We evaluated the response of the Earth land biomes to drought by correlating a drought index with three global indicators of vegetation activity and growth: vegetation indices from satellite imagery, treering growth series, and Aboveground Net Primary Production (ANPP) records. Arid and humid biomes are both affected by drought, and we suggest that the persistence of the water deficit (i.e., the drought timescale) could be playing a key role in determining the sensitivity of land biomes to drought. We found that arid biomes respond to drought at short time-scales; that is, there is a rapid vegetation reaction as soon as water deficits below normal conditions occur. This may be due to the fact that plant species of arid regions have mechanisms allowing them to rapidly adapt to changing water availability. Humid biomes also respond to drought at short time-scales, but in this case the physiological mechanisms likely differ from those operating in arid biomes, as plants usually have a poor adaptability to water shortage. On the contrary, semiarid and subhumid biomes respond to drought at long timescales, probably because plants are able to withstand water deficits, but they lack the rapid response of arid biomes to drought. These results are consistent among three vegetation parameters analyzed and across different land biomes, showing that the response of vegetation to drought depends on characteristic drought time-scales for each biome. Understanding the dominant time-scales at which drought most influences vegetation might help assessing the resistance and resilience of vegetation and improving our knowledge of vegetation vulnerability to climate change. drought impacts | NDVI | drought adaptation | Standardized Precipitation Evapotranspiration Index | drought index

Abstract:In this study we 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, we 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. We 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. We 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 index shows improved capability to identify drought impacts as compared with the SPI one. 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.

We use high quality climate data from ground meteorological stations in the Iberian Peninsula (IP) and robust drought indices to confirm that drought severity has increased in the past five decades, as a consequence of greater atmospheric evaporative demand resulting from temperature rise. Increased drought severity is independent of the model used to quantify the reference evapotranspiration. We have also focused on drought impacts to drought-sensitive systems, such as river discharge, by analyzing streamflow data for 287 rivers in the IP, and found that hydrological drought frequency and severity have also increased in the past five decades in natural, regulated and highly regulated basins. Recent positive trend in the atmospheric water demand has had a direct influence on the temporal evolution of streamflows, clearly identified during the warm season, in which higher evapotranspiration rates are recorded. This pattern of increase in evaporative demand and greater drought severity is probably applicable to other semiarid regions of the world, including other Mediterranean areas, the Sahel, southern Australia and South Africa, and can be expected to increasingly compromise water supplies and cause political, social and economic tensions among regions in the near future.

Near-surface wind speed trends recorded at 67 land-based stations across Spain and Portugal for 1961-2011, also focusing on the 1979-2008 subperiod, were analyzed. Wind speed series were subjected to quality control, reconstruction, and homogenization using a novel procedure that incorporated the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale Model (MM5)-simulated series as reference. The resultant series show a slight downward trend for both 1961-2011 (20.016

Since the 1980s anthropogenic aerosols have been considerably reduced in Europe and the Mediterranean area. This decrease is often considered as the likely cause of the brightening effect observed over the same period. This phenomenon is however hardly reproduced by global and regional climate models. Here we use an original approach based on reanalysis-driven coupled regional climate system modeling to show that aerosol changes explain 81±16% of the brightening and 23±5% of the surface warming simulated for the period 1980-2012 over Europe. The direct aerosol effect is found to dominate in the magnitude of the simulated brightening. The comparison between regional simulations and homogenized ground-based observations reveals that observed surface solar radiation and land and sea surface temperature spatiotemporal variations over the Euro-Mediterranean region are only reproduced when simulations include the realistic aerosol variations. Key Points A regional climate system model over the Euro-Mediterranean includes aerosols Aerosol changes are needed to reproduce observed climate trends since 1980 Aerosols play an essential role in the brightening and warming since 1980This work is a contribution to the HyMeX (HYdrological cycle in the Mediterranean EXperiment) and ChArMEx (Chemistry-Aerosol Mediterranean Experiment) program through INSU-MISTRALS support and the Med-CORDEX initiative (COordinated Regional climate Downscaling EXperiment Mediterranean region, www.medcordex.eu). This research has been supported by the French National Research Agency (ANR) project REMEMBER (contract ANR-12-SENV-001). Gridded temperature data sets, GISS and CRUTEM, have been provided, respectively, by the NASA Goddard Institute for Space Studies and the Met Office Hadley Center. HISTALP temperature data sets have been downloaded from http://www.zamg.ac.at/histalp. We also thank Brigitte Dubuisson and Anne-Laure Gibelin for the availability of homogenized temperature series in France, and we acknowledge the data providers in the ECA&D project. A. S. L. was supported by the "Secretaria per a Universitats i Recerca del Departament d'Economia i Coneixement, de la Generalitat de Catalunya i del programa Cofund de les Accions Marie Curie del 7e Programa marc d'R+D de la Unio Europea" (2011 BP-B 00078), the postdoctoral fellowship JCI-2012-12508, and the project NUCLIERSOL (CGL2010-18546

[1] This study analyzes the spatial and temporal changes in sunshine duration (SunDu) and total cloud cover (TCC) over the Iberian Peninsula (IP) and four subregions during 1961-2004 using high-quality, homogenized data sets. The analyses confirm that over most of the IP and in most seasons, SunDu and TCC variations are strongly negatively correlated, with absolute values $0.8-0.9. Somewhat weaker correlations (0.5-0.6) are found in the southern portion of the IP in summer. A large discrepancy between the SunDu and TCC records occurs from the 1960s until the early 1980s when the SunDu series shows a decrease that it is not associated with an increase in TCC. This negative trend or ''dimming'' is even more pronounced after removing the effects of TCC via linear regression. Since the early 1980s, the SunDu and TCC residual SunDu series exhibit an upward trend or ''brightening.'' In addition to the long-term dimming and brightening, the volcanic eruptions of El Chichon and Mount Pinatubo are clearly evident in the TCC residual SunDu record. The TCC and SunDu records over the IP are well correlated with sea level pressure (SLP), with above normal TCC and below normal SunDu corresponding to below normal SLP locally in all seasons. The TCC and SunDu related SLP changes over the IP in winter and spring are part of a larger-scale north-south dipole pattern that extends over the entire Euro-Atlantic sector. Other more regional atmospheric circulation patterns, identified from rotated principal component analysis, are also linked to TCC and SunDu variations over the IP. Finally and perhaps surprisingly, the TCC residual SunDu series exhibits a statistically significant relationship with a regional atmospheric circulation pattern during spring, summer, and autumn.

Abstract. Substantial changes in anthropogenic aerosols and precursor gas emissions have occurred over recent decades due to the implementation of air pollution control legislation and economic growth. The response of atmospheric aerosols to these changes and the impact on climate are poorly constrained, particularly in studies using detailed aerosol chemistry-climate models. Here we compare the HadGEM3-UKCA (Hadley Centre Global Environment Model-United Kingdom Chemistry and Aerosols) coupled chemistry-climate model for the period 1960-2009 against extensive ground-based observations of sulfate aerosol mass (1978-2009), total suspended particle matter (SPM, 1978(SPM, -1998

Abstract. In this study we analyzed the spatial distribution, temporal variability and trends in 13 reference evapotranspiration (ET 0 ) in Spain from 1961 to 2011. Twelve methods were analyzed to 14 quantify ET 0 from quality controlled and homogeneous series of various meteorological variables 15 measured at 46 meteorological stations. Some of the models used are temperature based (e.g., 16Thornthwaite, Hargreaves, Linacre), whereas others are more complex and require more 17 meteorological variables for calculation (e.g., Priestley-Taylor, Papadakis, FAO-Blaney-Criddle). 18The Penman-Monteith equation was used as a reference to quantify ET 0 , and for comparison 19 amongst the other methods applied in the study. No major differences in the spatial distribution of 20 the average ET 0 was evident among the various methods. At annual and seasonal scales some of the 21 ET 0 methods requiring only temperature data for calculation provided better results than more 22