It has been well known that extreme precipitation events in Portugal occurring during the extended winter months (October-March) are often associated with landslides and flooding in the major river basins [1
] and, with flash floods in small watersheds or urban areas [4
] which can produce socio-economic impacts. Thus, in order to prevent and manage the associated effects and risks, there is a clear need for better characterization of the associated extreme meteorological metrics and the nature of likely unusual strong atmospheric driving mechanisms.
The characterization of past floods and landslides with important human consequences in Portugal since 1865 to present has been undertaken recently, within the context of the DISASTER project [1
]. Using this database, some event specific studies [1
] have been published with the purpose of characterizing the respective meteorological drivers and the related weather circulation conditions present at both the meso- and synoptic-scales. Flood and landslide DISASTER events with significant economic and human impacts are usually associated with precipitation events co-located with specific Circulation Weather Types (CWTs) and sometimes combined with Atmospheric Rivers (ARs). CWTs are composite-averaged characteristics of the synoptic scale patterns of circulation occurring over a given region of the globe, estimated using a set of indices associated with the direction and vorticity of the geostrophic flow. Several studies have shown that daily precipitation in Portugal in winter is strongly related to the occurrence of the three wettest CWTs, namely the cyclonic (C), south-westerly (SW) and westerly (W) types. These three CWTs occur on average 20% of the winter season days but are responsible for more than 50% of the winter mean monthly precipitation [7
]. Most of the precipitation is produced by just a few CWTs that contribute to a large percentage of the mean monthly precipitation in most areas of the Iberian Peninsula. Their efficiency, however, can vary, with the Cyclonic and the Directional types (those with a western component) being the most efficient WTs [8
In addition, ARs are often responsible for extreme precipitation over the Iberian Peninsula [9
], being frequently associated with well-known DISASTER events, such as the December 1909 record flood in river Douro [1
]. ARs are transient filamentary regions of highly concentrated water vapour affected by strong low-level winds that occur within the warm conveyor belt of extratropical cyclones. These moisture transport structures typically occur as long (about 2000 Km) and narrow (300–500 km) bands of enhanced water vapour flux at the lower troposphere and are responsible for most of the water vapour transported meridionally across the mid-latitudes, as well as for most of the poleward water vapour transport [10
]. Additionally, due to their narrow nature, the ARs latitudinal location over the Iberian Peninsula is highly determinative of their contribution to extreme daily precipitation events over Minho, Douro and Tagus basins [9
In this work, we have selected the top ranked event in the DISASTER database regarding the total number of affected people not only in Portugal but also in the Tagus basin (18,578 and 9861, respectively), that occurred in 5–16 February 1979. This extreme event had a major socio-economic impact in Portugal when compared with all the DISASTER events that occurred during the entire 150-year long period of data spanning between 1865–2015. Despite the relevance of the February 1979 major floods in the lower section of the Tagus river basin, this event was never evaluated in detail, to the best of our knowledge, in terms of the human impacts assessment, characterization of the spatial and temporal extent of the precipitation event, and the atmospheric circulation pattern characteristics that led to its occurrence.
Accordingly, this work has two main goals: (1) to perform a comprehensive characterization of the socio-economic impacts and spatial distribution of the February 1979 hydro-geomorphologic DISASTER event over Portugal and (2) to analyse the atmospheric circulation conditions associated with such an extreme event and the spatial distribution of the precipitation anomalies over Western Iberia.
The 5–16 February 1979 event is the top ranked in the DISASTER database (since 1865) in terms of human impact for each one of the three Portuguese main hydrographic regions affected: Douro, Mondego and Tagus. This event was mainly characterized by progressive flooding occurring along the lowland sectors of each main river’s basin, with the majority of the cases and affected people concentrated along the Tagus river floodplain. The authors are aware that larger floods occurred in recent history, including record floods in Douro [1
] and Tagus [2
]. However, to the best of our knowledge, the 1979 event was the most extreme flood in the Tagus basin since major dams were built in these large international river basins, particularly between the 1950s and 1970s [24
]. The 1979 event was responsible for 62 damaging floods and five damaging landslides in Portugal. However, it did not cause any loss of human life, despite resulting in the greatest number of evacuated (4244) and displaced (14,322) people recorded in the DISASTER database.
In the last two centuries, the various regions of the Iberian Peninsula have suffered major extreme precipitation events with significant socio-economic impact. While it is beyond the scope of this work to provide an extensive compilation of all these events, we do want to highlight two other studies of regions of Spain in order to put the February 1979 event into a wider context. The estimation of extreme flash flood evolution in Barcelona County from 1351 to 2005 was undertaken by [25
], where the authors analysed all the floods that have occurred in Barcelona County (Catalonia) since the 14th century, as well as the flooded area, urban evolution, impacts and the weather conditions. The most severe events are mainly associated with flash floods that are often produced by a sudden thunderstorm that usually develops into a mesoscale convective system. In addition, [26
] studied the large-scale patterns of daily precipitation extremes on the Iberian Peninsula, where it was found that extreme precipitation events on the Western Iberian Peninsula mainly occurred under strong zonal flow, with a long Atlantic fetch generating moisture advection towards that area. On the eastern sector of the Iberian Peninsula, the most important pattern associated with extreme precipitation is characterized by a cut-off low system at mid-levels together with easterly moisture flow from the Mediterranean.
Moreover, we want to highlight three additional studies where flash flood events had a major socio-economic impact on the Iberian Peninsula. The deadliest storm affecting Portugal since at least the early 19th century took place on 25 and 26 November 1967, causing more than 500 fatalities [4
]. In addition, between 6–8 November 1982, there occurred one of the most catastrophic flash-flood events in the Eastern Pyrenees affecting Andorra and also Southern France and Eastern Spain with rainfall accumulations exceeding 400 mm in 24 h with 44 fatalities and widespread damage [27
]. Finally, on 5 November 1997, extreme precipitation with values higher than 150 mm in 24 h in Southern Portugal and Extremadura province in Spain produced major flash flood events, causing major socio-economic impact with 11 deaths in Portugal and 21 in Spain [20
While flash floods are mainly associated with heavy rain over a small period of time, persistent precipitation over several days can cause slow onset floods on river basins. In addition, slow onset floods struck Portugal in the last 200 years, such as the floods of the Tagus and Guadiana Rivers in December 1876 [2
] and Douro in 1909 [1
]. The first week of December 1876 was marked by extreme weather conditions that affected the south-western sector of the Iberian Peninsula, leading to an all-time record flow in these two large international rivers. As a direct consequence, several Portuguese and Spanish towns and villages located in the banks of both rivers suffered serious flood damage on 7 December 1876. These unusual floods were amplified by the particularly wet autumn months, with October 1876 presenting extremely high precipitation anomalies for all Western Iberian stations. The case of the record floods in Douro [1
] corresponds with the combination of persistent precipitation over several days, culminating with heavy rain on the 22 December 1909, leading to 89 casualties (57 due to floods and 32 due to landslides) and a further total of 3876 affected people, including fatalities, injured, missing, evacuated and homeless people.
The February 1979 event was considered a major flood in the Tagus valley in the 20th century (even when river flow was controlled by dams) due to the flooded area extension and the human and economic impact. This flood falls into the category of slow onset floods due to persistent precipitation over the two-week period, with rising water levels causing dikes to rupture, as well as flooding and isolating settlements, main roads and railroads. Also, there were recorded electric power and water supply failures in Lisbon and the surrounding municipalities. All these human damages (displaced and evacuated people) and services ruptures referenced in the newspapers were felt in the life of the population beyond the days of the event.
The February 1979 event is framed within a 25-day period of very high and abnormal precipitation totals for most of the Portuguese continental territory, at the daily, multi-day and hydrological year scales, as shown in Section 3.3
. In fact, this event corresponded to the third most significant event for the Iberian Peninsula and Portugal when using the 10-days accumulated ranking previous published [18
]. In this work, we have updated this accumulated ranking to a 15-day period, and when using a larger accumulated period, the results show that it was the top event in the Iberian Peninsula and the second top event in Portugal. However, when we considered 10-days or 15-days accumulated precipitation for the Tagus basin (Figure 8
b), this event was outside the top 20 ranking of events [18
]. This is explained by the highest daily precipitation magnitudes for the Tagus region appearing on the period spanning between 8–11 February, coinciding with the greatest amount of human damage registered on this basin during the event (9–13 February).
The major Portuguese Rivers and tributaries located in the south are characterized by irregular regimes, with floods during the rainy autumn and winter months and droughts during the dry summer months. The rivers’ regimes are directly related to the annual and monthly rainfall variability that characterizes a Mediterranean climate [20
]. Taking the example of the Tagus basin, after the dams construction, the river flow peaks reduced and also the frequency of floods [20
]. This is particularly true during the autumn months when the water storage capacity of dams is being restored after the prolonged dry summer period.
The regional atmospheric circulation affecting the Iberian Peninsula throughout this specific event was analysed by means of CWT and by the analysis of different meteorological fields. Regarding the CWT, the daily precipitation on the Iberian Peninsula during the winter season is strongly related to the occurrence of a few “wet” CWTs, namely the C, SW, NW and W [7
]. The analysis of Figure 7
b from 20 January until 16 February shows that the predominant CWTs affecting the Iberian Peninsula were the SW, W, NW and C (highlighted in bluish colours). These CWTs match well with the prolonged period of high daily precipitation and aggregate multi-day precipitation.
Moreover, close attention was also given to different meteorological fields that were used in other case studies, e.g., [1
], that provide meaningful insight into some of the meteorological mechanisms associated with an extreme event. Summarizing, this event was characterized by the passage of consecutive low-level pressure systems north-west of the Iberian Peninsula, while being reinforced by high North Atlantic moisture advection towards the Iberian Peninsula's western coast during the same period. The recurrent presence of persistent ARs reaching the Western Iberian Peninsula during or before extreme precipitation days for this event (Figure 10
) significantly contributed to the North Atlantic moisture transport towards the Iberian Peninsula's western coast and to the local impact of precipitation events, since it helped concentrate moisture at more localized areas.
While all this precipitable water content hovering over the Iberian Peninsula's western coast during the event was available to precipitate, there were indeed favourable meteorological conditions for triggering floods and deep landslides. For example, the aforementioned passage of the frontal systems fuelled by ARs that were associated with the large synoptic scale low pressure system storm track established over the Atlantic (Figure 10
), the local convective instabilities produced by low level wind convergences and upper level wind divergences over the continental Iberian Peninsula (Figure 11
) and the orographic forced convections developed in rugged terrains, most significantly in the centre and north regions of Portugal.
Similar to other extreme events, part of the widespread socio-economic impact was due to human poor decisions. During the February 1979 DISASTER event, dams were not capable to efficiently control river flow peaks, resulting in flooding along the lower Tagus valley. In particular, the huge Spanish Alcántara Dam (3162 hm3
of capacity, located close to the Portuguese border, Figure 1
) released significant amounts of water at the same time as the Castelo do Bode Dam (900 hm3
capacity, located in river Zêzere, the main tributary of Tagus within Portugal, Figure 1
). Thus, it seems that the two most likely factors that contributed to this outcome were the prolonged precipitation period during that winter and the poor coordination of the dams’ discharges between the Portuguese and Spanish authorities.