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In this study, we analyze the linkage between atmosphere and ocean modes and winter flood variability over the 20th century based on long‐term flow‐discharge series, historical archives, and tree‐ring records of past floods in the North Atlantic Basin (NAB). The most extreme winter floods occurred in 1936 and had strong impacts on either side of the Atlantic. We hypothesize that the joint effects of sea surface temperatures (SSTs) over the Atlantic and Pacific Oceans and the Arctic Oscillation (AO), which is closely related to the North Atlantic Oscillation, play a significant role when describing flood variability in North America and Europe since 1900. Statistical modeling supports the assumption that the response of flood anomalies over the NAB to AO phases is subsidiary of SST phases. Besides, we shed light on the extraordinarily winter flood of 1936 that was characterized by very high SSTs over both the Atlantic and Pacific (>98th percentile) and very low, negative values of AO (, This study was funded by the Institute for Environmental Sciences (University of Geneva) without any specific grant. S.B. acknowledges support from the FP7 project ERA‐CLIM2. J.A.B.C. thanks Sebastian Guillet, Christophe Corona, and Jaime Madrigal for their comments. G.B. appreciates the support of Fundación Biodiversidad (MITECO) through research project DAM‐ADAPT. D.B. and R.G.‐H. were supported by the Spanish Ministry of Economy and Competitiveness through the PALEOSTRAT (CGL2015‐69699‐R) project and the European Project 603557‐STRATOCLIM under program FP7‐ENV.2013.6.1‐2.

Trabajo presentado en el MedCLIVAR Conference: Bridging the Mediterranean Climates, celebrado en Belgrado (Serbia) del 17 al 21 de septiembre de 2018, Atmospheric blocking episodes are an important component of the intra-seasonal and interannual variability at mid-latitudes. Their impacts in the European continent, and in particular, in the Mediterranean basin, depend on: i) blocking location; ii) spatial characteristics; iii) temporal length. A characterization of Euro-Atlantic blocking occurrence within different sectors (Atlantic, European and Russian) was performed, focusing on the impacts in temperature and precipitation regimes. High-latitude blocks were distinguished from sub-tropical ridges using a novel ridge detection scheme. Ridges do not require wave-breaking occurrence as blockings do, although they are frequent precursors of wave-breaking and subsequent high-latitude blocking episodes. The distinct seasonal and regional impacts associated with different blocking/ridge locations were analyzed, as well as the dynamical mechanisms driving the temperature and precipitation responses associated to each pattern, namely: the role of cyclonic activity; moisture transport; large-scale atmospheric instability; balances between horizontal advection, subsidence and radiation budgets. Our analysis clarifies that increasing extreme heat episodes in southern Europe and Mediterranean areas should not be attributed to blockings, but rather to sub-tropical ridges. In northern Europe, both regimes are responsible for warm conditions in summer, due to enhanced radiative heating and increased subsidence. Opposite temperature responses are found for winter blocking/ridges. While blockings reinforce cold northerly advection and foster cold spells, ridge patterns are related with mild Atlantic flows associated. An opposite north-south dipole is also found regarding precipitation anomalies. While blocks force a split of the Jetstream and storm-track, ridges are characterized by a stronger zonal flow at higher latitudes. Accordingly, negative (positive) precipitation anomalies during blocks occur at higher (lower) latitudes. We also demonstrate how enhanced atmospheric instability and cyclonic activity occurs south of blocking centers, highlighting the importance of torrential regimes in Mediterranean areas. On the other hand, the presence of sub-tropical ridge enhances a northward deflection of moisture corridors, leading to very dry conditions in southern Europe, and significantly increasing the chances of drought occurrence in Mediterranean areas. Finally, we discuss how sub-tropical ridge frequency has been increasing, in the context of a warming atmosphere and an expanding Hadley Cell, and how the expected further expansion of the tropics poses a severe threat for water availability in the Mediterranean area., This work was supported by project IMDROFLOOD – Improving Drought and Flood Early Warning, Forecasting and Mitigation using real-time hydroclimatic indicators (WaterJPI/0004/2014) funded by FCT. Alexandre M. Ramos was also supported by a FCT postdoctoral grant (FCT/DFRH/ SFRH/BPD/84328/2012).

Trabajo presentado en el MedCLIVAR Conference: Bridging the Mediterranean Climates, celebrado en Belgrado (Serbia) del 17 al 21 de septiembre de 2018, This work describes the exceptional mega-heatwave that affected western and central Europe in June 2017 as an example of summers with an earlier occurrence of record-breaking megaheatwaves. To do so, we designed a novel algorithm that monitors mega-heatwaves by tracking the spatio-temporal evolution of extreme temperature patterns. The results indicate that the spatial extension, persistence and intensity of the June 2017 event were similar to those of top European mega-heatwaves of the reanalysis period. However, it occurred earlier than other well-known mega-heatwaves, such as that of 2003. The most affected area was southwestern Europe, where the event was the longest heatwave on record, causing the warmest temperatures from daily to seasonal scales, forest fires and human casualties. The megaheatwave was associated with a record-breaking subtropical ridge in mid-June with signatures more typical of July and August, which caused an unprecedented subtropical warm air intrusion. Using the analogue method, we found that the recorded temperature anomalies were higher than those expected from past flow analogues. This was partially due to changes in circulation. However, thermodynamical changes of the last decades made a much larger contribution to the exceptionality of the event.

Trabajo presentado en el EMS Annual Meeting: European Conference for Applied Meteorology and Climatology, celebrado en Budapest (Hungría), del 3 al 7 de 2018, Wind direction provided by ships’ logbooks is a consolidated instrumental source, as it has been measured with a compass for many centuries, thus yielding daily observations for the world’s oceans and seas. Nonetheless, ships’ logbooks have still been poorly exploited to reconstruct climate variability. We present four monthly indices of wind persistence, one for each cardinal direction: Northerly (NI), Easterly (EI), Southerly (SI) and Westerly (WI), based on daily wind direction observations taken aboard ships over the English Channel. These directional indices (DIs) are the longest observational record of atmospheric circulation to date, covering the 1685-2014 period. We show that the DIs are informative about the mid-latitude atmospheric circulation over the Atlantic and play an important role in the climate conditions of Europe. They show strong climatic signals in terms of temperature and precipitation anomalies over large areas of Europe throughout the year, especially during cold seasons. Statistical models including all DIs are able to explain a considerable amount of climate variability, improving in most cases that accounted for by the North Atlantic Oscillation (NAO). A Principal Component (PC) Analysis of the DIs reveals links with the main modes of atmospheric climate variability over the eastern Atlantic, namely the NAO and the Eastern Atlantic (EA) pattern. The combination of NAO and EA phases inferred from the DIs allows us to report periods dominated by “high-zonal” and “low-zonal” flow patterns during the last three centuries. These results show the potential of DIs to capture non-stationary signatures of the atmospheric circulation, characterize the European climate variability from monthly to multidecadal time scales. As such, they arise as powerful tools to explore the atmospheric circulation responses to slowly-varying climate drivers, as well as anthropogenic and natural forcings., This work was supported by FCT through project UID/GEO/ 50019/2013

Trabajo presentado en el EMS Annual Meeting: European Conference for Applied Meteorology and Climatology, celebrado en Budapest (Hungría), del 3 al 7 de 2018, We use daily wind direction observations from ships’ logbooks over the English Channel to construct atmospheric circulation indices on a monthly basis for the Late Maunder Minimum (LMM, 1685-1715). They include four wind directional indices (one for each cardinal direction) and 8-point wind roses providing the monthly persistence of the wind from a given direction. Firstly, the indices are used to characterize the atmospheric circulation during the LMM. The results reveal a pronounced increase of the meridional circulation during the LMM and a marked reduction in the frequency of the westerlies all year-round, as compared to present-day (1981-2010). In spite of this overall picture, the LMM hided contrasting spatial patterns in circulation, as well as, large decadal variability. In particular, we report increased northerlies contributing to colder winters in Europe during the first half of the LMM, but enhanced southerlies favoring milder winters in the second half of the LMM. Second, we infer the dynamically-driven temperature anomalies during the LMM based on flow (8-point wind roses) analogues of the 20th century. The results confirm the majority of the well-documented extremely cold winters. However, the analysis also uncovers other less documented cold winters and a substantial number of mild winters, most of which had been largely unnoticed. Extremely cold winters were associated with negative phases of a "high zonal" North Atlantic Oscillation (NAO) pattern, while moderately cold winter were rather dominated by negative phases of a "low zonal" NAO pattern. Therefore, our results indicate that the LMM was more heterogeneous than expected, displaying decadal variability, non-stationary signatures of the NAO pattern a relatively frequent mild winters., This work was supported by FCT through project UID/GEO/ 50019/2013.

Trabajo presentado en la European Geosciences Union General Assembly, celebrada en Viena (Austria), del 8 al 13 de abril de 2018, In recent decades, observations found in ships’ logbooks have shown to be a valuable source to explore past climate and its variability. Here, we present four wind directional indices and 8-point wind roses at monthly scales based on daily wind direction observations from ships over the English Channel. They are used to characterize the atmospheric circulation during the Late Maunder Minimun (LMM, 1685-1715), one of the few cold periods in recent centuries that persisted over decades. The comparison with the present-day (1981-2010) period reveals a pronounced increase of the meridional component of the atmospheric circulation during the LMM and a marked reduction in the frequency of westerly days all year-round. Nevertheless, our findings indicate that the LMM was more heterogeneous than previously thought, displaying contrasting spatial patterns in circulation, as well as, large decadal variability. In particular, we report an increase of northerly winds favoring colder winters in Europe during the first half of the LMM, but enhanced southerlies contributing to milder conditions in the second half of the LMM. The temperature inferred from the atmospheric circulation confirms the majority of extremely cold winters well documented in the literature, while uncovering other poorly documented cold winters and a substantial number of mild winters that had been unnoticed so far. Our results also suggest a non-stationarity of the climate impacts of the North Atlantic Oscillation (NAO) pattern within the LMM, with extremely cold winters being driven by negative phases of a "high zonal" NAO pattern and "low zonal" NAO patterns dominating during moderately cold winters., This work was supported by FCT through project UID/GEO/ 50019/2013.

Trabajo presentado en el AGU (American Geophysical Union) Fall Meeting: Advancing Earth and Space Science, celebrado en Washington D.C. (Estados Unidos), del 10 al 14 de diciembre de 2018, near-surface wind speed, which strongly depends on the local setting of the observational sites and the boundary layer parameterizations within the reanalysis model. There is considerable spatial heterogeneity in stagnation patterns (i.e. occurrence of stagnation days and events, duration of events) over the area of study. Therefore, we have used the k-means algorithm to regionalise the monthly frequency of stagnant days, obtaining five regions: Scandinavia (SCAN), Northern-Europe (NEU), Central-Europe (CEU), South-West (SW) and South-East (SE). The northern regions (SCAN and NEU) present low frequency of stagnation compared to the southern regions (SW and SE). We will present additional results from the temporal variability and synoptic patterns associated with air stagnation in these regions. Finally, we have assessed the impact of air stagnation on air quality. This is particularly strong over CEU. In this region, the anomalies of summer ozone and winter PM10 on stagnant vs. non-stagnant days exceed 20% and 50%, respectively. Persistent, widespread stagnation events have also been found to favour the build-up of these pollutants over most of the continent. We will discuss the distinct impacts of stagnation on both pollutants over the different regions, considering the main processes at play.

Trabajo presentado en la European Geosciences Union General Assembly, celebrada en Viena (Austria), del 8 al 13 de abril de 2018, Ships’ logbooks are the most important source of wind direction observations over the world’s oceans and seas for the pre-instrumental period. Moreover, wind direction in the English Channel is informative about the mid-latitude atmospheric circulation over the Atlantic and plays an important role in the climate conditions of Europe. We present four monthly indices of wind persistence, one for each cardinal direction: Northerly (NI), Easterly (EI), Southerly (SI) and Westerly (WI), based on daily observations taken aboard ships over the English Channel. These directional indices (DIs) are the longest observational record of atmospheric circulation to date, covering the 1685-2014 period. We show that the DIs constitute a powerful tool to characterize the atmospheric circulation over Europe from monthly to multidecadal time scales. Thus, they present a coherent climatic signal in terms of temperature and precipitation over most of Europe with zonal indices (WI-EI) affecting larger areas than meridional indices (NI-SI), especially during cold seasons. In fact, statistical models including all DIs are able to explain a considerable amount of climate variability, improving in most cases that accounted for by the North Atlantic Oscillation (NAO). Moreover, the combination of DIs is able to capture the non-stationarity of the NAO. In this sense, we report multidecadal fluctuations during the last centuries between a “high-zonal” and “low-zonal” dipole of the NAO, with no discernible trend for the last 330 years. These results show the potential of DIs to study European climate variability and its responses to slowly-varying climate drivers, as well as anthropogenic and natural forcings., This work was supported by FCT through project UID/GEO/ 50019/2013

Trabajo presentado en la European Geosciences Union General Assembly, celebrada en Viena (Austria), del 8 al 13 de abril de 2018, This work examines the exceptional characteristics of the mega-heatwave that affected western and central Europe in June 2017. A novel algorithm was designed to track the spatio-temporal evolution of extreme temperature patterns, being particularly well suited to monitor mega-heatwaves. We show that the intensity, extension and persistence of the June 2017 event were comparable to those of historical European mega-heatwaves but it occurred earlier. The most affected area was south-western Europe, where the event was the longest heatwave of the reanalysis period, and caused the warmest temperatures from daily to seasonal scales, devastating forest fires and human casualties. The peak of the mega-heatwave was characterized by an unprecedented subtropical warm air intrusion due to a record-breaking subtropical ridge that displayed signatures closer to those of July and August ridges. Using the analogue method, we found that the atmospheric circulation was the main triggering factor of the event. Nevertheless, the recorded temperature anomalies were higher than those expected from past flow analogues and thermodynamical changes of the last decades made a substantial contribution to the exceptionality of the event. This episode could be an actual manifestation of summers that are becoming longer and with an earlier occurrence of high-summer mega-heatwaves.

Trabajo presentado en la Stratosphere-troposphere Processes And their Role in Climate General Assembly, celebrada en Tokyo (Japón) del 1 al 5 de octubre de 2018, Major sudden stratospheric warmings (SSWs) represent one of the most abrupt phenomena of the boreal wintertime stratospheric variability, and constitute the clearest example of the stratosphere-troposphere coupling. A good representation of SSWs in climate models is required to reduce their biases and uncertainties in future projections. The ability of models to reproduce these phenomena is usually assessed with just one reanalysis. However, the number of reanalyses has increased in the last decade and their own biases can affect the model evaluation. In this study, we compare the representation of the main aspects of SSWs across reanalyses. The examination of their main characteristics in the pre- and post-satellite periods reveals that reanalyses behave very similarly in both periods. However, the spread of results is wider in the pre-satellite period than afterwards, particularly for the NCEP/NCAR reanalysis. The good agreement among reanalyses is also found for triggering mechanisms, tropospheric precursors and surface fingerprint and all datasets reproduce similarly the specific signatures of wavenumber 1 (WN1) and wavenumber 2 (WN2) SSWs. Differences in blocking activity prior to WN1 and WN2 events between reanalyses are much smaller than between blocking definitions.

Trabajo presentado en la Stratosphere-troposphere Processes And their Role in Climate General Assembly, celebrada en Tokyo (Japón) del 1 al 5 de octubre de 2018

Trabajo presentado en las 5ª Journée Climat et Impacts (2018), celebradas en Orsay (Francia), del 29 al 30 de noviembre de 2018, This work examines the global warming contribution to mega-heatwaves in western Europe. The exceptional June 2017 mega-heatwave (which affected western and central Europe) was used as a case study. This event shows that mega-heatwaves are not only more frequent and intense than in the past, but also are extending to earlier dates. So the associated mega-heatwave impacts (record-breaking temperatures, devasting wildfires, human causalities and economical losses) are extending out of high-summer. Using the analogue method, we found that a record-breaking subtropical ridge was the main triggering factor. However, the thermodynamical changes of the last decades made a substantial contribution to the exceptionality of the event.

Trabajo presentado en la European Geosciences Union General Assembly, celebrada en Viena (Austria), del 8 al 13 de abril de 2018, During 2016-2017, most of Western Europe has suffered a major drought. Associated impacts have been reported for a number of socio-economic sectors. Crops have been severely affected with the resulting reduction in agricultural production, especially for cereals, olives and almonds in Spain and Italy. Devastating fires in southern Europe have led to the evacuation of thousands of people and even human fatalities in Portugal. The energy sector has also been significantly impacted partly due to the drop in hydroelectrical production, which reached a ∼50% decrease in Spain and coincided with minimum wind energy production, rising the electricity bill to historical maxima. The standardised precipitation-evapotranspiration index (SPEI) is used to diagnose this major drought. It is compared to the standardized precipitation index (SPI) in order to highlight the contribution of meteorological variables other than precipitation. The results of the SPEI show that July 2016 - June 2017 has been the most severe 12-month drought of the reanalysis period over Western Europe. Unlike most mega-droughts in Europe, the event is characterized by an unusual spatial pattern, with dry conditions over northern and southern Europe. In fact, several countries such as France, Spain or Germany have experienced a record-breaking drought simultaneously. Nevertheless, the analysis of the SPI, which only accounts for the precipitation deficit, suggests that this has not been the driest event on record. Given the discrepancies between the results found for both drought indices, we focus on analysing individually the variables controlling evapotranspiration to understand their relative roles. Moreover, the atmospheric circulation and moisture sources have been explored during the driest months in order to assess the role of synoptic patterns in the spatio-temporal evolution of the drought.

La portada de la revista ha sido ilustrada por Javier Mariscal. Colaboran en este número 18 de la revista de eldiario.es Yayo Herrero, Ignacio Escolar, y contiene una entrevista a la escritora Naomi Klein. También escriben, bajo la coordinación científica de Fernando Valladares, investigador del CSIC, Miguel Álvarez Cobelas, Marta Peirano, José Luis Pelegrí, Jordi Martínez-Vilalta, Adrián Escudero, David Barriopedro, Antonio Lucio Gil, José Vicente Martí Boscà, Marta Barberá, Alfonso Saiz-López, Darío Pescador, Gonzalo Fanjul, Raúl Rejón. José Luis Gallego y Juan López de Uralde., Necesidad e implicaciones del acuerdo sobre el clima de París (COP21). Aunque se detengan las emisiones hoy, el planeta seguirá calentándose como consecuencia de lo ya vertido a a la atmósfera. La única diferencia estre las cuatro proyeccionessobre Cambio climático es si lo hará 0,9ºC o 5,4ºC. Este artículo ha sido publicado en 'El planeta atormentado', número 18 de la revista de eldiario.es, que reciben todos los socios (https://seguro.eldiario.es/socios/alta.html?redirectUrl=https://www.eldiario.es/&_ga=2.135896817.1999867352.1515399473-1930166648.1507027114).

Trabajo presentado en el EMS Annual Meeting: European Conference for Applied Meteorology and Climatology, celebrado en Dublin (Irlanda), de 4 al 8 de septiembre de 2017, Blocking high pressure systems are large-scale atmospheric circulation patterns with meteorological impacts that vary across regions and seasons, depending on the blocking location, spatial characteristics and temporal length. Blocking episodes are an important component of the intra-seasonal and inter-annual variability at mid-latitudes. We performed a thorough characterization of Euro-Atlantic blocking occurrence within different longitudinal sectors (Atlantic, European and Russian), and a comprehensive analysis of their seasonal and regional impacts in temperature and in precipitation regimes. In order to distinguish high-latitude blocking from other common high pressure systems affecting Europe, namely sub-tropical ridges, a novel ridge detection scheme was developed. Ridges do not require a wave-breaking occurrence as blockings do, although they are frequent precursors of wave-breaking and subsequent blockings. Thus, a novel characterization of the distinctive seasonal impacts associated with sub-tropical ridges occurring in different longitudinal sectors of the Euro-Atlantic region was performed. Finally, an assessment of the dynamical mechanisms behind the temperature and precipitation responses to blocking was performed. This includes for precipitation the role of cyclonic activity (storm-tracks and cut-off lows), moisture transport, and large-scale atmospheric instability, while for temperature the focus was on the role played by horizontal advection, subsidence and imbalances in various radiation budgets. This distinction clarifies that most extreme heat episodes in southern Europe and Mediterranean areas should not be attributed to blockings, but rather to ridges. In central and northern European areas, both regimes are responsible for warm conditions in summer, due to enhanced radiative heating and increased subsidence. During winter, blocking and ridges lead to opposite temperature responses. Blocks reinforce cold northerly advection in their eastern flank, promoting cold winter spells, especially those centered in the eastern Atlantic and western Europe, while mild Atlantic flows associated to ridge patterns result in warmer conditions – in Sousa et al., 2017b, Climate Dynamics. Blocking and ridges are associated with a marked north-south dipole in precipitation anomalies. While blocks force a split of the storm-track, ridges are associated with a stronger zonal flow at higher latitudes. Thus, negative (positive) precipitation anomalies during blocks occur at higher (lower) latitudes. Enhanced atmospheric instability and cyclonic activity south of blocking centers relate with increased rainfall in southern Europe, where torrential regimes significantly contribute to precipitation totals. This dipole reverses during ridges, which lead to dry conditions in southern Europe, and increase moderate rainfall classes northwards under enhanced zonal flow conditions – in Sousa et al., 2017a, Climate Dynamics., Publication supported by FCT- project UID/GEO/50019/2013 – IDL

Trabajo presentado en el Seminario Científico: Computación de Altas Prestaciones y Modelización de Ciencias de la Atmósfera y del Océano, celebrado en Santiago de Compostela (España), en el Instituto de Investigaciones Agrobiológicas de Galicia (IIAG) el 18 de julio de 2017

Trabajo presentado a la XXXVI Reunión Bienal de la Real Sociedad Española de Física, celebrada en Santiago de Compostela (España) del 17 al 21 de julio de 2017., The Late Maunder Minimum (LMM, 1685-1715) denotes the climax of the Little Ice Age (LIA, cf. 1400-1700), which was one of the coldest periods in the last millennium. The LMM is a period of great interest since it was one of the few cold periods in recent centuries that persisted for several decades. Here, we present the first direct instrumental evidence of the daily atmospheric circulation over the eastern Atlantic during the LMM based on wind direction observations taken aboard ships over the English Channel. To do so, we derived two sets of monthly atmospheric circulation indices based on the persistence of the wind direction in the four cardinal directions and in 8-point wind roses for the 1685-2014 period. They provide the longest observational records of the atmospheric circulation to date, allowing us to explore the variability of the atmospheric circulation in a wide range of time-scales. The analysis of these indices indicate that the LMM was characterized by a pronounced meridional circulation and a marked reduction in the frequency of westerlies all year round, as compared to the present (1981-2010). The winter circulation contributed the most to enhance the cold conditions through an overall increase of northerly winds. Nevertheless, our findings also show a LMM more heterogeneous than previously thought, displaying contrasting spatial patterns in circulation and temperature, as well as large decadal variability. Based on the circulation characteristic of each winter, we provide a new observational-based catalogue of winters for the LMM, reassessing the indirect evidences available in the literature about the temperature conditions of the LMM. Our assessment confirms the majority of extremely cold winters documented in the literature, but also uncovers the existence of additional undocumented cold winters and a substantial number of mild winters that had been unnoticed so far. The outcomes also suggest a non-stationarity of the North Atlantic Oscillation (NAO) pattern within the LMM, which has not been reported before, with extremely cold (moderate) winters being associated to negative phases of a “high-zonal” (“low-zonal”) NAO pattern.

Trabajo presentado a la XXXVI Reunión Bienal de la Real Sociedad Española de Física, celebrada en Santiago de Compostela (España) del 17 al 21 de julio de 2017.

Trabajo presentado en la European Geosciences Union General Assembly, celebrada en Viena (Austria), del 23 al 28 de abril de 2017, We present an analysis of the impact that high-latitude blocks and subtropical ridges exert on European air quality. For this purpose, a catalogue of blocks and ridges over the Euro-Atlantic region is used together with a gridded dataset of maximum daily 8-hour running average ozone (MDA8 O3) covering the 1998-2012 period as well as daily average near-surface PM10 observations from the European Environment Agency’s air quality database (AirBase) during 2000-2009. The response of the pollutant concentrations to the location of blocks and ridges with centres in two main longitudinal sectors (Atlantic, ATL, 30˚-0˚ W; European, EUR, 0˚-30˚ E) is examined, with a focus on summer-spring ozone and winter PM10. The impact of blocks and ridges on ozone is regionally and seasonally dependent. In particular, blocks within the EUR sector yield positive ozone anomalies of ∼5-10 ppb over large parts of central Europe in spring and northern Europe in summer. More than 20 % and 30 % of the days with blocks in that sector register exceedances of the 90th percentile of the seasonal ozone distribution over large regions during spring and summer, respectively. The impacts of ridges are subtle and more sensitive to their specific location, although they can trigger ozone anomalies of ∼5-10 ppb in Italy and the surrounding countries in summer, eventually exceeding the EU target value for maximum daily 8-hour mean ozone (120 µg m-3, approximately 60 ppb). During winter, PM10 over large regions of Europe presents completely opposite responses to blocks and ridges. The anticyclonic circulation associated with EUR blocking implies a collapse of the boundary layer as well as reduced wind speeds and precipitation occurrence, yielding large positive anomalies which on average exceed ∼10 µg m-3 over the whole continent, reaching ∼50 µg m-3 at some locations. Conversely, the enhanced zonal flow around 50˚-60˚ N and the increased occurrence of precipitation over northern-central Europe on days with ATL ridges favour the ventilation of the boundary layer and the impact of washout processes, reducing PM10 concentrations on average by around 7 µg m-3 and up to 45 µg m-3 for some locations. Finally, we have found that both patterns can partly control the interannual variability of winter mean PM10 in southern Germany, where this pollutant presents a correlation of 0.83 with EUR blocks and -0.65 with ATL ridges. The findings reported here can be exploited in the future to evaluate the modelled responses of air quality to circulation changes within chemical transport models (CTMs) and chemistry-climate models (CCMs).

Trabajo presentado en el 5th PAGES Open Science Meeting, celebrado en Zaragoza (España), del 9 al 13 de mayo de 2017, Last Millennium (LM) simulations are crucial to asses the relative roles of internal versus forced variability, the responses to natural and anthropogenic forcings, and the climate sensitivity of the Earth system. However, many open questions still remain regarding the ability of these models to reproduce the LM, since reconstructions suggest more complex patterns than those obtained from LM simulations. Among other factors, these discrepancies could be due to uncertainties in the forcings and/or in model physics. In particular, recent studies have demonstrated the influence of the stratosphere on the surface with important implications in future climate projections. However, most models employed in PMIP3 did not have a well-resolved stratosphere, thus neglecting its potential contribution to explain anomalous periods before the industrial era. PALEOSTRAT is a project funded by the Spanish government which investigates the impact of the stratosphere on the climate of the LM (850-1850 CE). This is addressed by means of a suite of LM simulations with the CESM model which only differ in the representation of the stratosphere, so that their comparisons will provide insights into the impact of the middle-atmosphere on the surface climate. Uncertainties related to external forcings will also be assessed by comparing business-as-usual LM model simulations with LM runs forced with novel external forcing histories and model implementations. This talk will summarize the influence of the stratosphere on the surface climate, its potential role in explaining past changes, the main objectives of PALEOSTRAT and its experimental design., PALEOSTRAT (CGL2015-69699-R) is funded by the Spanish Ministry of Economy and Competitiveness (MINECO)