feedbacks 16 , as well as anthropogenic global warming 17 , constitute key drivers of extreme heatwave variability. Notably, the influence of atmospheric teleconnection merits mention, where surface temperature anomalies during Northern Hemisphere summers are frequently linked with multiple atmospheric telecorrelations 18,19 , sometimes leading to simultaneous extreme events in multiple regions 20 . ...
Western Europe experienced an unprecedentedly hot July in 2022, which significantly impacted ecosystems and society. Our observational and numerical modeling study reveals that this event was influenced by anomalous North Atlantic and Eurasian jet streams. The northeastward shift of the North Atlantic jet stream, driven by sea surface temperature gradients, and the curving of the Eurasian jet stream, affected by rainfall anomalies in Pakistan, enhanced atmospheric subsidence over western Europe. This research highlights the crucial role of the synergistic behavior of the North Atlantic and Eurasian jet streams in driving extreme heat over Western Europe. Furthermore, CMIP6 climate model projections suggest that under the SSP585 scenario, similar jet stream configurations could lead to even more intense extreme temperatures (~7.02 ± 0.61 °C) compared to the current climatological mean.
... Conversely, anomalous cyclonic conditions offshore of Ireland have been suggested to influence heatwaves in Europe for a negative EA phase (e.g. Duchez et al., 2016). Using a clustering approach (e.g. ...
We apply causal effect networks to evaluate the influence of spring North Atlantic extratropical sea surface temperatures (NA-SSTs) on the summer East Atlantic (EA) pattern seasonal predictability during the period of 1908–2008. In the ECMWF Reanalysis of the 20th Century (ERA-20C), we find that the causal link from the meridional NA-SST gradient in spring (expressed by a meridional “SST index”) to the summer EA is robust during the period from 1958 to 2008, with an estimated causal effect expressed by a β coefficient of about 0.2 (a 1 standard deviation change in the spring SST index causes a 0.2 standard deviation change in the EA 4 months later). However, this causal link is not evident when analysing the entire period from 1908 to 2008. When performing the analysis on 45-year-long time series randomly sampled from this late period, we find the strength of the causal link to be affected by interannual variability, suggesting a potential modulation by an external physical mechanism. In addition to the summer EA, we find that the spring SST index has an estimated causal effect of about -0.2 on summer 2 m air temperatures over northwestern Europe. We then use different datasets from the Max Planck Institute Earth System Model in its mixed-resolution set-up (MPI-ESM-MR) to analyse the 1908–2008 period, focusing on a historical simulation and a 30-member initialised seasonal prediction ensemble. We specifically test the model's ability to reproduce the causal links detected in ERA-20C and evaluate their impact on the model's predictive skill for the European summer climate. We find that MPI-ESM-MR generally fails to reproduce the causal link between the spring SST index and the summer EA across the datasets. The 30-member initialised ensemble occasionally reproduces the causal link, though it typically underestimates its strength. We perform a predictive skill assessment conditioned on the spring SST index causal links for July–August sea level pressure, 500 hPa geopotential height, and 2 m air temperatures for predictions initialised in May. Our results suggest that while the overall impact may be limited, leveraging these causal links locally could help to constrain and improve the seasonal prediction skill of European summer climate.
... Rather than only a southward shift of the Westerlies during winter, we argue that this seasonal atmospheric difference is a plausible explanation for the overall dry conditions inferred by the summer-sensitive biomarker proxies across Central Europe during the Younger Dryas. This atmospheric pattern is very similar to multidecadal atmospheric summer oscillations in the North Atlantic 84 and agrees with recent observations of strong heat waves in Central Europe in relation to unusually cool winter SST anomalies in the North Atlantic 85 . On a broader spatial scale, summer blocking over Central Europe caused positive precipitation anomalies and cold temperatures in southeastern Europe and the eastern Mediterranean realm 79 . ...
It is generally accepted that a weakening of the North Atlantic thermohaline circulation caused the Younger Dryas cooling. Although the role of seasonality was emphasized previously, this aspect is rarely considered yet, and it remains elusive how this impacted hydroclimate during winters and summers across Central Europe. Here, we coupled biomarker-based δ¹⁸O and δ²H from Bergsee in southern Germany to reconstruct deuterium excess as a proxy for evaporation history from the Bølling-Allerød to the Preboreal. We compared this dataset with other biomarker isotope records in Central Europe. They are all lacking a strong isotopic depletion during the Younger Dryas, which is best explained by the summer sensitivity of the biomarker proxies: As Younger Dryas summers were relatively warm, there is an absence of the strong winter cooling signals recorded in annual water isotope records like Greenland or Lake Steißlingen. Lake evaporation at Bergsee together with other paleohydrological reconstructions draw a coherent picture of the Late Glacial hydroclimate, with strong evidence for warm and dry Younger Dryas summers. Rather than a southward shift of the Westerlies during winter, we suggest that a recently proposed feedback mechanism between North Atlantic sea ice extend, strong winter cooling and summer atmospheric blocking serves as a suitable explanation for summer dryness. Additional confidence to the robustness of these biomarker records is provided by the overall agreement of paleohydrological fluctuations during the Preboreal.
... Summer extreme temperatures over Europe are potentially predictable on seasonal time-scales. Indeed, the summer European climate is significantly influenced by slowly evolving components of the climate system, such as sea-surface temperatures (SSTs) in the North Atlantic (Duchez et al., 2016;Mecking et al., 2019), Indian Ocean (Black & Sutton, 2007), and Mediterranean Sea (Black et al., 2004), as well as European soil moisture (Hamilton et al., 2012;Prodhomme et al., 2016) and Arctic sea-ice (Screen, 2013). Moreover, the occurrence of summer European extreme temperatures has been associated with modes of low-frequency climate variability, such as the North Atlantic Oscillation (NAO; Folland et al., 2009;Alvarez-Castro et al., 2018), the East Atlantic (EA; Cassou et al., 2005;Wulff et al., 2017), and the El Niño-Southern Oscillation (ENSO; Luo & Lau, 2020;Martija-Díez et al., 2021). ...
... Consistent with previous studies (Di Capua et al., 2021;Duchez et al., 2016;Neddermann et al., 2019), these results highlight the relevance of the NAO and EA modes in influencing summer extreme temperatures and explaining their variance in Europe, particularly in northern, central, and eastern regions. Therefore, incorporating these modes into the subsampling strategy may be beneficial to improve the seasonal prediction of summer extreme temperatures in state-of-the-art SPSs. Figure 2a,c shows that the C3S MME successfully reproduces the key features of the summer NAO pattern in the observations, both in terms of spatial shape and amplitudes of Z500 anomalies. ...
Although summer extreme temperatures over Europe are potentially predictable on seasonal time‐scales, state‐of‐the‐art dynamical seasonal prediction systems (SPSs) exhibit low skills in predicting such events in central and northern Europe. This limitation arises from the underestimation of predictable components of climate variability in the model ensemble. However, recent studies suggest that the skills in predicting extratropical climate can be largely improved through statistical postprocessing techniques, which increase the signal‐to‐noise ratio in the model ensemble. In this study, we evaluate the potential for improving the seasonal prediction skills of European summer extreme temperatures in a multimodel ensemble (MME) of SPSs by applying a teleconnection‐based subsampling technique in the hindcast period 1993–2016. This technique is applied to the North Atlantic Oscillation (NAO) and East Atlantic (EA) modes, which are key drivers of summer extreme temperatures in Europe. Results show that the subsampling substantially improves the MME prediction skills of both the summer NAO and EA. Specifically, correlations between the observed and subsampled MME NAO indices improve from −0.38 to 0.77, and for the EA they improve from −0.11 to 0.84. Similarly, the root‐mean‐square error of the subsampled MME NAO (EA) index improves from 1.06 (1.02) to 0.65 (0.56). Moreover, retaining those ensemble members that accurately represent the NAO teleconnections enhances the MME prediction skills for the summer European climate, including the occurrence of summer extreme temperatures. This improvement is particularly pronounced in central and northern Europe; that is, the regions where current SPSs show the lowest skills in predicting European heat extremes. In contrast, selecting ensemble members that accurately represent the EA teleconnections does not improve the predictions of summer extreme temperatures. This is likely associated with the model deficiencies in realistically representing the spatial pattern of the summer EA and, thus, the physical processes driving summer extreme temperatures.
... European temperatures are indirectly linked to Arctic SIC [49] [50] through impacts on atmospheric circulation, but we assume no link to Antarctic SIC. North Atlantic SSTs affect the European climate, particularly in winter [51], but there is some evidence to suggest a lagged influence on summer heatwaves [52,53]. Meanwhile, global modes of climate variability, which influence distant continents via teleconnections, are represented by global SST patterns. ...
Heatwaves (HWs) are extreme atmospheric events that produce significant societal and environmental impacts. Predicting these extreme events remains challenging, as their complex interactions with large-scale atmospheric and climatic variables are difficult to capture with traditional statistical and dynamical models. This work presents a general method for driver identification in extreme climate events. A novel framework (STCO-FS) is proposed to identify key immediate (short-term) HW drivers by combining clustering algorithms with an ensemble evolutionary algorithm. The framework analyzes spatio-temporal data, reduces dimensionality by grouping similar geographical nodes for each variable, and develops driver selection in spatial and temporal domains, identifying the best time lags between predictive variables and HW occurrences. The proposed method has been applied to analyze HWs in the Adda river basin in Italy. The approach effectively identifies significant variables influencing HWs in this region. This research can potentially enhance our understanding of HW drivers and predictability.
... Using AA, we extract the large-scale patterns associated with extreme surface temperatures in the extratropical Northern Hemisphere. Well-studied events, such as the 2003 western European heatwave (Black et al., 2004;Duchez et al., 2016), the 2010 Eastern European or "Russian" heatwave (Dole et al., 2011;Barriopedro et al., 2011;Lau and Kim, 2012;Hauser et al., 2016), and the 2021 western North-America "heatdome" event (Bartusek et al., 2022;White et al., 2023), appear naturally from our analysis, as well as several less-well studied events. We will provide a plausible dynamical mechanism by linking the extreme events with hemispheric-scale wave patterns and continental-scale RWPs. ...
We study the hemispheric to continental scale regimes that lead to summertime heatwaves in the Northern Hemisphere. By using a powerful data mining methodology - archetype analysis - we identify characteristic spatial patterns consisting of a blocking high pressure systems embedded within a meandering upper atmosphere circulation that is longitudinally modulated by coherent Rossby Wave Packets. Periods when these atmospheric regimes are strongly expressed correspond to large increases in the likelihood of extreme surface temperature. Most strikingly, these regimes are shown to be typical of surface extremes and frequently reoccur. Three well publicised heatwaves are studied in detail - the June-July 2003 western European heatwave, the August 2010 "Russian" heatwave, and the June 2021 "Heatdome" event across western North America, and are shown to be driven by blocking high pressure systems linked to stalled Rossby Wave Packets. We discuss the implications of our work for long-range prediction or early warning, climate model assessment and post-event diagnosis.
... These blocking events, resulting from the weakened jet stream, have been identified as the causes of significant HW episodes in Western Europe (in 1994, 2003, 2006, 2010, 2017, and 2018) (Barriopedro et al., 2011;Rousi et al., 2022;Sánchez-Benítez et al., 2018). Yet, Duchez et al. (2016) demonstrated that the HW in Western Europe in 2015 was associated with SST anomalies in the North Atlantic (NA). In fact, a significant meridional temperature gradient in the NA-SST can induce stagnation in the upper-troposphere jet stream, which in turn leads to an inflow of cold air over the NA and of warm air over Western Europe, promoting the development of stagnant high pressures and extreme HWs in the Western European region. ...
... El Niño-Southern Oscillation (ENSO), as the dominant mode of tropical Pacific sea surface temperature (SST) variability, could modulate heatwave variations over Europe 20 , North America 21 , East Asia 22 , South Asia 23 , and Australia 24,25 . In addition, the European heatwaves in 2003 26 and 2015 27 were thought to be driven by SST anomalies in the Indian Ocean and the North Atlantic, respectively. The Pacific SST anomaly identified as the Pacific Extreme Pattern as a precursor was observed even 50 days in advance of the occurrences of hot days over the eastern United States 28 . ...
