Suchergebnisse

Changes in streamflow timing are studied in 27 mountain rivers in Spain, in the context of climate warming. The studied rivers are characterized by a highflows period in spring due to snowmelt, although differences in the role of snow and consequently in the timing of flows are observed amongst cases. We calculated for every year of the studied period (1976-2008) various hydrological indices that enable locating the timing of spring flows within the annual hydrologic regime, including the day of 75% of mass, and the day of spring maximum. The evolution of these indices was compared with that of seasonal precipitation and temperature, and trends in time were calculated. Results show a general negative trend in the studied indices which indicates that spring peaks due to snowmelt are shifting earlier within the hydrological year. Spring temperatures, which show a significant increasing trend, are the main co-variable responsible for the observed changes in the streamflow timing. In a second set of analyses we performed hydrological simulations with the SWAT model, in order to estimate changes in streamflow timing under projected warming temperatures. Projections show further shifting of spring peak flows along with a more pronounced low water level period in the summer. The simulations also allowed quantifying the role of snowfall-snowmelt on the observed changes in streamflow. © 2014 Elsevier B.V. ; This study was possible thanks to financial support from the Spanish Government (Ministry of Education) through the postdoctoral program "Ayudas de movilidad postdoctoral en centros extranjeros (Orden EDU/2728 /2011, de 29 de Septiembre)". It has been as well supported by the research projects: "Hidrología nival en el Pirineo Central Español: Variabilidad espacial, importancia hidrológica y respuesta a la variabilidad y cambio climático (CGL2011-27536/HID, Hidronieve)", financed by the Spanish Commission of Science and Technology and FEDER; and CTTP1/12 "Creación de un modelo de alta resolución espacial para cuantificar la esquiabilidad y la afluencia turística en el Pirineo bajo distintos escenarios de cambio climático", financed by the Comunidad de Trabajo de los Pirineos. We acknowledge the two anonymous reviewers for the constructive comments that helped improving the final version of the manuscript. ; Peer Reviewed

[EN] In this study we analyzed the spatiotemporal variability of streamflow droughts in the Iberian Peninsula from 1945 to 2005. Streamflow series belonging to 187 homogeneously distributed gauging stations across the study area were used to develop a standardized streamflow index (SSI), which facilitated comparison among regimes and basins, regardless of streamflow magnitudes. A principal component analysis was performed to identify homogeneous hydrological regions having common features based on the temporal evolution of streamflows. Identification of drought events was carried out using a threshold level approach. We assessed the duration and magnitude of drought episodes and the changes that occurred between two contrasting periods for each hydrological region. The results showed a trend toward increased drought severity in the majority of regions. Drought duration, magnitude and spatial coverage were found to depend mainly on climatic conditions and the water storage strategies in each basin. In some basins these strategies have altered river regimes, and in others created a high level of dependence on storage and water transfer rates. ; We would like to thank the Centro de Estudios Hidrograficos (CEDEX), Sistema Nacional de Informac¸ ´ aoˆ de Recursos H´ıdricos (SNIRH), Agencia Catalana de l'Aigua, ` Agencia Andaluza del Agua and Augas de Galicia for providing the hydrological data used in this study. This work has been supported by a pre-doctoral fellowship by the Aragon Government (Spain), ´ and by the research projects CGL2006-11619/HID, CGL2008- 01189/BTE, CGL2011-27574-C02-02 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, and "La nieve en el Pirineo Aragones y su respuesta a la variabilidad clim ´ atica" 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. ; Peer Reviewed

In this study we assessed the impact of meteorological droughts on streamflow droughts in 187 river basins of the Iberian Peninsula. For each basin we correlated the 1 mo to 48 mo standardized precipitation index (SPI) with the standardized streamflow index to assess the response time of streamflow droughts to the SPI, and to determine the best climatic time scale over which to monitor hydrological droughts. In addition, we analyzed the effects of dam construction and reservoir use on changes in the response of hydrological droughts. The analysis revealed 2 main types of hydrological drought response to meteorological drought: (1) a peak response pattern involving enhanced hydrological sensitivity at short SPI time scales, which largely characterized basins in the northern sector of the Iberian Peninsula, and (2) a sustained response pattern involving greater sensitivity to meteorological droughts at much longer time scales, which primarily characterized the central and southern basins of the Iberian Peninsula. Water management has significantly altered the natural hydrological response to meteorological droughts, delaying the impacts of precipitation deficits over long time scales, especially downstream of reservoirs used for irrigation purposes. © Inter-Research 2013. ; We thank the Centro de Estudios Hidrográficos (CEDEX), Sistema Nacional de Informaçâo de Recursos Hídricos (SNIRH), Agència Catalana de l'Aigua, Agencia Andaluza del Agua and Augas de Galicia for providing the hydrological data used in this study. This work has been supported by a pre-doctoral fellowship by the Aragón Government (Spain) and by the research projects CGL2006-11619/HID, CGL2008-01189/BTE, CGL2011- 27574-C02-02 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 Program of the European Commission, and 'La nieve en el Pirineo Aragonés y su respuesta a la variabilidad climática' and '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. ; Peer Reviewed

12 páginas, 9 figuras. ; In this study the climatic and hydrological trends across 88 sub-basins of the Ebro River basin were analyzed for the period 1950–2006. A new database of climate information and river flows for the entire basin facilitated a spatially distributed assessment of climate-runoff relationships. It constitutes the first assessment of water yield evolution across the whole Ebro basin, a very representative example of large Mediterranean rivers. The results revealed a marked decrease in river discharges in most of the sub-basins. Moreover, a number of changes in the seasonality of the river regime was found, resulting from dam regulation and a decrease in snowpack in the headwaters. Significant and positive trends in temperature were observed across most of the basin, whereas most of the precipitation series showed negative coefficients, although the decrease in magnitude was low. The time evolution of the residuals from empirical models that relate climate and runoff in each sub-basin provided evidence that climate alone does not explain the observed decrease in river discharge. Thus, changes in water yield are associated with an increase in evapotranspiration rates in natural vegetation, growth of which has expanded as a consequence of land abandonment in areas where agricultural activities and livestock pressure have decreased. In the lowlands of the basin the decrease in water yield has been exacerbated by increased water consumption for domestic, industrial and agricultural uses. Climate projections for the end of the 21st century suggest a reduced capacity for runoff generation because of increasing temperature and less precipitation. Thus, the maintenance of water supply under conditions of increasing demand presents a challenging issue requiring appropriate coordination amongst politicians and managers. ; This work has been supported by the research projects CGL2006-11619/HID, CGL2008-01189/BTE, and CGL2008-05112-C02-01/CLI financed by the Spanish Commission of Science and Technology and FEDER, EUROGEOSS (FP7-ENV-2008-1-226487), 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 y su respuesta a la variabilidad climática" financed by "Obra Social La Caixa" and the Aragón Government. ; Peer reviewed

17 páginas, 7 figuras ; [EN] In this study we analyzed the influence of the North Atlantic Oscillation (NAO) on the streamflow in 187 sub-basins of the Iberian Peninsula. Monthly and one-month lagged correlations were conducted to assess the spatio-temporal extent of the NAO influence on Iberian river discharges. Analysis of the persistence of the winter NAO throughout the year was also undertaken, together with analysis of streamflow anomalies during positive and negative NAO phases. Moving-window correlation analyses were conducted to assess potential changes in the temporal evolution of the NAO influence on Iberian streamflows. The results show that the NAO has a large impact on surface water resources throughout the Iberian Peninsula during winter, and in the Atlantic watershed during autumn. We showed that water resources management and snowmelt are causing the persistent dependence of streamflows on the previous winter NAO. We found that strongly positive streamflow anomalies occurred during winter, especially in the Atlantic watershed, and provide evidence of non-stationarity and spatial variability in the NAO influence on Iberian water resources. ; We would like to thank the Centro de Estudios Hidrográficos (CEDEX), Sistema Nacional de Información de Recursos Hídricos (SNIRH), Agencia Catalana de l'Aigua, Agencia Andaluza del Agua and Augas de Galicia for providing the hydrological data used in this study. We also want to thank to the Climate Research Unit for the North Atlantic Oscillation data. This work has been supported by a pre-doctoral fellowship by the Aragón Government (Spain), and by the research projects CGL2006-11619/HID and CGL2008-01189/BTE, financed by the Spanish Commission of Science and Technology and FEDER, EUROGEOSS (FP7-ENV-2008-1-226487), ACQWA (FP7-ENV- 2007-1-212250) financed by the VII Framework Programme of the European Commission, and "La nieve en el Pirineo Aragonés y su respuesta a la variabilidad climática" financed by "Obra Social La Caixa" and the Aragón Government. ; Peer reviewed

44 Pags., 4 Tabls., 12 Figs. The definitive version is available at: http://ascelibrary.org/heo/ ; In this study, the authors investigated an approach to calculate the standardized streamflow index (SSI), which allows accurate spatial and temporal comparison of the hydrological conditions of a stream or set of streams. For this purpose, the capability of six three-parameter distributions (lognormal, Pearson Type III, log-logistic, general extreme value, generalized Pareto, and Weibull) and two different approaches to select the most suitable distribution the best monthly fit (BMF) and the minimum orthogonal distance (MD), were tested by using a monthly streamflow data set for the Ebro Basin (Spain). This large Mediterranean basin is characterized by high variability in the magnitude of streamflows and in seasonal regimes. The results show that the most commonly used probability distributions for flow frequency analysis provided good fits to the streamflow series. Thus, the visual inspection of the L-moment diagrams and the results of the Kolmogorov-Smirnov test did not enable the selection of a single optimum probability distribution. However, no single probability distribution for all the series was suitable for obtaining a robust standardized streamflow series because each of the distributions had one or more limitations. The BMF and MD approaches improved the results in the expected average, standard deviation, and the frequencies of extreme events of the SSI series in relation to the selection of a unique distribution for each station. The BMF and MD approaches involved using different probability distributions for each gauging station and month of the year to calculate the SSI. Both approaches are easy to apply and they provide very similar results in the quality of the obtained hydrological drought indexes. The proposed procedures are very flexible for analyses involving contrasting hydrological regimes and flow characteristics. ; This work has been supported by research projects grant numbers CGL2008-01189/BTE and CGL2006-11619/HID financed by the Spanish Commission of Science and Technology and FEDER; EUROGEOSS grant number (FP7-ENV-2008-1-226487), and ACQWA grant number (FP7-ENV-2007-1-212250) financed by the VII Framework Programme of the European Commission; and "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" financed by "Obra Social La Caixa" and the Aragón government. ; Peer reviewed

41 Pag., 14 Fig. The definitive version is available at: http://www.sciencedirect.com/science/journal/00221694 ; The influence of climate variation on the availability of water resources was analyzed in the headwaters of the Tagus River basin using two drought indices, the standardized precipitation index (SPI) and the standardized precipitation evapotranspiration index (SPEI). This basin is highly regulated and strategic, and contains two hyperannual reservoirs that are the origin of the water supply system for Mediterranean areas of southeast Spain. The indices confirmed that drought conditions have prevailed in the headwaters of the Tagus River since the 1970s. The responses in river discharge and reservoir storage were slightly higher when based on the SPEI rather than the SPI, which indicates that although precipitation had a major role in explaining temporal variability in the analyzed parameters, the influence of temperature was not negligible. Moreover, the greatest response in hydrological variables was evident over longer timescales of the climatic drought indices. Although the effect of climate variability on water resources was substantial during the analyzed period, we also showed a major change in hydrological–climatic relationships in regulated systems including reservoir storage and outflow. These were closely related to changes in external demand following commencement of the water transfer system to the Júcar and Segura basins after the 1980s. The marked reduction in water availability in the basin, which is related to more frequent droughts, contrasts with the amount of water transferred, which shows a clear upward trend associated with increasing water demand in the Mediterranean basin. ; This work has been supported by the research projects CGL2006-11619/HID, CGL2008-01189/BTE, and CGL2008-1083/CLI 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, STRIVER (Strategy and methodology for Improved IWRM—An integrated interdisciplinary assessment in four twinning river basins), financed by the VI Framework Programme of the European Commission. "Las sequías climáticas en la cuenca del Ebro y su respuesta hidrológica" Financed by "Obra Social La Caixa" and the Aragón Government and "Programa de grupos de investigación consolidados" financed by the Aragón Government. ; Peer reviewed

Abstract. An extraordinary convective rainfall event, unforeseen by most numerical weather prediction models, generated a devastating flash flood (305 m3 s−1) in the town of Sant Llorenç des Cardassar, Mallorca, on 9 October 2018. Four people died inside this village, while casualties were up to 13 over the entire affected area. This extreme event has been reconstructed by implementing an integrated flash flood modelling approach in the Ses Planes catchment up to Sant Llorenç (23.4 km2), based on three components: (i) generation of radar-derived precipitation estimates, (ii) modelling of accurate discharge hydrographs yielded by the catchment (using FEST and KLEM models), and (iii) hydraulic simulation of the event and mapping of affected areas (using HEC-RAS). Radar-derived rainfall estimates show very high agreement with rain gauge data (R2=0.98). Modelled flooding extent is in close agreement with the observed extension by the Copernicus Emergency Management Service, based on Sentinel-1 imagery, and both far exceed the extension for a 500-year return period flood. Hydraulic simulation revealed that water reached a depth of 3 m at some points, and modelled water depths highly correlate (R2=0.91) with in situ after-event measurements. The 9 October flash flood eroded and transported woody and abundant sediment debris, changing channel geomorphology. Water velocity greatly increased at bridge locations crossing the river channel, especially at those closer to the Sant Llorenç town centre. This study highlights how the very low predictability of this type of extreme convective rainfall events and the very short hydrological response times typical of small Mediterranean catchments continue to challenge the implementation of early warning systems, which effectively reduce people's exposure to flash flood risk in the region.

42 Pag., 13 Fig. The definitive version is available at: http://www.agu.org/journals/jd/ ; In this study we analyzed the influence of the El Niño‐Southern Oscillation (ENSO) phenomenon on drought severity at the global scale. A unique aspect of the analysis is that the ENSO influence was quantified using a multiscalar drought indicator, which allowed assessment of the role of the ENSO phases on drought types affecting various hydrological, agricultural and environmental systems. The study was based on ENSO composites corresponding to El Niño and La Niña phases, which were obtained from the winter El Niño 3.4 index for the period 1901–2006. Drought was identified in a multiscalar way using the Standardized Precipitation Evapotranspiration Index (SPEI) and the global SPEIbase data set. The study revealed the differing impacts of the El Niño and La Niña phases on drought severity, the time scales of droughts, and the period of the year when the ENSO phases explained drought variability worldwide. In large areas of America and eastern Europe the role of ENSO events were evident at the shortest time scales (1–3 months) at the beginning of events, but in areas of South Africa, Australia and Southeast Asia the effects were more obvious some months later, and at longer time scales. We also identified areas where severe drought conditions are associated with more than 70% of ENSO events. The persistence of the drought signal at longer time‐scales (e.g., 6‐ or 12‐months) is not directly determined by the atmospheric circulation response to the SST anomalies, since the SPEI anomalies will be caused by the cumulative dry conditions in some specific months. Knowledge of how these effects differ as a function of the El Niño and La Niña phases, and how they propagate throughout the drought time scales could aid in the prediction of the expected drought severity associated with the ENSO. Lags detected during the study may help forecasting of dry conditions in some regions up to one year before their occurrence. ; 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 climáticas" Financed by "Obra Social La Caixa" and the Aragón Government. ; Peer reviewed

24 Pag., 5 Fig. The definitive version is available at: http://www.int-res.com/abstracts/cr/ ; We analysed the evolution of drought on the NW Iberian Peninsula from 1930 to 2006, and identified differences between the effects of precipitation variability and warming processes on drought severity and surface water resources. Two drought indices were used, one based on precipitation (the standardised precipitation index, SPI) and the other based on water balance, as reflected by the difference between precipitation and potential evapotranspiration, PET (the standardised precipitation evapotranspiration index, SPEI). The results show that precipitation has increased in the region, but a significant increase has also occurred in PET. No statistically significant differences were found over the study period for the severity of drought, as assessed by both the SPI and SPEI. In addition, although in the last 3 decades the mean duration of drought episodes has increased by approximately 1 mo as a consequence of the increase of the PET rates, the differences are not statistically significant. River discharge is mainly driven by precipitation variability, whereas warming processes did not have a noticeable influence on the streamflow variability between 1930 and 2006. The implications of global warming projected by global climate models on future drought severity and the availability of water resources are highly relevant in the near future. ; This study has been supported by the research projects CGL2008-01189/BTE and CGL2006-11619/ HID, financed by the Spanish Commission of Science and Technology; 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 hidro lógica' and 'La nieve en el Pirineo aragonés: Distribución espacial y su respuesta a las condiciones climáticas', financed by 'Obra Social La Caixa' and the Aragón Government; and the project INCITE09 383 278 PR, financed by Xunta de Galicia. ; Peer reviewed

In this study we analyze the observed trends for the period 1950-2006 in a number of climate indices related to the occurrence of winter warm events in the Ebro basin, northeast Iberian Peninsula. Climatic simulations using 12 regional climate models (RCMs) from the ENSEMBLES database enable calculation of the multi-model means for the projected evolution of these climatic indices for the time periods 2021-2050 and 2051-2080. The results based on observations indicate a significant increase in occurrence of warm and very warm days and nights, melting events at 2000m a.s.l. and the duration of warm spells across most of the study area. The number of warm spells comprising at least five consecutive warm days or nights also tends to increase, although the trend is not statistically significant for many of the observatories involved in the study. The RCMs project that the trends observed during the observation period will continue, and that the occurrence of warm day and night events and spells are very likely to increase during this century. In some cases the occurrence of warm events is projected to double during the period 2021-2050, and continue increasing for the period 2051-2080. For both the observed and projected periods, most of the indices show a greater increase in the occurrence of these events in the mountain areas of the basin (the Pyrenees and the Iberian mountains). © 2013 Royal Meteorological Society. ; We thank the Agencia Estatal de Meteorología for providing the temperature data used in this study. This work was supported by the research projects CGL2011-27574-CO2-02 and CGL2011-27536, financed by the Spanish Commission of Science and Technology and FEDER; and ACQWA (FP7-ENV-2007-1-212250), financed by the VII Framework Programme of the European Commission; Efecto de los escenarios de cambio climatico sobre la hidrología superficial y la gestión de embalses del PirineoAragonés, financed by Obra Social La Caixa and the Aragón Government and Influencia del cambioclimático en el turismo de nieve CTTP01/10, financed by the Comunidad de Trabajo de los Pirineos. ; Peer Reviewed

15 Pags.- 9 Figs.- 2 Tabls. ; We analyzed the relationships between meteorological drought and hydrological drought using very dense and diverse network of gauged natural drainage basins across the conterminous U.S. Specifically, this work utilized a dataset of 289 gauging stations, covering the period 1940–2013. Drainage basins were obtained for each gauging station using a digital terrain model. In addition to meteorological data (e.g., precipitation, air temperature and the atmospheric evaporative demand), we obtained a number of topographic, soil and remote sensing variables for each defined drainage basin. A hydrological drought index (the Standardized Streamflow Index; SSI) was computed for each basin and linked to the Standardized Precipitation Evapotranspiration Index (SPEI), which was used as a metric of climatic drought severity. The relationships between different SPEI time-scales and their corresponding SSI were assessed by means of a Pearson correlation coefficient. Also, the general patterns of response of hydrological droughts to climatic droughts were identified using a principal component analysis. Overall, results demonstrate a positive response of SSI to SPEI at shorter time-scales, with strong seasonality and clear spatial differences. We also assessed the role of some climatic and environmental factors in explaining these different responses using a predictive discriminant analysis. Results indicate that elevation and vegetation coverage are the main drivers of the diverse response of SSI to SPEI time-scales. Similar analyses were made for three sub-periods (1940–1964, 1965–1989 and 1989–2013), whose results confirm considerable differences in the response of SSI to SPEI over the past eighty years. ; This work was supported by the research project I-Link1001 (Validation of climate drought indices for multi-sectorial applications in North America and Europe under a global warming scenario) financed by CSIC, PCIN-2015-220 and CGL2014-52135-C03-01 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, and funded by FORMAS (SE), DLR (DE), BMWFW (AT), IFD (DK), MINECO (ES), ANR (FR) with co-funding by the European Union (Grant 690462). Marina Peña-Gallardo was granted 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. Jamie Hannaford was supported by the Belmont Forum project 'DrIVER', NERC Grant Number (grant NE/L010038/1). Marco Maneta acknowledges support from the USDA NIFA grant 2016-67026-25067. ; Peer reviewed

40 Pags., 11 Figs., 3 Tabls. The definitive version is available at: http://journals.ametsoc.org/loi/eint ; In this study, the authors provide a global assessment of the performance of different drought indices for monitoring drought impacts on several hydrological, agricultural, and ecological response variables. For this purpose, they compare the performance of several drought indices [the standardized precipitation index (SPI); four versions of the Palmer drought severity index (PDSI); and the standardized precipitation evapotranspiration index (SPEI)] to predict changes in streamflow, soil moisture, forest growth, and crop yield. The authors found a superior capability of the SPEI and the SPI drought indices, which are calculated on different time scales than the Palmer indices to capture the drought impacts on the aforementioned hydrological, agricultural, and ecological variables. They detected small differences in the comparative performance of the SPI and the SPEI indices, but the SPEI was the drought index that best captured the responses of the assessed variables to drought in summer, the season in which more drought-related impacts are recorded and in which drought monitoring is critical. Hence, the SPEI shows improved capability to identify drought impacts as compared with the SPI. In conclusion, it seems reasonable to recommend the use of the SPEI if the responses of the variables of interest to drought are not known a priori. ; This work has been supported by the research projects CGL2008-01189/BTE, CGL2011-27574-CO2-02, CGL2011–24185, CGL2011-26654 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. JJC acknowledges the support of ARAID. ; Peer reviewed

15 Pag., 3 Tabl., 10 Fig. ; This work provides a first assessment of the outstanding characteristics of the anomalous precipitation occurrence in the winter of 2010 over the Iberian Peninsula, as well as on the associated atmospheric driving mechanisms. Large areas of Iberia, those located in the western and southern sectors, registered a new historical maximum in winter precipitation values. Simultaneously, the most extreme, negative North Atlantic Oscillation (NAO) index for winter was recorded in 2010. The anomalous pressure gradient in the North Atlantic region steered a large number of low pressure systems via an unusually southern path, directly influencing Iberia and northern Africa. Storms were frequent, and a high number of days occurred with weather types prone to cause precipitation. In addition, the most extreme daily precipitation episodes were recorded during the period with the strongest negative NAO index. Global climate models for the entire 21st century show that strong negative NAO winters, similar to that which occurred in 2010, may be expected in the future. ; This work was supported by the research projects CGL2008-01189/BTE and CGL2006-11619/ HID, financed by the Spanish Commission of Science and Technology and FEDER; by EUROGEOSS (FP7-ENV-2008-1- 226487) and ACQWA (FP7-ENV-2007-1-212250), financed by the VII Framework Programme of the European Commission; and 'La nieve en el Pirineo aragonés: distribución espacial y su respuesta a las condiciones climáticas', financed by 'Obra Social La Caixa' and the Aragón Government. Ricardo Trigo's work was partially supported by the FCT (Portugal) through project ENAC (PTDC/AAC-CLI/103567/2008). ; Peer reviewed

18 Pags., 7 Figs. The definitive version is available at: http://www.springerlink.com/content/978-94-007-1371-0/ ; This chapter shows the influence of the North Atlantic Oscillation (NAO) on droughts in the entire Mediterranean region between 1901 and 2006. The analysis has been based on identification of positive and negative NAO winters and also detection of the anomalies of drought severity by means of the Standardized Precipitation Evapotranspiration Index (SPEI). The analysis is focussed on the winter NAO. Nevertheless, given that the SPEI drought indicator can be obtained at different time-scales, the study shows how the effects of the winter NAO on droughts are propagated for the following months when long time scales are considered. In general, during the positive phases, the negative SPEI averages are recorded in Southern Europe (the Iberian Peninsula, Italy and the Balkans), areas of Turkey and northwest Africa. On the contrary, the SPEI averages are found positive in northeast Africa. The opposite configuration, but with some differences in the spatial patterns and the magnitude of the SPEI averages, is found during the negative NAO years. The findings of this study should be of great applicability in terms of developing early warning systems. The established relationships between NAO phases and drought indices seem appropriate for drought prediction over large areas of the Mediterranean basin. ; 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, and "La nieve en el Pirineo aragonés: Distribución espacial y su respuesta a las condiciones climáticas" Financed by "Obra Social La Caixa" and the Aragón Government. ; Peer reviewed