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In recent years a strong relationship has been found between Atmospheric Rivers (ARs) and extreme precipitation and floods across western Europe, with some regions having 8 of their top 10 annual maxima precipitation events related to ARs. In the case of the Iberian Peninsula, the association between ARs and extreme precipitation days has also been well documented, particularly for western Iberia river basins.Since ARs are often associated with high impact weather, it is important to study their medium-range predictability. Here we perform such an assessment using the ECMWF ensemble forecasts up to 15 days, for events that made landfall in western Iberian Peninsula during the winters spanning between 2012/2013 and 2015/16. IVT and precipitation from the 51 ensemble members of the ECMWF Integrated Forecasting System (IFS) ensemble (ENS) were processed over a domain including western Europe and contiguous North Atlantic Ocean.Metrics concerning the ARs location, intensity and orientation were computed, in order to compare the predictive skill (for different prediction lead times) of IVT and precipitation analyses in the IFS. We considered several regional boxes over Western Iberia, where the presence of ARs is detected in analysis/forecasts, enabling the construction of contingency tables and probabilistic evaluation for further objective verification of forecast accuracy. Our results indicate that the ENS forecasts have skill to detect upcoming ARs events, which can be particularly useful to improve the prediction of associated hydrometeorological extremes. We also characterized how the ENS dispersion and confidence curves change with increasing forecast lead times for each sub-domain. We employed the standard ROC analysis to evaluate the probabilistic component of these predictions showing that for short lead times precipitation forecasts are more accurate than IVT forecasts, while for longer lead times this result is reversed (~10 days). Furthermore, we show that this reversal occurs at shorter lead times in areas where the ARs contribution is more relevant for winter precipitation totals (e.g. northwestern Iberia). AcknowledgementsThe work done was supported by the project Landslide Early Warning soft technology prototype to improve community resilience and adaptation to environmental change (BeSafeSlide) funded by Fundação para a Ciência e a Tecnologia, Portugal (FCT, PTDC/GES-AMB/30052/2017). A.M.R. was also supported by the Scientific Employment Stimulus 2017 from FCT (CEECIND/00027/2017).

Long filaments of high integrated water vapor transport (IVT) content, widely known as atmospheric rivers (ARs), play a relevant role in the water cycle being also associated with many extreme flooding events worldwide. In this work, we inspect whether similar structures can be found over the western Mediterranean. The methodology used here to detect these AR-like structures is based on standard ARs detection methods but imposing a strong IVT advection westward component. The ERA5 global reanalysis and Spain02 high resolution gridded observational dataset are used, covering the period 1979-2017, to analyze the composites of mesoscale features and associated impacts on rainfall.Results show that AR-like structures over the Mediterranean (abbreviated here Med-ARs) have relatively low incidence with an approximately once-per-year frequency. Nevertheless, these rare events are usually associated with extreme precipitation, often amplified by orographic features, contributing to more than 40% to the annual precipitation in some cases (Lorente-Plazas et al., 2020). During a typical Med‐AR, the value of IVT increases significantly due to high horizontal winds and water vapor contents. Med-ARs are always associated to the placement of a cutoff cyclone with the cold core over northwestern Africa and warmer air mass over northern Europe. The vertical structure of Med-ARs suggests an occluded front with a low-level jet in the warmer front where Med‐ARs reside and, moisture penetrating into high atmospheric levels where cold and warm front intersect leading to severe convection. To sum up, long filaments of IVT can be found over the western Mediterranean Sea, traveling in an east-west direction, playing a relevant role in hydrometeorological impacts. Although these structures share some features with ARs over the Pacific/Atlantic Ocean they present so many specific characteristics that can be also considered to constitute a variant of this well-established meteorological phenomenon. AcknowledgmentsThe author would like to acknowledge the financial support by Fundação para a Ciência e Tecnologia (FCT) through project UIDB/50019/2020 – IDL. A. M. Ramos was supported by the Scientific Employment Stimulus 2017 from FCT (CEECIND/00027/2017). ReferencesLorente-Plazas, R., Montavez, J. P., Ramos, A. M., Jerez, S., Trigo, R. M., & Jimenez-Guerrero, P. (2019). Unusual Atmospheric-River-like structures coming from Africa induce extreme precipitation over western Mediterranean Sea. Journal of Geophysical Research: Atmospheres, 124. doi: 10.1029/2019JD031280

Moisture transport and Atmospheric Rivers (ARs) over the Northeastern Atlantic are a very relevant process for the inter-annual variability of precipitation over Western Europe. Based on a long-term transient simulation (850-2100CE) from the CESM model, we have showed that moisture transport towards Western Europe (using the vertically integrated horizontal water vapor transport, IVT) has been increasing significantly since pre-industrial period, in a clear association with the global warming trend. Both current and projected changes (using RCP 8.5) significantly exceed the range given by inter-annual to inter-decadal internal/external variability observed during the last millennium.We have checked the emergence of the temperature, IVT and precipitation signals in Iberia and the UK, showing that while the first two have now clearly emerged from the pre-warming state, precipitation series are still slightly below that threshold. Nevertheless, projections clearly show an increase in rainfall at higher latitudes (i in phase with a warmer and moister atmosphere); and a decrease at lower latitudes decoupled from the overall increase in moisture availability. Additionally we have explored the role played by large-scale circulation and atmospheric dynamics for these contrasting projections. Overall, results show that a poleward migration of moisture corridors and ARs explain a significant fraction of these projected trends. Based on the Clausius–Clapeyron relation we have separated the thermodynamical from dynamical changes. We also show how that a significant increase in subtropical anticyclonic activity over Iberia is responsible for: i) dynamical circulation changes; ii) a shortening of the wet season; iii) to less efficient precipitation regimes in the region. These results highlight the urge to adapt to a drying trend in Mediterranean-type climates, as a consequence of Global Warming. The financial support for this work was possible through the following FCT project: HOLMODRIVE - North Atlantic Atmospheric Patterns influence on Western Iberia Climate: From the Lateglacial to the Present [PTDC/CTA-GEO/29029/2017]

The occurrence of an increasing number of high impact storms over southwestern Europe (e.g. Klaus, 23-24 January 2009 and Xynthia, 27-28 February 2010; Liberato et al. 2011; 2013) has led to the meteorological services of France (Météo-France), Portugal (IPMA) and Spain (AEMET) to assign names to storms, since 1st December 2017. This new list of named storms has the main objective to better inform the general public and media while contributing to increasing public awareness to high impact storms and associated warnings and timely safety recommendations. The Institute of Meteorology of the Freie Universität Berlin has named all pressure systems in Central Europe since 1954; since 1998, lows are given male names and highs are given female names in odd years, and vice versa in even years. This new list built by the southwestern Europe meteorological services has the main difference of naming only high impact storms.In this study an analysis of the extreme storms affecting the Iberian Peninsula during the 2017-2019 extended winters is performed. From nine named high impact storms during the 2017-2018 season, seven of them affected the Iberian Peninsula region; and from the thirteen high impact storms during 2018-2019 winter, ten of them affected the region. Firstly an assessment of the strong winds, heavy precipitation and socio-economic impacts is presented. Secondly, a characterization of the synoptic conditions and associated extratropical cyclones is performed. Finally, the events are ranked and classified into the groups previously defined on Karremann et al. (2016) and a variability assessment is made in order to understand how their magnitude and frequency of occurrence fits the identified multi-decadal variability.AcknowledgementsThe authors would like to acknowledge the financial support by Fundação para a Ciência e a Tecnologia, Portugal (FCT), through projects PTDC/CTA-MET/29233/2017 and UIDB/50019/2020 – IDL. A.M. Ramos is supported by Scientific Employment Stimulus 2017 from FCT (CEECIND/00027/2017).ReferencesKarremann et al. (2016) Atmos. Sci. Let., 17: 354-361 DOI: 10.1002/asl.665Liberato et al. (2011) Weather, 66: 330-334 DOI: 10.1002/wea.755Liberato et al. (2013) Nat. Hazards Earth Syst. Sci., 13: 2239-2251 DOI: 10.5194/nhess-13-2239-2013

August 2018 saw the warmest Iberian heatwave since that of 2003. Recent climate change has exacerbated this event making it at least »1°C warmer than similar events since 1950–83.

This chapter explores the global and regional footprints of ARs, which are just beginning to be recognized. This chapter begins by highlighting the global climatology of ARs using metrics such as frequency, duration, seasonality, and the fraction of precipitation that can be attributed to ARs. It then highlights regional climatologies of ARs, the unique ways in which ARs are manifested across each of these regions, and some of the high-impact events associated with them.

Weather- and climate-related extreme events such as droughts, heatwaves and storms arise from interactions between complex sets of physical processes across multiple spatial and temporal scales, often overwhelming the capacity of natural and/or human systems to cope. In many cases, the greatest impacts arise through the ‘compounding’ effect of weather and climate-related drivers and/or hazards, where the scale of the impacts can be much greater than if any of the drivers or hazards occur in isolation; for instance, when a heavy precipitation falls on an already saturated soil causing a devastating flood. Compounding in this context refers to the amplification of an impact due to the occurrence of multiple drivers and/or hazards either because multiple hazards occur at the same time, previous climate conditions or weather events have increased a system’s vulnerability to a successive event, or spatially concurrent hazards lead to a regionally or globally integrated impact. More generally, compound weather and climate events refer to a combination of multiple climate drivers and/or hazards that contributes to societal or environmental risk.Although many climate-related disasters are caused by compound events, our ability to understand, analyse and project these events and interactions between their drivers is still in its infancy. Here we review the current state of knowledge on compound events and propose a typology to synthesize the available literature and guide future research. We organize the highly diverse event types broadly along four main themes, namely preconditioned, multivariate, temporally compounding, and spatially compounding events. We highlight promising analytical approaches tailored to the different event types, which will aid future research and pave the way to a coherent framework for compound event analysis. We further illustrate how human-induced climate change affects different aspects of compound events, such as their frequency and intensity through variations in the mean, variability, and the dependence between their climatic drivers. Finally, we discuss the emergence of new types of events that may become highly relevant in a warmer climate.

The assessment of sources of moisture is key to the understanding of the hydrological cycle at different time scales, because it enables the establishment of source-sink relationships and the identification of the main moisture transport conveyors and associated processes, the result of which is precipitation. Gimeno et al. (2012) provided a comprehensive review of the state-of-the-art in the assessment of moisture source-sinks and how different approaches can contribute to improving our knowledge of this component of the Earth’s Climate System. Since then, a variety of studies have focused on more specific aspects of the moisture budget and the source-sink distribution across the globe by integrating observations, satellite-derived products, physical tracers and numerical modelling. Here, we summarise the main advances in the field related to the impact of the moisture source-sink relationship on rainfall distribution, and add to the scientific debate on the question of the residence time of water vapour. We also revisit some of the recent advances in the role of the major mechanisms of moisture transport at a global scale, mainly Atmospheric Rivers and Low-Level Jet systems (Gimeno et al., 2016), in terms of their effects on precipitation extremes. Finally, we set out some of the main challenges for future research. Gobierno de España/[LAGRIMA (RTI2018-095772-B-I00)]//España Gobierno de España/[EVOCAR (CGL2015-65141-R)]//España Xunta de Galicia/[ED481B 2018/062]//España Xunta de Galicia/[ED481B 2018/069]//España Xunta de Galicia/[ED481B 2019/070]//España Ministerio de Asuntos Económicos y Transformación Digital/[CGL2015-65141-R]/MINECO/España Fundação para a Ciência e a Tecnologia/[PTDC/CTA-MET/29233/2017]/FCT/Portugal Fundação para a Ciência e a Tecnologia/[FCT, CEECIND/00027/2017]/FCT/Portugal National Science Foundation/[Award AGS 1454089]/NSF/Estados Unidos Xunta de Galicia/[ED431C2017/64-GRC]//España UCR::Vicerrectoría de Investigación::Unidades de Investigación::Ciencias Básicas::Centro de Investigaciones Geofísicas (CIGEFI) UCR::Vicerrectoría de Docencia::Ciencias Básicas::Facultad de Ciencias::Escuela de Física

This chapter discusses the intensity of the precipitation ARs often bring, and their unique combinations of durations, geometries, winds, and temperatures which make ARs important and unique pieces of the climatology and landscapes that affect both natural systems and human societies. Mountains profoundly affect AR conditions and processes, for instance, especially because ARs transport vast quantities of water vapor, which can result in huge amounts of precipitation. These vast quantities pose risks (flooding) and provide benefits (water resources) as well as affect aquatic ecosystems, terrestrial landscapes, surface winds, and coastal sea levels. Section 5.5 reviews the distinct regional patterns of AR effects across North America, Europe, South America, New Zealand, and the Arctic and Antarctica. Section 5.6 summarizes the key meteorological and land characteristics of ARs that most strongly determine the consequences of their landfalls and inland penetrations. Improvements in forecasting AR storms and conditions offer significant opportunities for improved ecosystem, resource, and disaster management across all sorts of landscapes and societal needs. But because ARs have their own physical and statistical properties relative to other storms and weather phenomena, recognition of their special roles in these systems will allow more targeted responses to both the risks and benefits of ARs.

A Lagrangian analysis is applied to identify the main moisture source areas associated with atmospheric rivers (ARs) making landfall along the west coast of South Africa during the extended austral winter months from 1980 to 2014. The results show that areas that provide the anomalous uptake of moisture can be categorized into four regions: (1) the South Atlantic Ocean between 10°S and 30°S, (2) a clear local maximum in the eastern South Atlantic, (3) a continental source of anomalous uptake to the north of the Western Cape, and (4) over South America at a distance of more than 7000 km from the target region. It emerges that the South American moisture source can be linked to a particular phase of the South American low-level jet, known as a no Chaco jet event (NCJE), which transports moisture to the western and central South Atlantic basin. Concisely, we provide strong evidence that the two margins of the South Atlantic Ocean appear connected by two meteorological structures, with the NCJE playing a key role of transporting moisture from South America to the western and central South Atlantic basin, feeding the AR that transports some of the moisture to the west coast of South Africa.

The explosive cyclogenesis of extratropical cyclones and the occurrence of atmospheric rivers are characteristic features of a baroclinic atmosphere, and are both closely related to extreme hydrometeorological events in the mid-latitudes, particularly on coastal areas on the western side of the continents. The potential role of atmospheric rivers in the explosive cyclone deepening has been previously analysed for selected case studies, but a general assessment from the climatological perspective is still missing. Using ERA-Interim reanalysis data for 1979–2011, we analyse the concurrence of atmospheric rivers and explosive cyclogenesis over the North Atlantic and North Pacific basins for the extended winter months (ONDJFM). Atmospheric rivers are identified for almost 80% of explosive deepening cyclones. For non-explosive cyclones, atmospheric rivers are found only in roughly 40% of the cases. The analysis of the time evolution of the high values of water vapour flux associated with the atmospheric river during the cyclone development phase leads us to hypothesize that the identified relationship is the fingerprint of a mechanism that raises the odds of an explosive cyclogenesis occurrence and not merely a statistical relationship. These new insights on the relationship between explosive cyclones and atmospheric rivers may be helpful to a better understanding of the associated high-impact weather events. Jorge Eiras-Barca was financially supported by the Spanish government (MINECO) and Xunta de Galicia (CGL2013-45932-R, GPC2015/014 – ERDF), and contributions by the COST action MP1305 and CRETUS Strategic Partnership (AGRUP2015/02). Alexandre M. Ramos was supported through a postdoctoral grant (SFRH/BPD/84328/2012) from the Portuguese Science Foundation (Fundação para a Ciência e a Tecnologia, FCT). Alexandre M. Ramos and Ricardo M. Trigo were supported by the project IMDROFLOOD – Improving Drought and Flood Early Warning, Forecasting and Mitigation using real-time hydroclimatic indicators (WaterJPI/0004/2014), funded by Fundação para a Ciência e a Tecnologia, Portugal (FCT). Joaquim G. Pinto thanks AXA Research Fund for support SI

A climatology of atmospheric rivers (ARs) impinging on the west coast of South Africa (29°–34.5°S) during the austral winter months (April–September) was developed for the period 1979–2014 using an automated detection algorithm and two reanalysis products as input. The two products show relatively good agreement, with 10–15 persistent ARs (lasting 18 h or longer) occurring on average per winter and nearly two-thirds of these systems occurring poleward of 35°S. The relationship between persistent AR activity and winter rainfall is demonstrated using South African Weather Service rainfall data. Most stations positioned in areas of high topography contained the highest percentage of rainfall contributed by persistent ARs, whereas stations downwind, to the east of the major topographic barriers, had the lowest contributions. Extreme rainfall days in the region are also ranked by their magnitude and spatial extent. The results suggest that although persistent ARs are important contributors to heavy rainfall events, they are not necessarily a prerequisite. It is found that around 70% of the top 50 daily winter rainfall extremes in South Africa were in some way linked to ARs (both persistent and nonpersistent). Overall, the findings of this study support similar investigations on ARs in the North Atlantic and North Pacific.

This work aims at characterizing the exceptional drought that affected Madeira Archipelago (Portugal) during the 2011–2012 hydrological year while including some major impacts but also the main atmospheric circulation mechanism behind the event. Precipitation records from six meteorological stations are used to assess the extreme drought episode by means of a decile classification. The assessment of the drought duration and severity is further corroborated by the Standardized Precipitation Index (SPI) computed for the 3, 6, 12 and 24 months’ time scales which confirmed that the 2012 drought event was one of the events with higher drought intensity at all scales, being classified as extreme at the 6-month time sale (SPI6 Vegetation activity is assessed through anomalies of NDVI (Normalized Difference Vegetation Index), confirming several large sectors of Madeira under vegetative stress. The southern sector of Madeira Island suffered up to seven months (out of nine) of extremely negative anomalies. From an operational point of view, obtained results reveal the ability of the developed methodology to monitor vegetation stress and droughts in Madeira. The extreme dryness of the Island favoured an unusually intense summer fire season of 2012 (between June and September) in Madeira being the year with highest number of fires in the last decade (with robust data). Furthermore, the main fire hotspots of the 2012 fire season are mostly coincident with the areas affected by drought. The large-scale atmospheric circulation responsible for the setting and intensification of the drought is evaluated using reanalysis data. An extensive high pressure anomaly (maximum above 12 hPa in December) persisted over the North Atlantic during the extended winter months (October to April), centred between the Azores Islands and the UK. This feature is in agreement with a positive NAO index (2.25 in December), a negative EA index (−1.76 in January and −1.73 in February) and compatible with enhanced north-easterly trade winds over the region. As a consequence of this uncharacteristic dynamics there was a deficit of moisture availability over the region as evaluated by the vertically integrated horizontal water vapour transport with a negative anomaly up to −120 kg m−1 s−1.

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., We acknowledge the E-OBS dataset from the EU-FP6 project ENSEMBLES (http://ensembleseu.metoffice.com) and the data providers in the ECA&D project (http://www.ecad.eu). PMS was supported by the projects IMDROFLOOD – Improving Drought and Flood Early Warning, Forecasting and Mitigation using real-time hydroclimatic indicators (WaterJPI/0004/2014) and HOLMDRIVE – North Atlantic Atmospheric Patterns influence on Western Iberia Climate: From the Lateglacial to the Present [PTDC/CTA-GEO/29029/2017] through Fundação para a Ciência e a Tecnologia, Portugal (FCT). RMT was partially supported by national funds through Fundação para a Ciência e a Tecnologia, Portugal (FCT) under project FireCast (PCIF/GRF/0204/2017). This research was partially funded by the European Union’s Horizon 2020 research and innovation programme under Grant agreement No 690462 within the frame of the ERANET “European Research Area for Climate Services –ERA4CS” 2016 Call. AMR was supported by the Scientific Employment Stimulus 2017 from FCT (CEECIND/00027/2017). We thank the Editor and reviewers for their helpful comments on this manuscript.

It is now clear that a large fraction of extreme precipitation and flood events across Western Europe are triggered by Atmospheric Rivers (ARs). The association between ARs and extreme precipitation days over the Iberian Peninsula has been well documented for western river basins. Since ARs are often associated with high impact weather, it is important to study their medium-range predictability. Here we perform such an assessment using the ECMWF ensemble forecasts up to 15 days for events that made landfall in western Iberian Peninsula during the winters spanning between 2012/2013 and 2015/16. IVT and precipitation from the 51 ensemble members of the ECMWF Integrated Forecasting System (IFS) ensemble (ENS) were processed over a domain including western Europe and contiguous North Atlantic Ocean. Metrics concerning the ARs location, intensity and orientation were computed, in order to compare the predictive skill (for different prediction lead times) of IVT and precipitation analyses in the IFS. We considered several regional boxes over Western Iberia, where the presence of ARs is detected in analysis/forecasts, enabling the construction of contingency tables and probabilistic evaluation for further objective verification of forecast accuracy. Our results indicate that the ensemble forecasts have skill to detect upcoming ARs events, which can be particularly useful to better predict potential hydrometeorological extremes. We also characterized how the ENS dispersion and confidence curves change with increasing forecast lead times for each sub-domain. The probabilistic evaluation, using ROC analysis, shows that for short lead times precipitation forecasts are more accurate than IVT forecasts, while for longer lead times this reverses (~10 days). Furthermore, we show that this reversal occurs for shorter lead times in areas where the ARs contribution is more relevant for winter precipitation totals (e.g. northwestern Iberia).

Weather extremes have gained great attention to the general public and policy makers recently. Extratropical cyclones, frontal systems and atmospheric rivers are central components of weather over ...

ISSN:0899-8418 ISSN:1097-0088

We review the major conceptual models of atmospheric moisture transport, which describe the link between evaporation from the ocean and precipitation over the continents. We begin by summarizing some of the basic aspects of the structure and geographical distribution of the two major mechanisms of atmospheric moisture transport, namely low-level jets (LLJs) and atmospheric rivers (ARs). We then focus on a regional analysis of the role of these mechanisms in extreme precipitation events with particular attention to the intensification (or reduction) of moisture transport and the outcome, in terms of precipitation anomalies and subsequent flooding (drought), and consider changes in the position and occurrence of LLJs and ARs with respect to any associated flooding or drought. We then conclude with a graphical summary of the impacts of precipitation extremes, highlighting the usefulness of this information to hydrologists and policymakers, and describe some future research challenges including the effects of possible changes to ARs and LLJs within the context of future warmer climates.

Atmospheric Rivers (ARs) are elongated bands of high water vapor concentration extending to the midlatitudes, which can be associated with intense precipitation and floods over continental areas. We analyze ARs reaching Europe in simulations from six Coupled Model Intercomparison Project Phase 5 (CMIP5) global climate models (GCMs) to quantify possible changes during the current century, with emphasis in five western European prone coastal areas. ARs are represented reasonably well in GCMs for recent climate conditions (1980–2005). Increased vertically integrated horizontal water transport is found for 2074–2099 (RCP4.5 and RCP8.5) compared to 1980–2005, while the number of ARs is projected to double on average for the same period. These changes are robust between models and are associated with higher air temperatures and thus enhanced atmospheric moisture content, together with higher precipitation associated with extratropical cyclones. This suggests an increased risk of intense precipitation and floods along the Atlantic European Coasts from the Iberian Peninsula to Scandinavia.

In this study we seek to investigate the role of atmospheric water vapour on the intensification of extra-tropical cyclones over the North Atlantic Ocean and more specifically to investigate the linkage between atmospheric rivers' conditions leading to the explosive development of extra-tropical cyclones. Several WRF-ARW simulations for three recent extra-tropical storms that had major negative socio-economic impacts in the Iberian Peninsula and south-western Europe (Klaus, 2009; Gong, 2013 and Stephanie, 2014) are performed in which the water vapour content of the initial and boundary conditions are tuned. Analyses of the vertically integrated vapour transport show the dependence of the storms' development on atmospheric water vapour. In addition, results also show changes in the shape of the jet stream resulting in a reduction of the upper wind divergence, which in turn affects the intensification of the extra-tropical cyclones studied. This study suggests that atmospheric rivers tend to favour the conditions for explosive extra-tropical storms' development in the three case studies, as simulations performed without the existence of atmospheric rivers produce shallow mid-latitude cyclones, that is, cyclones that are not so intense as those on the reference simulations.

An objective method for ranking extreme precipitation episodes in different time scales over the Iberian Peninsula (IP) is presented. It relies on the extensive use of the high-resolution (0.2°) gridded daily precipitation database available for the IP, spanning from 1950 to 2008. The sum of the normalized anomalies is computed over different time scales (2–10 days) to allow ranking the different anomalous precipitation multi-day periods. The magnitude of a precipitation episode is given taking into account the area (in percentage) that has precipitation anomalies above two standard deviations (std) and the mean values of these anomalies over this area. With these criteria, we are able to evaluate the spatial extent of the precipitation episodes as well as their spatially integrated intensity. Different precipitation rankings are built for the entire IP, for Portugal and independently for six of the major river basins in the IP. The different rankings correctly detect and categorize the most extreme precipitation episodes which occurred on the various domains. For each domain, in general, results show that few events dominate the top ten of a particular ranking; specific extreme events at shorter time scales (e.g. 2–3 days) may be absent from the top rankings at longer time scales (e.g. 10 days). When comparing the different domains, the top ten events are generally different from each other, thus highlighting the spatial variability of extreme precipitation in the IP. Moreover the methodology used to build the rankings imply that some events are considered extreme due to particularly high precipitation totals, while other episodes are more dependent on extensive areas affected by less extreme precipitation values. The different rankings revealed to be an useful tool for future studies in identifying the meteorological impacts of extreme precipitation episodes at the regional scale covering relatively large areas.

An automated atmospheric river (AR) detection algorithm is used for the North Atlantic Ocean basin, allowing the identification of the major ARs affecting western European coasts between 1979 and 2012 over the winter half-year (October to March). The entire western coast of Europe was divided into five domains, namely the Iberian Peninsula (9.75° W, 36–43.75° N), France (4.5° W, 43.75–50° N), UK (4.5° W, 50–59° N), southern Scandinavia and the Netherlands (5.25° E, 50–59° N), and northern Scandinavia (5.25° E, 59–70° N). Following the identification of the main ARs that made landfall in western Europe, a Lagrangian analysis was then applied in order to identify the main areas where the moisture uptake was anomalous and contributed to the ARs reaching each domain. The Lagrangian data set used was obtained from the FLEXPART (FLEXible PARTicle dispersion) model global simulation from 1979 to 2012 and was forced by ERA-Interim reanalysis on a 1° latitude–longitude grid. The results show that, in general, for all regions considered, the major climatological areas for the anomalous moisture uptake extend along the subtropical North Atlantic, from the Florida Peninsula (northward of 20° N) to each sink region, with the nearest coast to each sink region always appearing as a local maximum. In addition, during AR events the Atlantic subtropical source is reinforced and displaced, with a slight northward movement of the sources found when the sink region is positioned at higher latitudes. In conclusion, the results confirm not only the anomalous advection of moisture linked to ARs from subtropical ocean areas but also the existence of a tropical source, together with midlatitude anomaly sources at some locations closer to AR landfalls.

The Iberian Peninsula (IP) is recurrently affected by dust transport from the Sahara Desert and from the semi-arid Sahel regions. African dust is one of the most important sources of particulate matter in the southern Mediterranean. Therefore, it is vital to understand the underlying processes that lead to episodes of air pollution associated to the occurrence of dust intrusions. This work proposes to make an extended characterization of the preferential circulation weather patterns associated to the onset of dust events affecting the IP between 2006 and 2016. Saharan dust intrusions were analysed and an automatic objective classification procedure was used to classify circulation weather patterns associated to dust events. The spatial distribution of intrusion episodes is not homogeneous throughout the IP, occurring less frequently at northern and northwestern locations than at central and southern sites. Moreover, days with Saharan dust intrusions were more frequent in summer months, and more probable to occur under regimes with a southerly component. Finally, two extreme events with high concentration of particulate matter were analysed relatively to their life-cycle and particle trajectories. The distinct extreme episodes can be associated to different synoptic situations. However, and despite different large-scale configurations, a south or south-easterly component over the region is responsible for the establishment of a dust transport from the Saharan region towards Iberia, and thus leading to the intrusion onset. These results were supported by the calculation of back-trajectories which allowed to source apportioning the particles' origin, through a clear trajectory of air parcels originating from northern Africa in both events. The proposed framework can be useful to the prediction of dust and air pollution events based on the forecast of circulation weather patterns, as the results show that these events across the IP are mainly induced by specific patterns.

In the most recent decades, central Europe and the Danube River Basin area have been affected by an increase in the frequency and intensity of extreme daily rainfall, which has resulted in the more frequent occurrence of significant flood events. This study characterised the link between moisture from the Mediterranean Sea and extreme precipitation events, with varying lengths that were recorded over the Danube River basin between 1981 and 2015, and ranked the events with respect to the different time scales. The contribution of the Mediterranean Sea to the detected extreme precipitation events was then estimated using the Lagrangian FLEXPART dispersion model. Experiments were modelled in its forward mode, and particles leaving the Mediterranean Sea were tracked for a period of time determined with respect to the length of the extreme event. The top 100 extreme events in the ranking with durations of 1, 3, 5, 7, and 10 days were analysed, and it was revealed that most of these events occurred in the winter. For extreme precipitation, positive anomalies of moisture support from the Mediterranean were found to be in the order of 80% or more, but this support reached 100% in summer and spring. The results show that extreme precipitation events with longer durations are more influenced by the extreme Mediterranean anomalous moisture supply than those with shorter lengths. However, it is during shorter events when the Mediterranean Sea contributes higher amounts of moisture compared with its climatological mean values, for longer events, this contribution decreases progressively (but still doubles the climatological moisture contribution from the Mediterranean Sea). Finally, this analysis provides evidence that the optimum time period for accumulated moisture to be modelled by the Lagrangian model is that for which the extreme event is estimated. In future studies, this fine characterisation could assist in modelling moisture contributions from sources in relation to individual extreme events.

This study identifies and characterizes the importance of the Atmospheric Rivers in the extreme precipitation episodes that strike the Iberian Peninsula and Portugal during the extended summer months (April to September) between 1950 and 2007. The extreme precipitation days are ranked taking into account a daily gridded precipitation database for the Iberian Peninsula at a 0.2° resolution. The ranking is based on the magnitude of the extreme precipitation days considering not only on the area affected above the 95th climatological percentile but also by the precipitation intensity within the anomalous area. The Atmospheric Rivers detection scheme is used for the North Atlantic Ocean basin that allows the identification of the persistent Atmospheric Rivers that impact the Iberian Peninsula for the extended summer months. It is shown, that there is a relationship between the Atmospheric Rivers and the extreme precipitation days in Portugal especially during the transition months of April, May and September. On the contrary when analysing the entire Iberia Peninsula the impact of ARs is considerably reduced. Moreover, the impacts of the Atmospheric Rivers is considerably higher for the top ranked events in Portugal but decreases when considering less intense extreme precipitation days.

A Lagrangian analysis is applied to identify the main moisture source areas associated with atmospheric rivers (ARs) making landfall along the west coast of South Africa during the extended austral winter months from 1980 to 2014. The results show that areas that provide the anomalous uptake of moisture can be categorized into four regions: (1) the South Atlantic Ocean between 10°S and 30°S, (2) a clear local maximum in the eastern South Atlantic, (3) a continental source of anomalous uptake to the north of the Western Cape, and (4) over South America at a distance of more than 7000 km from the target region. It emerges that the South American moisture source can be linked to a particular phase of the South American low-level jet, known as a no Chaco jet event (NCJE), which transports moisture to the western and central South Atlantic basin. Concisely, we provide strong evidence that the two margins of the South Atlantic Ocean appear connected by two meteorological structures, with the NCJE playing a key role of transporting moisture from South America to the western and central South Atlantic basin, feeding the AR that transports some of the moisture to the west coast of South Africa.

Author(s): Shields, Christine A; Rutz, Jonathan J; Leung, Lai-Yung; Ralph, F Martin; Wehner, Michael; Kawzenuk, Brian; Lora, Juan M; McClenny, Elizabeth; Osborne, Tashiana; Payne, Ashley E; Ullrich, Paul; Gershunov, Alexander; Goldenson, Naomi; Guan, Bin; Qian, Yun; Ramos, Alexandre M; Sarangi, Chandan; Sellars, Scott; Gorodetskaya, Irina; Kashinath, Karthik; Kurlin, Vitaliy; Mahoney, Kelly; Muszynski, Grzegorz; Pierce, Roger; Subramanian, Aneesh C; Tome, Ricardo; Waliser, Duane; Walton, Daniel; Wick, Gary; Wilson, Anna; Lavers, David; Collow, Allison; Krishnan, Harinarayan; Magnusdottir, Gudrun; Nguyen, Phu

The DISASTER database records 1969 unique hydro-geomorphologic cases (floods and landslides) that generated human damages in Portugal during a 150 year-period (1865–2015. The main purposes of this work are the following: to present a national disaster events catalogue; to analyse the atmospheric conditions and to discuss the role of rainfall as main driving force of the hydro-geomorphological disaster events. The main Circulation Weather Types (CWTs) and the presence of Atmospheric Rivers (ARs) of each day of the disaster events were classified using the 20CR dataset (1865–2014) and the NCEP/NCAR reanalysis (2015). These datasets were used to compute the daily cumulative precipitation and the long term climatological year accumulated precipitation 10th, 50th, 90th and 95th percentile spanning 1865–2015 and the accumulated rainfall conditions after and before each disaster event. In the central and northern Portugal Disaster events are mainly driven by southwest (SW), west (W), northwest (NW) and Cyclonic (C) Weather Types, whereas the CWTs with a southern and eastern component [east (E), southeast (SE) and south (S)] are the main drivers in the south part of the country. Cyclonic CWT is widely associated with convective systems that can generate flash floods and urban floods often responsible for fatalities and injured people. CWTs with a westerly component can be associated to prolonged wet periods that can induce flood events in the main rivers and landslides in the mountain areas. The methodology used to build this disaster event catalogue can be applicable to other study areas, namely the North Atlantic and the Mediterranean regions, where the flood and landslides extremes are related to the atmospheric circulation.

The large number of floods and landslides that occurred on 5–16 February 1979 in Portugal was a major hydro-geomorphologic extreme event according to the DISASTER database in terms of number of displaced people. The February 1979 event is the top ranked episode in terms of the total number of evacuated people (4244), displaced people (14,322) and also on the number of days of event duration (12 days) for the period 1865–2015. In this event, 62 damaging floods and five damaging landslides causing eight fatalities were recorded in Portugal. This event was driven by an unusually intense atmospheric forcing mechanism acting at different time scales. Despite the intense magnitude and the widespread impact on the population, this event has not been studied in detail. In this study, we show that the precipitation period of February 1979 had produced several multi-day accumulated precipitation events over the Portuguese continental territory, ranking among the top 10 events observed between 1950–2008. Additionally, most of the precipitation from this event occured in days in which atmospheric circulation was dominated by “wet” circulation weather types (CWTs), namely, cyclonic (C), west (W) or southwest (SW) types.

This study deals with the question of how winegrowing in Spain may be altered by anthropogenic climate change. The present state and expected future development of three bioclimatic indices relevant for winegrowing were assessed by observation, and four regional climate models from the EU-ENSEMBLES project were investigated. When comparing the 2061–2090 scenario period to the 1961–1990 reference period, the models unanimously indicate a significant increase in the mean of the two considered thermal indices over the entire study region. However, for the index based on temperature and precipitation, the models are heavily biased when verified against observations and generally disagree on the size of the projected future change. For this index, unanimous model agreement was only found for northwestern Spain where all models indicated a significant decrease in the mean. From these results, regional climate change is expected to negatively affect the quality of wine in the growing regions of central and southern Spain, and the Ebro valley, whereas positive effects should be expected in the northwest. No significant changes in the risk of mildew infestation are to be expected except for the northwest, where this risk is projected to decrease., This work was supported by the Xunta de Galicia under Research Grant No. 10PXIB383169PR and co-financing by European Regional Development Fund (FEDER). Alexandre M. Ramos was supported by the Portuguese Foundation for Science and Technology (FCT) through Grant FCT/DFRH/SFRH/BPD/84328/2012. S. Brands would like to thank the Consejo Superior de Investigaciones Científicas JAE-PREDOC programme for financial support.

The present work aims at evaluating the impacts of a climate change scenario on the hydrography and dynamics of the Iberian Upwelling System. Using regional ocean model configurations, the study domain is forced with three different sets of surface fields: a climatological dataset to provide the control run; a dataset obtained from averaging several global climate models (GCM) that integrate the Intergovernmental Panel for Climate Change (IPCC) models used in climate scenarios, for the same period as the climatological dataset; and this same dataset but for a future period, retrieved from the IPCC A2 climate scenario. After ascertaining that the ocean run forced with the GCM dataset for the present compared reasonably well with the climatologically forced run, the results for the future run (relative to the respective present run) show a general temperature increase (from +0.5 to +3 °C) and salinity decrease (from −0.1 to −0.3), particularly in the upper 100–200 m, although these differences depend strongly on season and distance to the coast. There is also strengthening of the SST cross-shore gradient associated to upwelling, which causes narrowing and shallowing of the upwelling jet. This effect is contrary to the meridional wind stress intensification that is also observed, which would tend to strengthen the upwelling jet.

In this work we reassess the impacts of blocking patterns on precipitation regimes in the Iberian Peninsula, distinguishing between north Atlantic and European blocking. For this we take full advantage of the recently developed high-resolution datasets for the Iberian countries. Precipitation anomalies during blocking events obtained with this dataset allow a much finer regional characterization of the impacts in both average and extreme daily precipitation, particularly when compared to widely used low-resolution reanalysis datasets. Blocked patterns induce a negative-positive dipole of precipitation anomalies from northwest to southeast Iberia. Increases are widespread during Atlantic blocks and pronounced in southern and eastern areas of Iberia, while during European blocks they are more spatially restricted, with increases above 50 % in coastal Mediterranean areas, which represents a considerable fraction of the annual precipitation. Blocking impacts in precipitation are nearly opposite to those found during strong zonal flow situations, but there are also some asymmetries in the precipitation responses. A significant increase in cyclones and cut-off lows frequency southwards of blocking structures is related to precipitation excesses over southern and eastern areas, where dynamical factors and local processes play a crucial role. On the contrary, precipitation deficits in northwest Iberia during blocking episodes are better explained by a reduction in north Atlantic frontal activity and simultaneous decreases in large-scale moisture advection towards northern Iberia. We show that these anomalies during blocking result from changes in precipitation amount rather than from increases in rainy days, pointing to more extreme rainfall regimes, particularly in southeastern Iberia. Finally, an Extreme Value Analysis was performed, fitting Generalized Pareto Distributions to precipitation extremes. Results show that the different extreme precipitation regimes of northwest and Mediterranean regions are partially determined by opposite anomalies of the zonal flow. Thus, heavy precipitation events in Mediterranean areas are usually short-lived and frequently associated with blocking conditions, while in northwest Iberia the total accumulations during rainfall episodes are more important for triggering extreme events and they are mainly related to strong westerly flows.

An automated atmospheric rivers (ARs) detection algorithm is used for the North Atlantic Ocean basin that allows the identification and a comprehensive characterization of the major AR events that affected the Iberian Peninsula over the 1948–2012 period. The extreme precipitation days in the Iberian Peninsula and their association (or not) with the occurrence of ARs is analyzed in detail. The extreme precipitation days are ranked by their magnitude and are obtained after considering 1) the area affected and 2) the precipitation intensity. Different rankings are presented for the entire Iberian Peninsula, for Portugal, and for the six largest Iberian river basins (Minho, Duero, Tagus, Guadiana, Guadalquivir, and Ebro) covering the 1950–2008 period. Results show that the association between ARs and extreme precipitation days in the western domains (Portugal, Minho, Tagus, and Duero) is noteworthy, while for the eastern and southern basins (Ebro, Guadiana, and Guadalquivir) the impact of ARs is reduced. In addition, the contribution from ARs toward the extreme precipitation ranking list is not homogenous, playing an overwhelming role for the most extreme precipitation days but decreasing significantly with the less extreme precipitation days. Moreover, and given the narrow nature of the ARs, the location of the ARs over each subdomain is closely related to the occurrence (or not) of extreme precipitation days.

An assessment of extreme precipitation events (EPEs) is performed using the high-resolution (0.2°) gridded daily precipitation database available for the IP, the accumulated precipitation from ERA-Interim reanalysis by EMCWF at 6-hour intervals, and the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis (TMPA) dataset, based on multisatellite estimates of precipitation and gauges measurements, for the common period since 1998. This study presents an analysis and validation of the extreme precipitation characteristics over IP, using both satellite and ground observations. Results show that there is a good general agreement between total precipitation analysis from observational gridded and TRMM datasets, both temporal and spatially, although TRMM TMPA results are underestimated when compared to observations and ERA Interim data.

Western Iberia is frequently struck by intense mid-latitude cyclones coming from the North Atlantic basin and often impinging extreme weather over large swaths of the Iberian Peninsula (IP). The spatial distribution and characterization of past floods and landslides with important social consequences in Portugal for the period 1865-2015 was performed within the context of the DISASTER project (Zezere et al., 2014). From this database, a major hydro-geomorphologic event was selected, the February 1979, in order to study its atmospheric forcings and to analyze its societal impacts. The February 1979 event is a top ranked event in the DISASTER database regarding the total number of affected (18578), displaced (14322) and evacuated (4244) people in Portugal and in the Tagus basin (7677, 4816 and 2834, respectively). Most of the days considered in this event produced daily precipitation values over or within the 90th-95th percentile of the corresponding long term daily precipitation series (available at high resolution between 1950 and 2008). Most of the event precipitation occurred in days characterized by wet Circulation Weather Types, i.e. cyclonic (C), west (W) or southwest (SW) types, which agrees with the assessment of wet days obtained by Trigo and DaCamara (2000) and Ramos et al. (2014) for the IP domain. Also, throughout this period, the North Atlantic Ocean is crossed several times by narrow and prolonged bands of high moisture concentration, with cores above 9 g/kg, that originate near the Caribbean islands and move towards extratropical latitudes by the influence of southwestern low-level jets of medium or high intensity. These are mostly persistent Atmospheric Rivers (ARs) reaching the western IP coast and affecting most of the month of February until the 16th. Overall, regarding the large-scale circulation, a deep low-pressure system located over the North Atlantic and reaching western IP, allowed for the frequent passage of frontal systems over the territory which was responsible for this precipitation event. In addition, local convective instabilities and strong moisture transport from the Tropical Atlantic produced an extremely intense 15-day precipitation event over western IP, that establishes as the meteorological trigger of the February 1979 Disaster event. Acknowledgements: This work was financed by national funds through FCT - Portuguese Foundation for Science and Technology, I.P., under the framework of the project FORLAND Hydro-geomorphologic risk in Portugal: driving forces and application for land use planning (PTDC/ATPGEO/1660/2014). A. M. Ramos was also supported by a FCT postdoctoral grant (FCT/DFRH/ SFRH/BPD/84328/2012).

The Danube River Basin is the second longest catchment basin in Europe and exhibits intense climatological diversity. In recent decades, the frequency and intensity of daily precipitation extremes have suffered from an increment in many parts of the world, including Central and Eastern Europe. Wet spells are defined by the number of consecutive rainy days with different thresholds. The identification of wet spells and their trends in the rainfall time is very important for many sectors, such as agriculture, ecology, hydrology and water resources. Wet spells can lead to extreme events and cause floods and other disasters. In this study, we will attempt to characterise global precipitation in the context of wet spells and associated precipitation depth of wet spells in the Danube River Basin area using daily precipitation data, as well as analysing different approaches to identifying wet spells. The ten most intense wet spells were detected, and the most intense, which occurred on 23 September 1996, was studied in depth in terms of precipitation and associated anomalies, the synoptic situation and the anomalous transport of moisture using a Lagrangian approach. The existence of a marked west-east dipole in the field of sea level pressure between the Atlantic Ocean and the eastern Mediterranean leads to the anomalous moisture transport from the Northern Atlantic Ocean to the Mediterranean Sea, where a higher available amount of moisture existed, and subsequently penetrated within the low positioned over the Danube River Basin. In addition, an Atmospheric River was also responsible for the wet conditions in the Danube River Basin. The combination of all these factors was responsible for the extreme precipitation linked with the wet spell.

This work evaluates the performance of several global climate models (GCMs) as forcing of a regional ocean model configuration centered in the Iberian Basin. The study is divided in two parts. First, the output of nine GCMs is analyzed based on the fields needed to force the ocean model (Regional Ocean Modelling System—ROMS). GCMs differ greatly between them and their performance depends on the field. In the second part, the two GCMs with the worst performances in both extremes of the ensemble are used as forcing for two ROMS simulations, with the purpose of assessing the range of uncertainty comprised in this set of GCMs. Two other ROMS runs are setup: one climatologically forced control run, and one forced with the average of all the nine GCMs—the ensemble mean. Results show that the tendency of overestimation/underestimation of the forcings is reflected in the modeled hydrography, both at the surface and deeper layers down to 500 m. Nevertheless, in terms of circulation, all four runs reproduce the Azores Current, as well as the coastal transition zone seasonality (winter poleward flow and summer upwelling-associated equatorward flow). The CGCMs output performance as forcing depends on the forcing variable: one performs well for one or more variables, but badly for others, and which field is well or badly reproduced varies for each CGCM. Therefore, there is not a single CGCM having the best forcing for all variables. Hence, our results indicate that the most adequate approach consists of using the ensemble mean as forcing rather than using an individual model. This is supported by the general low overall (i.e. for all forcing variables) errors of the ensemble mean regarding the control climatological dataset, and the good comparison of the ensemble-forced ROMS run with the control run.

Flood events is the natural hazard that originated more damages and fatalities in the Iberian Peninsula in the last decades. While most 20th century extreme precipitation and flood episodes in Iberia have been documented, the same does not hold for most events that took place during the 19th century. This article describes the unusually high precipitation and associated impacts recorded during the 1855/1856 hydrological year. We combine newspaper reports, early instrumental precipitation series and sea level pressure (SLP) reconstructed gridded fields. The early instrumental precipitation time series includes 11 observatories that were not previously digitized and preceded the implementation of the official meteorological observation network in Spain. We show that high values of precipitation were mostly recorded during the months of September, October 1855 and January 1856, with most of the flooding and damages occurring in the last month. The use of daily circulation weather types and monthly differences of SLP are particularly useful to explain the heavy precipitation in October and January, which were clearly associated with unusual high frequencies of wet weather types. However, SLP patterns cannot explain the September records, which could be associated to upper cold air intrusions in the Iberian Peninsula.

Precipitation extremes in mainland Portugal (south-western Europe) using daily precipitation data recorded in the period 1941–2007 (67 years) at 57 meteorological stations scattered across the area are studied at an annual scale. Trends in selected precipitation annual indices that are calculated from these data are investigated, in particular trends in the intensity, frequency and duration of extreme precipitation events. Special attention is dedicated to local and regional variability. The spatial correlations between the annual trends in mean precipitation and in the extremes are analysed. Moreover, the relationships between the variability of the North Atlantic Oscillation (NAO) index and several indices related to the frequency and intensity of the precipitation at the 57 stations were also investigated. Results show that several stations have predominantly negative tendencies in the precipitation indices, although the majority of stations did not show statistically significant change over time in the 1941–2007 period. At the regional level, the decreasing trend in the simple daily precipitation intensity index is the only one statistically significant at the 5 % level and appears to be related to the predominance of the positive phase of the NAO. For the period 1976–2007, the proportion of the total precipitation attributed to heavy and very heavy precipitation events increased and, consequently, daily precipitation events show a tendency to become more intense. Moreover, correlation analysis show that the most extreme events could be changing at a faster absolute rate in relation to the mean than more moderate events.

The relationship between circulation weather types (WTs) and the spatial variability of precipitation across the Iberian Peninsula were studied using a high density, quality controlled, homogenized monthly precipitation database with approximately 3000 stations and interpolated to a 10 km grid. The circulation WTs were computed using an objective version of the Lamb classification centered on the Iberian Peninsula. A total of 26 WTs were selected for the period 1948-2003. Daily WTs were grouped to obtain their monthly frequencies, and used as potential independent variables in a linear least-square non-negative regression model with a forward stepwise selection. Results show the impact of each WT on precipitation in the Iberian Peninsula with a spatial resolution which had never been achieved before and additionally were obtained on a monthly scale highlighting the large seasonal cycle observed in each class, and including significantly different patterns in winter and summer. Nevertheless, results confirm that most of the precipitation in the Iberian Peninsula is produced by just a few WTs, with W, SW and C being the most influential. The association between WTs and precipitation is more robust in winter months and for the western IP areas, while it is lower during summer months and for the eastern IP areas. Spatial analysis revealed that precipitation on the Mediterranean coastland is mostly related with easterly flows (NE, E, SE and their hybrid counterparts), while on the Cantabrian coastland. N and NW flows are the most influential WTs. In general, cyclone-related types are the least frequent ones and also the most efficient in generating precipitation; while anticyclone-related types have the highest frequencies, but also the lowest contribution to total monthly rainfall in the Iberian Peninsula.

This study aims mostly at understanding seasonal variations in the intensity, frequency and duration of extreme precipitation events in mainland Portugal. For this purpose, selected precipitation indices that mainly focus on extremes were calculated at the seasonal scale for daily data recorded in the period 1941–2007 at 57 meteorological stations scattered across the area. These indices were explored for trends at the local and regional levels. The results show that there are marked changes in precipitation indices at the seasonal scale. Trends in spring and autumn precipitation have opposite signals. In spring, statistically significant drying trends are found together with a reduction in extremes. In autumn, wetting trends are detected for all indices, although overall they are not significant at the 5 % level. In addition, the relationship between seasonal extreme precipitation indices and atmospheric large-scale modes of low-frequency variability is analysed by means of a seasonal correlation analysis. Four modes of low-frequency variability are explored. Results confirm that, over mainland Portugal, the North Atlantic Oscillation is one of the most important teleconnection patterns in any season and the mode of variability that has the greatest influence on precipitation extremes in the area, particularly in the winter and autumn.

This study provides a thorough assessment of recent changes in near-surface air temperature in mainland Portugal at both the local and regional level, focusing on extreme events (maximum and minimum) at a seasonal scale. It examines trends in selected specific indices that are calculated from daily air temperature data from 23 measuring stations scattered across the territory, recorded between 1941 and 2006. The results show overall warming trends over mainland Portugal that are consistent with the dominant global warming and reflect an increase in both maximum and minimum air temperature. When we split the study period into two sub-periods, 1945–1975 and 1976–2006, the partial trend analysis reveals that the first sub-period is mostly characterized by cooling followed by an even stronger tendency towards warmer benchmarks in all the indices evaluated, in particular for the warm-related temperature extremes in spring and summer. The changes observed in seasonal patterns confirm the well-known asymmetries in the climate in mainland Portugal and suggest that they are likely to be aggravated. There are changes associated with extreme temperatures, in particular, the significant increase in the frequency and duration of heat waves, and the increase in the frequency of hot days and tropical nights, especially in spring and summer; moreover there is a significant decrease in the frequency of cold waves and frost days. Teleconnections associated with changing patterns of temperature are also investigated. The results show that, over mainland Portugal, cold-related air temperature extremes have been associated with the East Atlantic mode in autumn, whereas warm-related extremes have been associated with the Scandinavia teleconnection pattern in spring, summer and autumn. However, the most prominent Northern Hemisphere pattern, the North Atlantic Oscillation, exerts limited influence, which is felt mostly in winter and spring.