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A procedure for classifying daily summer temperature extremes in northeastern Spain into homogenous regions has been presented and evaluated. This procedure employed daily temperature series from a dense network of 128 weather stations spanning the period from 1960 to 2006. Characteristics of temperature extremes included temperature frequency (e.g., warm days), intensity (e.g., warmest day), and duration (e.g., maximum length of hot spell). Following the results of the principal components analysis and Ward's method of clustering, the study area was divided into four homogenous sub-regions in terms of both the geographic and climatic meanings: the Mediterranean region, the mainland and the Cantabrian region, the moderately elevated areas westward and southward, and the mountainous region. Based on an internal cluster validation measure (Silhouette width), the quality of clustering was evaluated and ensured. The temporal evolution of the long-term (1960-2006) temperature extremes clearly showed a different behavior amongst these sub-regions. The Mediterranean and the highly elevated regions revealed the strongest signals in both daytime and nighttime extremes. For mainland areas, considerable differences in the behavior of the daytime and nighttime temperature extremes were evident. The influence of atmospheric circulation on spatial and temporal variability of temperature extremes was also explored. The variability of summer temperature extremes in NE Spain appears to be mainly driven by the Scandinavian (SCA), the Western Mediterranean Oscillation (WeMO), and the East Atlantic (EA) patterns, with a tendency toward increasing during the positive (negative) phases of the EA (WeMO and SCA) circulation modes. In such a region with complex geography and climate, regionalization of summer temperature extremes can be advantageous for extracting finer-scale information, which may prove useful for the vulnerability assessments and the development of local adaptation strategies in areas such as health, ecosystems and agriculture. © 2012 Springer-Verlag Wien. ; We would like to thank the Spanish Meteorological State Agency for providing the temperature database used in this study. This work has been supported by the research projects CGL2008-01189/BTE, CGL2011-27574-CO2-02, and CGL2011-27536 financed by the Spanish Commission of Science and Technology and FEDER, EUROGEOSS (FP7-ENV-2008-1-226487), and ACQWA (FP7-ENV-2007-1-212250) financed by the VII Framework Programme of the European Commission "Efecto de los escenarios de cambio climático sobre la hidrología superficial y la gestión de embalses del Pirineo Aragonés" financed by "Obra Social La Caixa" and the Aragón Government and Influencia del cambio climático en el turismo de nieve, CTTP01/10, financed by the Comisión de Trabajo de los Pirineos. ; Peer Reviewed

21 páginas, 13 figuras. ; [EN] Spatial and temporal characteristics of extreme temperature events in northeastern Spain have been investigated. The analysis is based on long-term, high-quality, and homogenous daily maximum and minimum temperature of 128 observatories spanning the period from 1960 to 2006. A total of 21 indices were used to assess changes in both the cold and hot tails of the daily temperature distributions. The presence of trends in temperature extremes was assessed by means of the Mann-Kendall test. However, the autocorrelation function (ACF) and a bootstrap methodology were used to account for the influence of serial correlation and cross-correlation on the trend assessment. In general, the observed changes are more prevalent in hot extremes than in cold extremes. This finding can largely be linked to the increase found in the mean maximum temperature during the last few decades. The results indicate a significant increase in the frequency and intensity of most of the hot temperature extremes. An increase in warm nights (TN90p: 3.3 days decade−1), warm days (TX90p: 2.7 days decade−1), tropical nights (TR20: 0.6 days decade−1) and the annual high maximum temperature (TXx: 0.27 °C decade−1) was detected in the 47-yr period. In contrast, most of the indices related to cold temperature extremes (e.g. cold days (TX10p), cold nights (TN10p), very cold days (TN1p), and frost days (FD0)) demonstrated a decreasing but statistically insignificant trend. Although there is no evidence of a long-term trend in cold extremes, significant interdecadal variations were noted. Almost no significant trends in temperature variability indices (e.g. diurnal temperature range (DTR) and growing season length (GSL)) are detected. Spatially, the coastal areas along the Mediterranean Sea and the Cantabrian Sea experienced stronger warming compared with mainland areas. Given that only few earlier studies analyzed observed changes in temperature extremes at fine spatial resolution across the Iberian Peninsula, the results of this work can improve our understanding of climatology of temperature extremes. Also, these findings can have different hydrological, ecological and agricultural implications (e.g. crop yields, energy consumption, land use planning and water resources management). ; We are indebted to the anonymous reviewers for their constructive comments which were most helpful in improving the paper. We would like to thank the "Agencia Estatal de Meteorolog´ıa" for providing the temperature data used in this study. This work has been supported by the research projects CGL2008-01189/BTE and CGL2006-11619/HID financed by the Spanish Commission of Science and Technology and FEDER, EUROGEOSS (FP7-ENV-2008-1-226487) and ACQWA (FP7- ENV-2007-1- 212250) financed by the VII Framework Programme of the European Commission, "Las sequ´ıas clim´aticas en la cuenca del Ebro y su respuesta hidrol´ogica" and "La nieve en el Pirineo aragon´es: Distribuci´on espacial y su respuesta a las condiciones clim´atica" Financed by "Obra Social La Caixa" and the Arag´on Government. ; Peer reviewed

[EN] This paper gives the complete details of the protocols applied for developing a spatially and temporarily high-resolution dataset of temperature for northeastern Spain. Our methodologies used data from a large number of observatories (1583) spanning some portions of the period between 1900 and 2006. The raw dataset was first tested for internal and external consistency to check data quality. To improve data completeness, a linear regression model was then utilized to infill gaps in the daily temperature series using the best correlated data from nearby sites. Discontinuities in the reconstructed series were determined by combining the results of three homogeneity-relative tests: the Standard Normal Homogeneity Test (SNHT), the Eastrerling and Peterson two-phased regression method and the Vincent test. To assess the possible impact of data homogenisation on trends and statistical properties of the final series, a set of tests (e.g. semivariance models and L-moment statistics) was applied to the series before and after correction. Semivariance models suggest a significant improvement in the spatial dependence of the corrected dataset on both seasonal and annual timescales. Also, L-moments gave no evidence of significant changes in the probability distribution of daily temperature series after correction. Taken together, the newly compiled dataset seems to be more robust and reveals more coherent spatial and temporal patterns of temperature compared with the original dataset. From the temporal and spatial perspectives, the new dataset comprises the most complete register of temperature in northeast Spain (1900-2006), with a reasonably spatial coverage. Accordingly, this database can provide a more reliable base for studying temperature changes and variability in the region. This dataset can also be of particular relevance to a number of meteorological, ecological, hydrological and agricultural applications on local, regional and continental scales. © 2011 Royal Meteorological Society. ; We are indebted to the anonymous reviewers for their constructive comments which were most helpful in improving this paper. We would like to thank the Agencia Estatal de Meteorologia for providing the temperature data used in this study. This work has been supported by the research projects CGL2006-11619/HID, CGL2008- 01189/BTE, CGL2011-27574-CO2-02, CGL2011-27753- CO2-01 and CGL2011-27536 financed by the Spanish Commission of Science and Technology; and also FEDER, EUROGEOSS (FP7-ENV-2008-1-226487) and ACQWA (FP7-ENV-2007-1- 212250) financed by the VII Framework Programme of the European Commission, La nieve en el Pirineo Aragones y su respuesta a ´ la variabilidad climatica, and Efecto de los escenarios ´ de cambio climatico sobre la hidrolog ´ ´ıa superficial y la gestion de embalses del Pirineo Aragon ´ es, financed by ´ Obra Social La Caixa and the Aragon Government and ´ Influencia del cambio climatico en el turismo de nieve, ´ CTTP01/10, financed by the Comision de Trabajo de los ´ Pirineos. ; Peer Reviewed

35 Pags., 1 Tabl., 9 Figs. The definitive version is available at: http://onlinelibrary.wiley.com/journal/10.1002/(ISSN)1097-0088 ; Most applications of the extreme value (EV) theory have assumed stationarity, i.e. the statistical properties of the process do not change over time. However, there is evidence suggesting that the occurrence of extreme events is not stationary but changes naturally, as it has been found for many other climate variables. Of paramount importance for hazard analysis is whether the observed precipitation time series exhibit long-term trends or cycles; such information is also relevant in climate change studies. In this study, the theory of non-stationary extreme value (NSEV) analysis was applied to data series of daily precipitation using the peaks-over-threshold (POT) approach. A Poisson/generalized Pareto (P/GP) model, in which the model parameters were allowed to vary linearly with time, was fitted to the resulting series of precipitation event's intensity and magnitude. A log-likelihood ratio test was applied to determine the existence of trends in the model parameters. The method was applied to a case study in northeast Spain, comprising a set of 64 daily rainfall series from 1930 to 2006. Statistical significance was achieved in less than 5% of the stations using a linear non-stationary model at the annual scale, indicating that there is no evidence of a generalized trend in extreme precipitation in the study area. At the seasonal scale, however, a significant number of stations along the Mediterranean (Catalonia region) showed a significant decrease of extreme rainfall intensity in winter, while experiencing an increase in spring. ; This research has been supported by the following projects: CGL2008-00831/BTE, CGL2006-11619/HID, CGL2008-01189/BTE and CGL2008-1083/CLI funded by the Spanish Commission of Science and Technology and FEDER; EUROGEOSS (FP7-ENV-2008-1-226487) and ACQWA (FP7-ENV-2007-1-212250) funded by the VII Framework Programme of the European Commission; 'Las sequías climáticas en la cuenca del Ebro y su respuesta hidrológica' and 'La nieve en el Pirineo Aragonés: distribución espacial y su respuesta a las condiciones climáticas' funded by 'Obra Social La Caixa' and the Aragón Government; and 'Programa de grupos de investigación consolidados' funded by the Aragón Government. ; Peer reviewed

A monthly global dataset of a multiscalar drought index is presented and compared in terms of spatial and temporal variability with the existing continental and global drought datasets based on the Palmer drought severity index (PDSI). The presented dataset is based on the standardized precipitation evapotranspiration index (SPEI). The index was obtained using the CRU TS3.0 dataset at a spatial resolution of 0.5°. The advantages of the new dataset are that; i) it improves the spatial resolution of the unique global drought dataset at a global scale; ii) it is spatially and temporally comparable to other datasets, given the probabilistic nature of the SPEI, and, in particular; iii) it enables identification of various drought types, given the multiscalar character of the SPEI. The dataset is freely available on the web page of the Spanish National Research Council (CSIC) in three different formats (NetCDF, binary raster, and plain text). ; This work has been supported by the research projects CGL2008-01189/BTE and CGL2006-11619/HID financed by the Spanish Commission of Science and Technology and FEDER, EUROGEOSS (FP7-ENV- 2008-1-226487) and ACQWA (FP7-ENV-2007-1- 212250) financed by the VII Framework Programme of the European Commission, "Las sequías climáticas en la cuenca del Ebro y su respuesta hidrológica" and "La nieve en el Pirineo aragonés: Distribución espacial y su respuesta a las condiciones climática" Financed by "Obra Social La Caixa" and the Aragón Government ; Peer reviewed

The definitive version is available at: http://www.agu.org/journals/jgr ; We used a novel method that combined probabilistic analysis and spatial modeling assisted by GIS to analyze the risk of extreme precipitation in northeast Spain related to three atmospheric circulation configurations: the North Atlantic Oscillation (NAO), the Mediterranean Oscillation, and the Western Mediterranean Oscillation (WeMO). The analysis was performed at an event-based scale using data obtained from daily atmospheric circulation indices. The maximum intensity and total precipitation magnitude recorded during positive and negative circulation events were obtained from the daily records of 174 observatories between 1950 and 2006. The series of both maximum intensity and magnitude for positive and negative phases of the three atmospheric circulation indices follow a generalized Pareto (GP) distribution. A regression-based interpolation procedure was used to generate distributed maps of GP parameters, enabling us to determine the probability of the magnitude and maximum intensity of precipitation and the quantile precipitation for any return period associated with the positive and negative phases of the three atmospheric circulation patterns. A high spatial variability in precipitation risk was found, depending on the positive/negative phases of the three atmospheric circulation patterns. Different phases of the circulation indices show contrasting effects on the two analyzed parameters. Thus the most extreme daily precipitation during winter months is expected for negative WeMO events, representing a markedly different result from those obtained for other events. In contrast, negative NAO events record the most extreme precipitation magnitude risk per event, although this is mainly restricted to mountainous areas. ; This work was supported by projects financed by the Spanish Commission of Science and Technology (CGL2005–04508/BOS and CGL2008–01189/BTE), the 7th framework program of the European Commission (projects ACQWA (FP7-ENV-2007-1-212250) and EUROGEOSS (FP7-ENV-2008-1-226487)), and the "Programa de grupos de investigación consolidados" financed by the Aragón Government. ; Peer reviewed

14 Pags.- 5 Figs. Under Creative Commons License CC BY-NC-ND ; The standardized precipitation evapotranspiration index (SPEI) is one of the well‐established drought metrics worldwide. It is simply computed using precipitation and atmospheric evaporative demand (AED) data. Although AED is considered a key driver of drought variability worldwide, it could have less impact on drought in specific regions and for particular times as a function of the magnitude of precipitation. Specifically, the influence of the AED might overestimate drought severity during both normal and humid periods, resulting in "false alarms" about drought impacts on physical and human environments. Here, we provided a global characterization of the sensitivity of the SPEI to changes of the AED. Results demonstrate that the contribution of AED to drought severity is largely impacted by the spatial and temporal variability of precipitation. Specifically, the impact of AED on drought severity was more pronounced during periods of low precipitation, compared to wet periods. Interestingly, drought severity in humid regions (as revealed by SPEI) also showed low sensitivity to AED under drier conditions. These results highlight the skill of SPEI in identifying the role of AED in drought evolution, especially in arid and semiarid regions whose climate is characterized typically by low precipitation. This advantage was also evident for humid environments, where SPEI did not overestimate drought severity due to the increased AED. These findings highlight the broader applicability of SPEI to accurately characterize drought severity worldwide. ; This work was supported by the research projects CGL2017‐82216‐R and PCI2019‐103631, financed by the Spanish Commission of Science and Technology and Fondo Europeo de Desarrollo Regional (FEDER); CROSSDRO project financed by the AXIS (Assessment of Cross(X)—sectoral climate Impacts and pathways for Sustainable transformation), JPI‐Climate cofunded call of the European Commission and INDECIS which is part of ERA4CS, an ERA‐NET initiated by JPI Climate, and funded by FORMAS (SE), DLR (DE), BMWFW (AT), IFD (DK), MINECO (ES), and ANR (FR) with cofunding by the European Union (Grant 690462). Dhais Peña‐Angulo received a "Juan de la Cierva" postdoctoral contract (FJCI‐2017‐33652 from Spanish Ministry of Economy and Competitiveness, MEC). ; Peer reviewed

The propagation of the pandemic times, especially during COVID-19, has caused millions of morbidity and mortality cases across the world, forcing people to change their lifestyles and governments to take different measures to slow down the spread of the disease. Consequently, electricity demand and consumption patterns across other socioeconomic sectors were affected by the pandemic. This study aims to investigate the impact of the COVID-19 pandemic on spatiotemporal electricity demand and consumption across six socioeconomic sectors in Doha City, Qatar. The electricity demand and consumption were assessed for three time periods in the pandemic year (pre-lockdown, lockdown, and post-lockdown) compared to electricity consumption prior to the pandemic year (2017–2019). The empirical analysis was performed on a geographically visualized map to determine the areas with high and low electricity consumption. The pattern has been compared before and during the pandemic with previous years. The results show that electricity consumption has increased in the residential and governmental sectors and declined in the industrial and commercial sectors during the lockdown period compared to the post-lockdown period, particularly in the summer season. Mapping the hot/cold spots areas and the temporal analysis of the changing patterns of electricity demand and consumption could provide useful insight to decision-makers for targeted interventions. ; This publication was made possible by an NPRP award [NPRP13S- 0206-200272] from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors. The open access publication of this article was funded by the Qatar National Library (QNL).

2 .pdf files: 1) Article (18 Pags.- 2 Tabls.- 10 Figs.); 2) Supplement (20 Pags.- 19 Figs.). © Author(s) 2019. This work is distributed under the Creative Commons Attribution 4.0 License (http://creativecommons.org/licenses/by/4.0/). ; Assessing the probability of occurrence of drought is important for improving current drought assessment, management and mitigation measures, and strategies across Spain. This study employed two well-established drought indices, the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI), to characterize drought duration and magnitude at different timescales over Spain. In order to map the drought hazard probability, we applied the extreme value theory and tested different thresholds to generate peak-over-threshold (POT) drought duration and magnitude series. Our results demonstrate that the generalized Pareto (GP) distribution performs well in estimating the frequencies of drought magnitude and duration. Specifically, we found a good agreement between the observed and modelled data when using upper percentiles to generate the POT series. Spatially, our estimations suggest a higher probability of extreme drought events in southern and central Spain compared to the northern and eastern regions. Also, our study found spatial differences in drought probability estimations as a function of the selected drought index (i.e. SPI vs. SPEI) and timescale (i.e. 1, 3, 6, and 12 months). Drought hazard probability maps can contribute to the better management of different sectors (e.g. agriculture, water resources management, urban water supply, and tourism) at national, regional, and even local scale in Spain. ; This work was supported by the following research projects: CGL2014-52135-C03-01 and PCIN-2015-220 financed by the Spanish Commission of Science and Technology and FEDER, 1560/2015; Herramientas de monitorización de la vegetación mediante modelización ecohidrológica en parques continentales financed by the Red de Parques Nacionales; IM-DROFLOOD financed by Water Works 2014; a co-funded call of the European Commission and INDECIS, which is part of ERA4CS; and an ERA-NET initiated by JPI Climate, funded by MINECO with co-funding by the European Union (grant 690462). Marina Peña-Gallardo was funded by the Spanish Ministry of Economy and Competitiveness, Miquel Tomas-Burguera was supported by a doctoral grant by the Spanish Ministry of Education, Culture and Sport, and Ahmed El Kenawy was supported by a postdoctoral Juan de la Cierva contract. ; Peer reviewed

33 Pags.- 1 Tabl.- 16 Figs. Creative Commons Attribution License. ; We analyzed potential land degradation processes in semiarid regions worldwide using long time series of remote sensing images and the Normalized Difference Vegetation Index (NDVI) for the period 1981 to 2011. The objectives of the study were to identify semiarid regions showing a marked decrease in potential vegetation activity, indicative of the occurrence of land degradation processes, and to assess the possible influence of the observed drought trends quantified using the Standardized Precipitation Evapotranspiration Index (SPEI). We found that the NDVI values recorded during the period of maximum vegetation activity (NDVImax) predominantly showed a positive evolution in the majority of the semiarid regions assessed, but NDVImax was highly correlated with drought variability, and the trends of drought events influenced trends in NDVImax at the global scale. The semiarid regions that showed most increase in NDVImax (the Sahel, northern Australia, South Africa) were characterized by a clear positive trend in the SPEI values, indicative of conditions of greater humidity and lesser drought conditions. While changes in drought severity may be an important driver of NDVI trends and land degradation processes in semiarid regions worldwide, drought did not apparently explain some of the observed changes in NDVImax. This reflects the complexity of vegetation activity processes in the world's semiarid regions, and the difficulty of defining a universal response to drought in these regions, where a number of factors (natural and anthropogenic) may also affect on land degradation. ; This work was supported by the research projects CGL2011-27574-CO2-02, CGL2011-24185, CGL2014-52135-C03-01 and Red de variabilidad y cambio climático RECLIM (CGL2014-517221-REDT) financed by the Spanish Commission of Science and Technology and FEDER and "LIFE12 ENV/ES/000536-Demonstration and validation of innovative methodology for regional climate change adaptation in the Mediterranean area (LIFE MEDACC)" financed by the LIFE programme of the European Commission. Cesar Azorin-Molina was supported by the JCI-2011-10263 grant. Miquel Tomas-Burguera was supported by a doctoral grant by the Ministry of Economy and Competitiveness and Natalia Martin-Hernandez was supported by a doctoral grant by the Aragón Regional Government. We acknowledge support by the CSIC Open Access Publication Initiative through its Unit of Information Resources for Research (URICI). ; Peer reviewed

Abstract. Assessing the probability of occurrence of drought is important for improving current drought assessment, management and mitigation measures, and strategies across Spain. This study employed two well-established drought indices, the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI), to characterize drought duration and magnitude at different timescales over Spain. In order to map the drought hazard probability, we applied the extreme value theory and tested different thresholds to generate peak-over-threshold (POT) drought duration and magnitude series. Our results demonstrate that the generalized Pareto (GP) distribution performs well in estimating the frequencies of drought magnitude and duration. Specifically, we found a good agreement between the observed and modelled data when using upper percentiles to generate the POT series. Spatially, our estimations suggest a higher probability of extreme drought events in southern and central Spain compared to the northern and eastern regions. Also, our study found spatial differences in drought probability estimations as a function of the selected drought index (i.e. SPI vs. SPEI) and timescale (i.e. 1, 3, 6, and 12 months). Drought hazard probability maps can contribute to the better management of different sectors (e.g. agriculture, water resources management, urban water supply, and tourism) at national, regional, and even local scale in Spain.

We mapped – for the first time – the probability of occurrence of drought over Spain, with the overriding aim of improving current drought assessment, management and mitigation measures and strategies across the region. We employed two well-established drought indices: the Standardized Precipitation Index (SPI) and the Standardized Precipitation Evapotranspiration Index (SPEI). Drought characteristics (i.e. duration and severity) were characterised at 1-, 3-, 6- and 12-month, implying that drought event is attained only when the index values are lower than zero. We applied the extreme value theory to map drought hazard probability. Following this procedure, we tested different thresholds to generate the peak-over-threshold drought severity and magnitude series, besides evaluating different three-parametric distributions and thresholds to fit these series. Our results demonstrate that the Generalized Pareto distribution performs well in estimating the frequencies of drought magnitude and duration, with good agreement between the observed and modelled data when using upper percentiles to generate the peak-over-threshold series. Spatially, our estimations suggest a higher probability of extreme drought events in southern and central areas of Spain, compared to northern and eastern regions. Nevertheless, there are strong differences in drought probability estimations between drought indices (i.e. SPI and SPEI), as well as among drought timescales. ; This work was supported by the research projects CGL2014-52135-C03-01 and PCIN-2015-220 financed by the Spanish Commission of Science and Technology and FEDER, 1560/2015: Herramientas de monitorización de la vegetación mediante modelización ecohidrológica en parques continentales financed by the Red de Parques Nacionales, IMDROFLOOD financed by the Water Works 2014 co-funded call of the European Commission and INDECIS, which is part of ERA4CS, an ERA-NET initiated by JPI Climate, and funded by MINECO with co-funding by the European Union 30 (Grant 690462).

18 Pags.- 8 Tabls.- 9 Figs. ; We analysed recent evolution and meteorological drivers of the atmospheric evaporative demand (AED) in the Canary Islands for the period 1961–2013.We employed long and high-quality time series of meteorological variables to analyse current AED changes in this region and found that AED has increased during the investigated period. Overall, the annual ETo, which was estimated by means of the FAO- 56 Penman–Monteith equation, increased significantly by 18.2mmdecade1 on average, with a stronger trend in summer (6.7mmdecade1). In this study we analysed the contribution of (i) the aerodynamic (related to the water vapour that a parcel of air can store) and (ii) radiative (related to the available energy to evaporate a quantity of water) components to the decadal variability and trends of ETo. More than 90% of the observed ETo variability at the seasonal and annual scales can be associated with the variability in the aerodynamic component. The variable that recorded more significant changes in the Canary Islands was relative humidity, and among the different meteorological factors used to calculate ETo, relative humidity was the main driver of the observed ETo trends. The observed trend could have negative consequences in a number of water-depending sectors if it continues in the future. ; This work was supported by the research projects PCIN-2015-220, CGL2014-52135-C03-01, and Red de variabilidad y cambio climático RECLIM (CGL2014-517221-REDT), financed by the Spanish Commission of Science and Technology and FEDER; "IMDROFLOOD-Improved Drought and Flood Early Warning, Forecasting and Mitigation using real-time hydroclimatic indicators", supported through the Water Joint programme Initiative "Water Challenges for a Changing World"; and Water Works 2014 Cofunded call and "LIFE12 ENV/ES/000536-Demonstration and validation of innovative methodology for regional climate change adaptation in the Mediterranean area (LIFE MEDACC)", financed by the LIFE programme of the European Commission. Cesar Azorin-Molina (JCI-2011-10263), Arturo Sanchez-Lorenzo (JCI-2012-12508) and Marina Peña-Gallardo received grants from the Spanish Ministry of Economy and Competitiveness; Natalia Martin-Hernandez was supported by a doctoral grant by the Aragón Regional Government; and Miquel Tomas-Burguera was supported by a doctoral grant by the Ministerio de Educación, Cultura y Deporte. ; Peer reviewed

23 Pags.- 13 Figs. © Author(s) 2018. This work is distributed under the Creative Commons Attribution 3.0 License. ; We analyzed changes in surface relative humidity (RH) at the global scale from 1979 to 2014 using both observations and the ERA-Interim dataset.We compared the variability and trends in RH with those of land evapotranspiration and ocean evaporation in moisture source areas across a range of selected regions worldwide. The sources of moisture for each particular region were identified by integrating different observational data and model outputs into a Lagrangian approach. The aim was to account for the possible role of changes in air temperature over land, in comparison to sea surface temperature (SST), but also the role of land evapotranspiration and the ocean evaporation on RH variability. The results demonstrate that the patterns of the observed trends in RH at the global scale cannot be linked to a particular individual physical mechanism. Our results also stress that the different hypotheses that may explain the decrease in RH under a global warming scenario could act together to explain recent RH trends. Albeit with uncertainty in establishing a direct causality between RH trends and the different empirical moisture sources, we found that the observed decrease in RH in some regions can be linked to lower water supply from land evapotranspiration. In contrast, the empirical relationships also suggest that RH trends in other target regions are mainly explained by the dynamic and thermodynamic mechanisms related to the moisture supply from the oceanic source regions. Overall, while this work gives insights into the connections between RH trends and oceanic and continental processes at the global scale, further investigation is still desired to assess the contribution of both dynamic and thermodynamic factors to the evolution of RH over continental regions. ; This work was supported by the EPhysLab (UVIGO-CSIC Associated Unit), PCIN-2015-220, CGL2014-52135-C03-01, and CGL2014-60849-JIN financed by the Spanish Commission of Science and Technology, and FEDER, IMDROFLOOD financed by the Water Works 2014 co-funded call of the European Commission and INDECIS, which is part of ERA4CS, an ERA-NET initiated by JPI Climate. This study was also funded by FORMAS (SE), DLR (DE), BMWFW (AT), IFD (DK), MINECO (ES), and ANR (FR) with co-funding from the European Union (grant 690462). ; Peer reviewed