Flood Patterns in a Mediterranean Region (1301–2012) Printer-friendly Version Interactive Discussion the Role of Climatic Factors in Evolving Flood Patterns in a Mediterranean Region (1301–2012) Flood Patterns in a Mediterranean Region (1301–2012) Printer-friendly Version Interactive Discussion

Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Abstract Data on flood occurrence and flood impacts for the last seven centuries in the northeast Iberian Peninsula have been analysed in order to characterise long-term trends, anomalous periods and their relationship with different climatic factors such as precipitation , general circulation and solar activity. Catastrophic floods do not present a 5 statistically significant trend, whereas extraordinary floods have seen a significant rise, especially from 1850 on, and were responsible for the total increase in flooding in the region. This rise can be mainly attributed to small coastal catchments, which have experienced a marked increase in developed land and population, resulting in changes in land use and greater vulnerability. Changes in precipitation alone cannot explain the 10 variation in flood patterns, although a certain increase was shown in late summer–early autumn, when extraordinary floods are most frequently recorded. The relationship between North Atlantic circulation and floods is not as strong, due to the important role of mesoscale factors in heavy precipitation in the northwest of the Mediterranean region. However it can explain the variance to some extent, mainly in relation to the 15 catastrophic floods experienced during the autumn. Solar activity has some impact on changes in catastrophic floods with cycles related to the Quasi-Biennial Oscillation and the Gleissberg solar cycle. In addition, anomalous periods of high flood frequency in autumn generally occurred during periods of increased solar activity. The physical influence of the latter in general circulation patterns, the high troposphere and the 20 stratosphere, has been analysed in order to ascertain its role in causing floods.


Introduction
Floods are the natural hazard with the largest socio-economic impact in the world, and they are responsible for the highest number of deaths and the most damage caused by natural hazards worldwide (Munich Re, 2006;IPCC, 2012).This problem is exacerbated by the acceleration of the hydrological cycle and other extreme events, which Figures are considered a consequence of climate change.Some studies show that water extremes (floods and droughts), which currently have a return period of 100 yr, may recur every 10 to 50 yr by 2070 in some regions in Europe (Lehner et al., 2006).Precipitation intensity is also projected to increase in some regions, leading to more flooding (Dankers and Feyen, 2009).However, keeping the previous IPCC assessment report in mind (IPCC, 2013), these results are not representative enough for a full understanding of the impact of climatic change on future floods, due to the complexity of the different factors involved in causing floods and the impact they have, as well as significant questions about future changes to precipitation.
With regard to present trends in precipitation extremes and floods, the latest IPCC report on extremes (IPCC, 2012) states that there is "limited to medium" evidence available to assess climate-driven changes to the magnitude and frequency of floods on a regional scale, with "low agreement" evidence and "low confidence" on a global scale for the signs of said changes.In the case of the Iberian Peninsula (IP) there are some controversial results, as for the rest of the Mediterranean area, with regard to signs of changes in precipitation.This is due to the diverse periods considered by different authors, the regions in question, and the varying approaches taken as the methodology applied (Barrera and Llasat, 2004;Llasat and Quintas, 2004;González-Hidalgo et al., 2009;Pryor et al., 2009;Turco and Llasat, 2011).Some of them point to a decrease in the intensity of daily precipitation and an increase in the number of days with light rain (García et al., 2007;Rodrigo and Trigo, 2007;Rodrigo, 2010;López-Moreno et al., 2010) for much of the Iberian Peninsula.On the contrary, other studies show that less-intensive rainy days are more frequent and intensive precipitation episodes have increased along the Mediterranean coast (Alpert et al., 2002;Goodess and Jones, 2002), but that these results cannot be generalised (Altava-Ortiz et al., 2010).These findings are consistent with the "low-agreement" findings of the IPCC (2012), which is notable in the Mediterranean region where mesoscale mechanisms and convective precipitation play a major role in the temporal and spatial distribution of precipitation.Introduction

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Full Apart from the low significance and agreement in precipitation trends, the complexity associated with flooding requires further analysis of the relationship between changes to floods and rainfall, and the evolution of climatic and hydrologic parameters (Di Baldassarre et al., 2009;Heine and Pinter, 2012;Remo et al., 2012;Hall et al., 2013).Most of these authors stress the importance of changes to land use (Naef et al., 2002), or possible changes in runoff coefficients (Sivapalan et al., 2005;Mouri et al., 2011).In short, and as indicated by Mertz et al. (2012), changes to the river itself, to the basin and to the atmosphere should be considered as possible physical causes for the changes to the flood regime.The problem is exacerbated when flood series are built from proxy data (i.e.flood impacts), where case factors such as vulnerability, exposure and perception play a more important role.For instance, the trends observed in changes to floods in the northwest of the Mediterranean Region could be related principally to changes in vulnerability and land use, and also to changes in perception and exposed assets (Barrera et al., 2006;Barredo et al., 2012;Llasat et al., 2013).
In this context, analysis of long-term homogeneous flood series and the corresponding causes is necessary in order to have a better understanding of how they evolve.That is the aim of this paper.Subsequently, based on early works on the evolution of floods in the northeast of the IP from the 14th century until 2002, the historical flood database of Catalonia (northeastern Spain) has been updated until 2012 to review and identify any significant long-term trends or anomalies, and to analyse the potential relationship with climatic and non-climatic factors.This database, together with data on precipitation and information on solar activity and general circulation, is presented in Sect. 2. The methodology on building flood index series and carrying out statistical analysis on the same is explained in Sect.3. Finally, the main results obtained on flood evolution and the potential relationship between the same and different climatic factors is presented and discussed in Sect. 4. Introduction

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Full 2 Area of study and data Catalonia (northeast Spain) is characterised by a complex topography that has a considerable impact on the region's climatology and atmospheric circulation patterns: there are two mountain ranges with average heights of around 500 m a.s.l.(littoral zone) and 1500 m a.s.l.(pre-littoral zone), located parallel to the coastline, and the Pyrenees, with summits above 3000 m a.s.l.(Fig. 1).These orographic factors, together with the influence of the Mediterranean Sea and the associated Mediterranean Air Mass (Jansà, 1997), as well as the Atlantic influence on the northwestern side of the region, produce high climatic and meteorological contrasts between the different areas.Subsequently, precipitation is characterised by significant spatial and temporal variability.Annualmean rainfall varies from 400 mm in the south to 1300 mm in the north, while extreme daily values can surpass 300 mm, mainly in areas located near the coast or in the Pyrenees.
Heavy rainfall is usually due to convective precipitation normally caused by mesoscale systems and multicellular structures that occur in the late summer, autumn and spring.While isolated thunderstorms associated with convective events are a typical feature of summer weather, convection embedded in stratiform precipitation associated with slight convective events is more frequent during late autumn and winter (Llasat, 2001;Rigo and Llasat, 2004;Barnolas et al., 2010).The annual cycle of convective rainfall shows that maximum levels fall between May and November, with the highest rainfall in August at 64 % of convective precipitation (Llasat et al., 2007).
As a result, this region frequently experiences floods.More than 40 % of municipalities in Catalonia have a high or very high flood risk, according to INUNCAT, a Civil Protection Plan covering Flood Risks in Catalonia (DGPC, 2012).Generally, flash floods have an impact on coastal torrential basins and cause some level of damage.They are associated with very convective and locally concentrated heavy rainfall where accumulated precipitation does not surpass 100 mm.On certain occasions, heavy rainfall produced by multicellular or mesoscale systems can be more extensive in terms of Introduction

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Full duration and area covered, producing more than 200 mm in less than 3 h or surpassing 400 mm in 24 h.The coastal fringe, where the majority of the population is concentrated, is the most flood-prone area due to the presence of numerous small torrential catchments, delta plains and river or stream mouths (Barnolas and Llasat, 2007).In these populated areas the relationship between human activity and flooding (in terms of the environment, land use changes, vulnerability, etc.) is very complex.In addition, some of the catchments are characterised by a non-permanent flooding regime, which means it is not possible to gauge data on discharge flows.
In this paper we have used flood chronologies for the Ebro, Segre, Ter and Llobregat rivers, the Maresme and Barcelona regions (with non permanent flow), which are representative of the key climatic and hydrological regions within Catalonia (Fig. 2).Data measured from the 14th century until 2002 was provided within the framework of the SPHERE project (EVG1-CT-1999-00010, Barriendos et al., 2003;Llasat et al., 2005).This database has been updated to cover until the year 2012, following the Ph.D. by Barrera-Escoda (2008) and research carried out by the authors until the present day.
Flood data has been obtained from documentary sources (from the 14th to the 20th century), and newspapers and technical reports (from the end of 19th century).Flood data basically consists of the recorded date for the flood and details of the damage caused.The locations of all of the flood series are shown in Fig. 2, while the main characteristics of each are shown in Table 1.
Instrumental data is only available for the last two centuries.The monthly (from 1786 onwards) and daily (from 1854 onwards) rainfall series for Barcelona (Barriendos et al., 1997;Barrera-Escoda, 2008) have been updated until the year 2012.Taking into account its location and climate, Barcelona is a good representative for precipitation behaviour along the Catalan coastal region.This series is the longest set of instrumental data available for rainfall in the western Mediterranean.A representative annual mean average areal precipitation series for the North-Eastern Iberian Peninsula (NEIP), with records for over 100 yr, has been also used to complete the analysis.A seasonal reconstruction  for the North Atlantic Oscillation (NAO) developed by Luterbacher et al. (2002) was used in order to analyse the relationship between floods and general circulation.The NAO is a large-scale seesaw in atmospheric mass between the subtropical high and the polar low.It is also the dominant mode of winter climate variability in the North Atlantic region ranging from central North America to Europe and covering much of northern Asia, although it has an impact on every season.Therefore, it is a good indicator for general circulation in Europe (Hurrell et al., 2003).
Finally, solar activity is taken into account using annually resolved 10 Be measurements for the past 600 yr (1389-1994) taken from the North Greenland Ice Sheet Project (NGRIP) 1997 S2 ice core (Berggren et al., 2009).This data is used to study the possible relationship with flood frequency.It is currently the only available series for annually resolved solar activity and covers almost the same period as the flood data.

Methodology
The criterion for flood classification (Barriendos et al., 2003;Llasat et al., 2005) is as follows: -Ordinary or small floods (ORD): do not cause rivers to overflow their banks, cause some damage if activities are being carried out in or near the river at the time, and cause minor damage to hydraulic installations.
-Extraordinary or intermediate floods (EXT): cause the overtopping of riverbanks, inconveniences in the daily life of the local population, and damage to structures near the river or torrent with possible partial destruction.
-Catastrophic or large floods (CAT): cause the overtopping of riverbanks and lead to serious damage to or destruction of hydraulic installations, infrastructures, paths and roadways, buildings, livestock, crops, and so on.Introduction

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Full This classification allows us to compare historic floods and those that were documented with instrumental records.It also matches similar criteria used in other European countries such as in the study by Sturm et al. (2001).
For each type of flood and location, a flood frequency index has been compiled to show the annual scale.Ordinary floods have not been considered due to overall heterogeneity (most of them were not recorded throughout history).Each flood series is normalised by taking into account the annual-mean value and standard deviation of flood occurrence for the 1901-2000 period as follows: where n is the yearly flood occurrence; m is the annual-mean value; and s is the standard deviation.Finally, in order to show the changes in the flood indices clearly, all the values are smoothed by low-pass Gaussian filters of 11 and 31 yr (Llasat et al., 2005;Barrera et al., 2006).A representative flood index for each category and for Catalonia as a whole has been developed by averaging out the normalised flood series.
Temporal trends are calculated using the flood index series (not smoothed) with a linear regression, and the significance level is tested following a Monte Carlo method (Lizevey and Chen, 1983).Temporal correlations are calculated using Pearson's linear coefficient, and are applied to the raw data without being smoothed.Spectral analysis is carried out by means of Tukey's power spectrum with a confidence level of 95 %, computed using unsmoothed data.The anomalous period was defined by only taking into account the catastrophic flood index series for Catalonia, smoothed using a lowpass Gaussian filter of 31 yr.The anomalous values were those with a flood index greater than or equal to 0.1 (Llasat et al., 2005).
The relationship between flood evolution and climatic factors (solar activity and general circulation) is analysed through Pearson's linear coefficient applied to data smoothed by a 31 yr low-pass Gaussian filter, in order to discover the relationship between long-term changes.Introduction

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Flood variability
Inter-annual flood distribution shows that autumn is the season with the greatest number of floods (54 %), followed by summer (21 %), with the highest number in October (21 %), followed by September (20 %) (Fig. 3).Catastrophic floods are mainly concentrated between September and November, while August also records a high frequency of extraordinary floods (23 % of catastrophic floods are recorded during the summer).These summer events are usually associated with coastal flash-floods produced in short, torrential watercourses, which cause extraordinary damage (Llasat et al., 2013).Barcelona is a good example, with a flood percentage of 29 % during the summer.
The temporal evolution of the annual catastrophic flood index for Catalonia (Fig. 4a) shows a greater inter-decadal variability up until the 19th century than for the last century alone (σ = 0.26 vs. σ = 0.22).This evolution also shows different periods of high and low catastrophic flood frequency.The periods with the lowest frequency of catastrophic floods are the mid-14th and mid-17th century, the beginning of the 18th century and the end of the 20th century.On the other hand, seven different anomalous periods of high flood frequency are highlighted in Fig. 4a: (1) 1325-1334 (Late Middle Age Oscillation), (2) 1541-1552 (Mid-16th century Oscillation, Brázdil et al., 1999), (3) 1591-1623 (Beginning of Little Ice Age Oscillation, LIA, Jones et al., 2001), (4) 1725-1729 (what we propose calling the "Enlightened" Oscillation), (5) 1761-1790 ("Maldà" Oscillation, Barriendos and Llasat, 2003), ( 6) 1833-1871 (End of LIA, Llasat et al., 2005), and finally (7) [1895][1896][1897][1898][1899][1900][1901][1902][1903][1904][1905][1906][1907][1908][1909][1910] (what we propose calling the "Modernist" Oscillation).Similar anomalous periods were found in other flood series in central Europe (Glaser and Stangl, 2004), southern France (Lang et al., 2000;Lang and Coeur, 2002) and northern Italy (Camuffo and Enzi, 1996), especially the 3rd, 5th and 6th oscillations.1790 and 1790-1840, which match to a certain extent with the 2nd, 3rd, 4th, 5th and 6th oscillations found in Catalonia.Trend analysis of temporal evolution for flood indices in Catalonia shows that catastrophic floods do not present a statistically significant trend, whereas extraordinary floods have seen a significant increase, especially from 1850 onwards (Fig. 4b).Extraordinary floods are responsible for the total increase in flooding in Catalonia (Fig. 4c).Due to the diversity of the catchments studied in this paper, we have separated them into inland basins (Ebro, Segre and Ter catchments) and coastal basins (Llobregat estuary, and the Barcelona and Maresme regions) for a more in-depth analysis.This means Fig. 5 shows how small and torrential coastal basins have seen a significant increase in flood frequency (+0.11/100 yr), which is two times or more the magnitude of those in inland basins.Thus, coastal basins are mainly responsible for the increase in flood frequency, especially for extraordinary floods (+0.08/100 yr).In particular, the detailed analysis of urban floods in the city of Barcelona from 1351 onwards shows a significant trend (+0.26/100 yr) that is mainly due to the increase in extraordinary floods in the summer (+0.13/100 yr).This trend is due to the abrupt change that occurred in the middle of the 19th century (Fig. 6), which could either be related to the end of the LIA and a possible corresponding increase in convective precipitation, or the notable urban changes in the city (Barrera et al., 2006).
The large reservoirs built to generate electricity and supply water for agriculture in the Oscillation (QBO ∼ 28-29 months ∼ 2.33-2.42yr), which is present in almost all temporal series involving climatological and meteorological variables (Baldwin et al., 2001).Two less significant periodicities of 4.2 and 2.2 yr are also shown, which could be also associated with the QBO.

Floods vs. rainfall
Autumn precipitation contributes more to annual precipitation less than autumn floods contribute to the annual total, but it is nonetheless the rainiest season in most of Catalonia.In Barcelona for example, autumn precipitation represents 37 % of annual precipitation, followed by spring (25 %), for the period 1786-2012.Summer is the driest season, contributing to just 18 % of total annual precipitation in Barcelona.However, summer precipitation is mainly convective (nearly 65 % in August, see Llasat, 2001), and is usually associated with thunderstorms or localised heavy rain that produces flash floods in coastal water streams.
On an annual scale, the temporal evolution of precipitation anomalies in Barcelona since 1786 and the same variable for areal precipitation in the NEIP do not show any significant long-term trends (Fig. 8).These same findings are shown for changes to annual maximum daily precipitation in Barcelona (Barrera et al., 2006), and for the number of days that exceed different daily precipitation thresholds (20, 30, 50 and 100 mm in one day; Fig. 9).Subsequently, when considering this common non-significant trend, we can state that annual extreme precipitation has not increased.These results can be extrapolated to the entire Catalan coastal region, although the precipitation series is smaller (Llasat et al., 2009).Similarly, Turco and Llasat (2011) analysed the evolution of extreme precipitation through the ETCCDI (Expert Team on Climate Change Detection and Indices, Zhang et al., 2011), in Catalonia from 1951 to 2003, and did not find any significant increase in total precipitation, neither in the highest precipitation amount in a five-day period (RX5DAY), or in the mean precipitation amount on a wet day (SDII) or in the proportion of heavy rainfall (R95p).Working on a seasonal scale and focusing on the city of Barcelona, some trends are shown if the confidence level is reduced to 90 %.

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Full In this case, a positive trend of +0.12 mm yr −1 can be found for summer precipitation in the city, which corresponds to a certain extent with the increase in extraordinary floods in the summer (Fig. 6).These results match the increase in precipitation in August in the city, with a significant value of +2.73 mm yr −1 for the 1850-1984 period, as shown by Altava-Ortiz et al. (2010).
The temporal correlation between total annual floods and the number of days exceeding thresholds of 20 mm d −1 , 30 mm d −1 , 50 mm d −1 and 100 mm d −1 is relatively low for the 1854-2012 period, which shows the most significant correlation for the number of days exceeding 50 mm (+0.24).Barrera et al. (2006) outlined that urban growth in the city of Barcelona has had an impact on flood vulnerability and the frequency of extraordinary floods from the 14th century onwards, especially from the late 19th and early 20th century.For this period, the correlation between rainfall thresholds and floods is at a low.On the other hand, from the 1940s onwards the correlation is higher, reaching values of +0.56 for a threshold of 50 mm.In comparison, the correlation between the previous thresholds and the catastrophic flood index also shows the same pattern for the city of Barcelona.The correlation between annual precipitation and the annual index for different types of floods (1786-2012) is also very low, reaching the highest value for the total number of floods (r = 0.19).However, it is interesting to note that there is a common period with a high increase in both precipitation and flood frequency (1833-1871, the end of the LIA, see Fig. 8a).In the case of catastrophic flood index the highest correlation is recorded for 1862-1892 (r = 0.59).
The end of the LIA is also characterised by an increase in both flood frequency and precipitation throughout Catalonia.The comparison between the regional evolution of annual-mean areal precipitation for the NEIP (1898-2012; Fig. 8b) and the different flood indices for Catalonia, show higher correlations than for the city of Barcelona, but they are nonetheless still low.The highest correlation is shown with the extraordinary flood index (r = 0.39).Introduction

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Full As a result, rainfall patterns alone cannot explain the changes in floods in the region.Extraordinary floods are more frequently related to flood vulnerability and land-use, while catastrophic floods are associated with climatic factors.

General circulation
While there is no single atmospheric synoptic pattern associated with floods in Catalonia, the analysis of recent and past floods suggests a predominant southern circulation for autumn floods.This implies a relation with a negative NAO phase (Trigo et al., 2004;Llasat et al., 2005), while summer events would be more closely associated to northern circulation in low levels in the region, with a meso-low in eastern Catalonia.In this latter instance the NAO phase would be more likely to be positive when taking into account the position of the Azores Anticyclone during the summer.However, the correlation between NAO and precipitation in the Mediterranean region for the summer season is very low and not significant, due to the important role of local convective developments.For this reason, it does not make sense to analyse the potential relationship between NAO and summer floods.
Following the previous discussion, the influence of general circulation on catastrophic floods in Catalonia has only been analysed for the autumn season (accounting for 55 % of all catastrophic floods, see Fig. 3), by means of the NAO reconstruction set out by Luterbacher et al. (2002) for the years 1500 to 2000 (Fig. 10).The long-term temporal correlation between these variables is fairly low with a significant value of +0.16.The low level of correlation is not unusual because some floods are isolated local events produced by short heavy rainfall that can take place under positive or negative NAO phases, depending more on mesoscale features than synoptic patterns.Besides this, the frequency of flood events is extremely low, and this affects the significance of any potential correlation.On the other hand, the relationship between the NAO and precipitation may change over time (Knippertz et al., 2003;Trigo et al., 2004;Beranová Introduction

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Full and Huth, 2008).For instance, the "Maldà" oscillation was an unusual period with both floods and droughts, mainly associated with a positive NAO phase (Barriendos and Llasat, 2003).However, the most important peak in floods was at the beginning and end of the LIA, and this so called "Modernist" oscillation corresponds to a negative NAO phase.The beginning of the LIA took place during the minimum value period of the NAO reconstruction, which shows a long-term negative phase for the entire duration of the LIA.This behaviour does not correspond at all with the rest of the series, which never shows such negative values.In addition, some periods of relatively high flood frequency occur within periods of abrupt changes to the NAO.

Solar variability
As previously mentioned, the most significant oscillations are shown in different basins in Europe and are correlated with the main phases of the LIA (Camuffo and Enzi, 1996;Brázdil et al., 1999;Pfister, 1999;Benito et al., 2003;Llasat et al., 2005;Glaser et al., 2010).The first oscillation (1325-1334; Fig. 4a) is situated between the Wolf Minimum and the Late Middle Age Solar Maximum (Vaquero, 2004).The most significant period of high flood frequency (Beginning of LIA Oscillation) was recorded near maximum solar activity levels that started in 1620 at the beginning of the LIA.During the Maunder Minimum (1640-1710) flooding activity decreased; the Dalton Minimum (1795-1830) coincided with the Maldà Oscillation, which was characterised by high climatic irregularity accompanied by hydrologic extremes.The maximum period of flood frequency for the end of the LIA corresponds with the highest levels of solar activity recorded between 1849 and 1875.
On the other hand, in Central Europe (Brázdil et al., 1999), the periods with the most flooding were recorded in the mid-16th century and in the late Maunder Minimum (1675-1715), corresponding to periods with less solar activity.This suggests that the regional component is very important, and that there is a more significant relationship between solar activity and higher flood frequency in Mediterranean countries than in Central Europe.Other authors (Borgmark, 2005;Versteegh, 2005;Wilhelm et al., 2012) 9158 Introduction

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Full also attribute the periodicities found in many geological flood records to extraterrestrial forcings, such as centennial and decadal solar cycles.
Figure 11a shows a comparison of the annual evolution in solar activity (1389-1994) taken from the 10 Be concentration, and the catastrophic flood index series for Catalonia (1301-2012). 10Be concentration is high for periods of low solar activity and low for periods of high solar activity.The long-term temporal correlation between them (data smoothed by a 31 yr low-pass Gaussian filter) is −0.32,where a negative value would imply that periods with lower 10 Be concentration (greater solar activity) would be related to periods with higher flooding activity.If the analysis focuses solely on autumn (SON; Fig. 12b), the long-term temporal correlation between solar activity and floods is higher, reaching a value of −0.42.It is also interesting to note that a significant change in flood occurrence is associated with transient periods between solar maxima and minima, and periods of solar maximum.In addition to this observation, we should mention that the reconstruction of solar activity from 10 Be concentration does not give the exact dates of the maxima due to dating uncertainties (Berggren et al., 2009).
The possible link between floods and solar activity is somewhat controversial (Benito et al., 2004;Vaquero, 2004).If we consider the accepted hypothesis that floodproducing mechanisms in the past are similar to those in the present, then marked clusters of historical floods could be associated with changes to the climatic pattern on both a regional and global scale, and, in turn, to changes in solar activity.Although this potential relationship is still a challenge for the scientific community and merits further research, some studies have revealed the influence of solar activity in North Atlantic atmospheric and ocean circulations (Moffa-Sánchez et al., 2014).Following on from this, solar activity would have an impact on the development and trajectory of Atlantic perturbations that could arrive to Europe; low solar irradiance would promote the development of frequent and persistent atmospheric blocking events, with a quasi-stationary high-pressure system in the eastern North Atlantic, which would modify the flow of westerly winds.This kind of pattern could be identified by a positive NAO phase, and would make it more difficult for low-pressure systems and the associated perturbations Introduction

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Full to arrive in southern Europe.On the other hand, this would favour their arrival in central and northern Europe.Given that floods produced by heavy rainfall are associated with different circulation patterns, the effect of solar variability could change from one region to another.Furthermore, maximum solar activity is associated with low intensity cosmic rays in the stratosphere, which results in greater ozone production in some regions and subsequent warming (Ermolli et al., 2013).Besides the influence of solar activity on general circulation and winds, this differential warming of the stratosphere might influence the development of potential vorticity and the dynamics near the tropopause.Therefore, it could have an important role in heavy rainfall.

Conclusions and discussion
A reviewed flood index series for twelve places in the northeastern Iberian Peninsula (1301-2012) has been updated for this paper.Seasonal distribution shows that catastrophic floods are mainly concentrated in the autumn, while extraordinary floods generally occur in the late summer and early autumn.Autumn is the rainiest season, while summer is the driest.The latter season is characterised by the contribution of convective precipitation to total precipitation, usually caused by thunderstorms or local heavy rainfall giving way to flash floods.There are no significant trends for the oldest available precipitation series in Catalonia, at either extreme.Although the flood index series does not show any notable trends for catastrophic floods (the most severe ones), there is a statistically significant trend for extraordinary floods, which implies that a significant increase in the total number of floods has been found.This increase is mainly associated with the floods recorded in small and torrential basins located near the coast (+0.11/100 yr), and associated with extraordinary floods.In the case of urban floods in Barcelona since 1351, the flood index trend is +0.26/100 yr, mainly due to the contribution of extraordinary floods in the summer.This trend can be explained by the abrupt change that occurred in the mid-Introduction

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Full 19th century, which is probably rooted in both climatic and human causes.Besides this, a significant trend in summer precipitation (+0.12 mm yr −1 ) has been discovered, albeit with a lesser significance (90 % confidence level).Notwithstanding, the increase in extraordinary floods in small coastal basins is mainly related to human activity, which implies a significant change in flood vulnerability and land use, as was recently stated by the IPCC (2012).These basins have experienced a marked increase in developed land over the last century, and especially over the last 30 yr.
There is some correlation (+0.24) between the annual total flood index and the number of days exceeding 50 mm in Barcelona, which rises to +0.56 when the period of study is reduced to 1940-2012.The correlation between annual precipitation and the annual catastrophic flood index (1786-2012) is low (r = 0.17), but it changes over time with the highest values shown for 1862-1892 (r = 0.59).
There are seven different anomalous periods of high catastrophic flood for Catalonia: (1) 1325-1334 (Late Middle Age Oscillation), ( 2 The correlation between the NAO and the catastrophic flood index evolution in autumn, from the year 1500 to 2000, is fairly low, with a value of +0.16.This value is not representative due to the correlation changes over time: in the second half of the 17th century it reaches +0.54 while in its first half of the same century it reaches just −0.35.This is not unusual in itself, given that the present correlation between the NAO and precipitation in this specific region and season does not reach −0.50.This is due to the important role of Mediterranean circulation, as well as mesoscale factors.However, the most significant flood oscillations occur at the beginning and end of the LIA, and this so-called "Modernist" oscillation coincides with a strong negative NAO phase, as for the last anomalous period in the 20th century.Introduction

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Full Spectral analysis shows two key periodicities for the annual catastrophic flood index.The first one (71 yr) is related to the Gleissberg solar cycle, and the second one (2.6 yr) is related to the Quasi-Biennial Oscillation.Long-term temporal correlation (−0.32, and −0.42 if only the autumn season is taken into consideration) between solar activity calculated using 10 Be concentration and the catastrophic flood index series , shows that major solar activity is associated with periods of higher flooding activity.Temporal evolution shows that the first anomalous period was recorded near maximum solar activity, starting in 1620 at the beginning of the LIA.The sixth such period is shown at the end of the LIA, in the mid-19th century.The maximum for the Maldà Oscillation (which also recorded an increase in droughts) coincided with the Dalton Minimum, and was characterised by a predominantly positive NAO phase.Recent studies have revealed the possible influence of solar activity through changes to solar irradiance and changes in cosmic rays and solar particles as they arrive in the stratosphere.In the first case, changes in solar irradiance over the North Atlantic would be amplified through atmospheric feedbacks including the Atlantic Meridional Overturning Circulation, which would in turn affect the formation of persistent atmospheric blocking events.The latter factor would also affect the predominant circulation patterns (i.e.NAO), with the consequent differential regional influence for heavy precipitation.The maxima of solar activity would be associated with less frequent blocking, a negative NAO phase, and an increase in heavy rainfall in southern Europe.Besides this, less interaction between cosmic rays and the ozone in the stratosphere during periods of maximum solar activity would increase ozone presence, diminish UV radiation arriving on the Earth surface, and increase the stratospheric temperature in some regions, with a consequent impact on the dynamics of the high atmosphere (wind circulation, potential vorticity, etc.).
Although aspects related to vulnerability, exposure and changes in land use have been found to be responsible for an increase in floods (IPCC, 2012), it is crucial to have a deeper knowledge of the evolution of heavy rainfall.Additionally, a daily reconstruction of general circulation indices should be developed in order to improve Introduction

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Full  Full Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Most recently, Glaser et al. (2010) have identified four common anomalous periods of high flood frequency for central and eastern Europe: 1540-1600, 1640-1700, 1730-Discussion Paper | Discussion Paper | Discussion Paper | main drainage basins in Catalonia during the second half of the 20th century (between the 1960s and 1970s) have probably lessened the flood hazards in the Inland Basins.Elements such as further mitigation measures, changes in land use, exposed assets and climatic factors should be also considered.A larger population living in flood-prone areas and exposed assets in coastal regions could be one of the key factors responsible for the rise in extraordinary floods in the region.Spectral analysis applied to the annual catastrophic flood index series for Catalonia shows two main significant periodicities (Fig. 7) of 71 and 2.6 yr.The first is related to the Gleissberg solar cycle (∼ 70-100 yr) and the second to the Quasi-Biennial Introduction Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | ) 1541-1552 (Mid-16th century Oscillation), (3) 1591-1623 (Beginning of LIA), (4) 1725-1729 (what we propose calling the "Enlightened" Oscillation), (5) 1761-1790 ("Maldà" Oscillation), (6) 1833-1871 (End of LIA), and finally (7) 1895-1910 (what we propose calling the "Modernist" Oscillation).The majority are also shown for other Mediterranean regions.

Table 1 .
Main characteristics of the updated flood chronologies: basins, locations, sub-basin surface at the location, temporal coverage and the number of extraordinary (EXT), catastrophic (CAT) and total (TOT = EXT + CAT) floods.