The role of snow cover in the Northern Hemisphere winter to summer transition

This paper examines the role of North Hemisphere snow cover in the linkage between the winter North Atlantic Oscillation (NAO) and the summer Northern Annular Mode (NAM). This transition is partially supported by the persistence of the NAO-induced snow cover anomalies and the asymmetric thermal distribution induced by summer snow cover. We define an index of subpolar temperature difference which links winter NAO with the subsequent summer NAM. The index is also significant in the linkage between summer and winter climates and can be used as an useful predictor of the upcoming winter NAO

European summer temperatures since Roman times

The spatial context is criticalwhen assessing present-day climate anomalies, attributing them to potential forcings and making statements regarding their frequency and severity in a long-term perspective. Recent international initiatives have expanded the number of high-quality proxy-records and developed new statistical reconstruction methods. These advances allow more rigorous regional past temperature reconstructions and, in turn, the possibility of evaluating climate models on policy-relevant, spatiotemporal scales. Here we provide a new proxy-based, annually-resolved, spatial reconstruction of the European summer (June-August) temperature fields back to 755 CE based on Bayesian hierarchical modelling (BHM), together with estimates of the European mean temperature variation since 138 BCE based on BHM and composite-plus-scaling (CPS). Our reconstructions compare well with independent instrumental and proxy-based temperature estimates, but suggest a larger amplitude in summer temperature variability than previously reported. Both CPS and BHM reconstructions indicate that the mean 20th century European summer temperature was not significantly different from some earlier centuries, including the 1st, 2nd, 8th and 10th centuries CE. The 1st century (in BHM also the 10th century) may even have been slightly warmer than the 20th century, but the difference is not statistically significant. Comparing each 50 yr period with the 1951-2000 period reveals a similar pattern. Recent summers, however, have been unusually warm in the context of the last two millennia and there are no 30 yr periods in either reconstruction that exceed the mean average European summer temperature of the last 3 decades (1986-2015 CE). A comparison with an ensemble of climate model simulations suggests that the reconstructed European summer temperature variability over the period 850-2000 CE reflects changes in both internal variability and external forcing on multi-decadal time-scales. For pan-European temperatures we find slightly better agreement between the reconstruction and the model simulations with high-end estimates for total solar irradiance. Temperature differences between the medieval period, the recent period and the Little Ice Age are larger in the reconstructions than the simulations. This may indicate inflated variability of the reconstructions, a lack of sensitivity and processes to changes in external forcing on the simulated European climate and/or an underestimation of internal variability on centennial and longer time scales

Tracking Iberian heatwaves from a new perspective

This paper presents a new heatwave (HW) detection algorithm that identifies spatially coherent HW patterns on synoptic scales and their temporal evolution, yielding the main characteristics (extension, intensity or persistence) of HW events (HWEs). The algorithm has been applied to temperature data from the ERA-Interim reanalysis in order to derive a catalogue of Iberian HWEs for the extended summers (June to September) of the 1979–2017 period. The results indicate mean frequencies of five Iberian HWEs and 16 summer days with HW conditions over Iberia (Iberian HWDs), with significant positive trends in both diagnostics. The analysis of the life-cycle reveals that more than half of the HWEs correspond to events that originated within the region. Although Iberian HWEs last more than one week on average, they tend to be transient, persisting for about three days in the region (Iberian phase), where they reach maximum intensity and extension, and evolving later to other areas. In order to identify recurrent patterns of occurrence, a clustering of Iberian HWEs was performed based on their mean temperature fields. Four clusters were obtained: Atlantic, Subtropical, European and Mediterranean events, which display distinctive characteristics and spatio-temporal evolution, causing HW conditions in western, southern, northern and eastern Iberia, respectively. Interestingly, Mediterranean events largely explain the overall trends in Iberian HWEs and HWDs. The connection between Iberian HWEs and atmospheric circulation patterns as summarized in four Weather Regimes (WRs) was also investigated. During the Iberian phase, HWEs are preferentially associated with ridge conditions in western Europe, with small variations in this WR determining different regional HWEs. However, the four types of regional Iberian HWEs tend to occur under different WRs during their pre- and post-Iberian phases, and show different relationships with WRs on seasonal scales. Using an impact-oriented metric for HWEs that accumulates the intensity of HW conditions over the areas affected by the event through its life-cycle, the top 10 Iberian HWEs were identified. They include well-known recent events such as those of August 2003, and June 2017. Flow analogues of the most outstanding Iberian HWEs reveal that recent warming has contributed to double their extension and intensity, making them more exceptional than they would have been in the past

A multi-parametric perspective of the North Atlantic eddy-driven jet

© The Author(s) 2022. This research is part of the CSIC Interdisciplinary Thematic Platform (PTI) Clima y Servicios Climáticos (PTI-CLIMA) and POLARCSIC (PTI-POLAR) activities. The valuable comments of two anonymous reviewers helped to improve the manuscript. Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. This work was supported by the Spanish Ministerio de Ciencia, Innovación y Universidades through the JeDiS (RTI2018-096402-B-I00) project. MGB was also supported by the Spanish Ministerio de Ciencia e Innovación (Grant PRE2019-090618).The North Atlantic eddy-driven jet (EDJ) is an essential component of the Euro-Atlantic atmospheric circulation. It has been typically described in terms of latitude and intensity but this is not enough to fully characterize its variability and complex EDJ confgurations. Here, we present a set of daily parameters of the EDJ based on low-tropospheric zonal wind data for the 1948–2020 period. They describe the intensity, sharpness, location, edges, tilt and other zonal asymmetries of the EDJ, therefore dissecting its structure beyond the latitudinal regimes. This allows for assessments of specifc EDJ aspects and a multi-parametric treatment of EDJ confgurations in a manageable way. Overall, variations in EDJ parameters refect distinctive patterns of eddy forcing and wave breaking, with anticyclonic eddies playing a major role in shaping the EDJ structure. A multimodal behavior of the EDJ is only detected in latitude, which largely infuences the longitudinal position of the EDJ. Other aspects of the EDJ are less constrained by the latitude and display a variety of confgurations. Four multi-parametric states (northern, central, tilted and split EDJs) provide a satisfactory description of recurrent patterns of the EDJ. They participate in meridional migrations of the EDJ, but yield less dramatic transitions than viewed from the latitudinal perspective. Finally, the EDJ parameters help to better understand the EDJ infuence on European climate. In many regions, latitude and intensity contain limited information on near-surface anomalies, and their signals can be masked by the additional efect of other EDJ parameters.Depto. de Física de la Tierra y AstrofísicaFac. de Ciencias FísicasTRUEMinisterio de Ciencia, Innovación y Universidades through the JeDiSMinisterio de Ciencia e Innovaciónpu

New observational insights into the atmospheric circulation over the Euro‑Atlantic sector since 1685

Wind direction kept in ships’ logbooks is a consolidated but underexploited observational source of relevant climatic information. In this paper, we present four indices of the monthly frequency of wind direction, one for each cardinal direction: Northerly (NI), Easterly (EI), Southerly (SI) and Westerly (WI), based on daily wind direction observations taken aboard ships over the English Channel. These Directional Indices (DIs) are the longest observational record of atmospheric circulation to date at the daily scale, covering the 1685–2014 period. DIs anomalies are associated with near-surface climatic signals over large areas of Europe in all seasons, with zonal indices (WI and EI) and meridional indices (NI and SI) often afecting different regions. Statistical models including all DIs are able to explain a considerable amount of European climate variability, in most cases higher than that accounted for by the North Atlantic Oscillation. As such, the DIs are able to reproduce the known European climatic history and provide new insights of certain episodes from monthly to multi-decadal time scales such as the warm winter decade of 1730–1739 or the extremely cold 1902 summer. The DIs show the potential to better constrain the atmospheric circulation response to external forcings and its associated anomalies. In particular, we provide frst observational evidences of all year-round atmospheric circulation signals following the strongest tropical volcanic eruptions of the last three centuries. These signatures are more complex than previously thought and suggest that the well-reported winter warming and summer cooling cannot be simply interpreted in terms of changes in zonality

Selection of optimal proxy locations for temperature field reconstructions using evolutionary algorithms

In the Era of exponential data generation, increasing the number of paleoclimate records to improve climate feld reconstructions might not always be the best strategy. By using pseudo-proxies from diferent model ensembles, we show how biologically-inspired artifcial intelligence can be coupled with diferent reconstruction methods to minimize the spatial bias induced by the non-homogeneous distribution of available proxies. The results indicate that small subsets of records situated over representative locations can outperform the reconstruction skill of the full proxy network, even in more realistic pseudo-proxy experiments and observational datasets. These locations highlight the importance of high-latitude regions and major teleconnection areas to reconstruct annual global temperature felds and their responses to external forcings and internal variability. However, low frequency temperature variations such as the transition between the Medieval Climate Anomaly and the Little Ice Age are better resolved by records situated at lower latitudes. According to our idealized experiments a careful selection of proxy locations should be performed depending on the targeted time scale of the reconstructed fel

Saharan air intrusions as a relevant mechanism for Iberian heatwaves: the record breaking events of August 2018 and June 2019

The summers of 2018 and 2019 were characterized by unusually warm conditions over Europe. Here, we describe the intense heatwaves striking the Iberian Peninsula in early August 2018 and late June 2019. The 2018 episode was relatively short-lived but outstanding in amplitude, particularly in western Iberia. Similar to previous mega-heatwaves, the 2019 event was long-lasting and affected large areas of western and central Europe, including eastern Iberia. During these events, many absolute temperature records were broken in western and eastern Iberia, respectively (some of them standing since 2003). In both cases, a cyclonic circulation off the coast in the northeastern Atlantic and a strong subtropical ridge pattern over the affected area promoted the advection of an anomalously warm air mass. This paper highlights the role of these very warm, stable and dry air intrusions of Saharan origin in the western and eastern Iberia heatwave events. Using a thermodynamical classification based on the geopotential height thickness and potential temperature, we show how the magnitude and poleward extension of these Saharan intrusions were unprecedented in the period since 1948. The relationship between Iberian heatwaves and Saharan warm air intrusions is discussed in the long-term context, showing a closer link in southern sectors of the Peninsula. However, a consistent poleward trend in the latitudinal extension of these subtropical intrusions reveals their increasing relevance for heatwaves in northern sectors of Iberia and western Europe. This overall trend is accompanied by an apparent “see-saw” in the occurrence of subtropical intrusions between eastern and western Iberia on multi-decadal scales

Impact of climate change on Spanish electricity demand

This paper evaluates the influence that climate change could exert on electricity demand 15patterns in Spain conditioned on the level of warming, with special attention to the 16seasonal occurrence of extreme demand days. For this purpose, assuming the currently 17observed electricity demand–temperature relationship holds in the future, we have gen-18erated daily time series of pseudo-electricity demand for the recent past and the twenty-19first century by using simulated temperatures from statistical downscaling of global 20climate model experiments. We show that both the frequency and severity of extreme 21electricity demand days at the national level are expected to increase, even for low levels 22of regional warming. Moreover, the occurrence of these extremes will experience a 23seasonal shift from winter to summer due to the projected temperature increases in both 24seasons. Under a RCP8.5 scenario of greenhouse gas emissions, the extended summer 25season (June–September) will concentrate more than 50% of extreme electricity demand 26days by the mid-century, increasing to 90% before the end of the century. These changes 27in electricity demand have considerable spatial heterogeneity over the country, with 28northwestern Spain experiencing the seasonal shift later than the rest of the country, 29due to the relatively mild summer temperatures and lower projected warming there

June 2017: The Earliest European Summer Mega-heatwave of Reanalysis Period

This paper examines the characteristics of the heatwave that affected western and central Europe in June 2017. Using a novel algorithm, we show that its extension, intensity, and persistence were comparable to those of other European mega-heatwaves, but it occurred earlier in the summer. The most affected area was Iberia, which experienced devastating forest fires with human casualties and the warmest temperatures of the reanalysis period from daily to seasonal scales. The peak of the mega-heatwave displayed an unprecedented warm air intrusion due to a record-breaking subtropical ridge with signatures closer to those of July and August. The atmospheric circulation was the main triggering factor of the event. However, thermodynamical changes of the last decades made a substantial contribution to the event, by increasing the likelihood of surpassing high-temperature thresholds. This episode could be a good example of a coming future, with high-summer mega-heatwaves occurring earlier