Atmospheric variability linked to large floods in the lower Ebro River basin

The study analyzes the atmospheric variability that caused the largest floods affecting the town of Tortosa in the mouth of the Ebro River (northeast Iberian Peninsula). The Tortosa flood database and flood marks in the nearby town of Xerta are used to define the more relevant flooding episodes (discharges > 2900 m³s^-1) of the 1600-2005 period. We explore the atmospheric variability based on low-frequency patterns and synoptic types applying a multivariable analysis to grids at sea-level pressure and geopotential at 500 hPa provided by the 20th Century V3 Reanalysis Project for the instrumental period (since 1836). Output from the Last Millennium Ensemble Project and climate reconstructions (Luterbacher et al., 2002) were used to analyze the sea-level pressure over the pre-instrumental period (before 1836).

The synoptic analysis includes 33 flood episodes since 1836. Four synoptic types are related to floods in Tortosa, characterized by low-pressure systems that interact with the Mediterranean warm air-mass and promote the atmosphere destabilization. The prevailing synoptic conditions that favor heavy rainfall explains well the hydrological reconstruction of the flood events. The synoptic types are characterized by Atlantic low-pressure systems that interact with the Mediterranean warm air mass that destabilizes the atmosphere due to temperature differences between the surface (warm and moist air from the Mediterranean Sea) and the middle levels of the troposphere. Furthermore, due to  high-pressure systems located in Central Europe causing stagnation of the synoptic configuration, long-lasting rainfall can occur over the Ebro basin.

We detected four clusters of high-frequency flooding in Tortosa since 1600: 1617-1643, 1710-1787, 1825-1884, and 1907-1985. Most of these are related phases of high solar variability, which highlight atmospheric and hydrological instability in periods of rapid climate change. The low-frequency atmospheric variability connected to these flood periods is related to the positive phase of the NAO, relative high values of solar activity and positive Northern Hemisphere temperature anomalies. This provides evidence that complex solar processes might provoke changes in temperature and variability in the atmospheric circulation. The NAO shows that the major floods in the region are related to the zonal atmospheric circulation. These atmospheric disturbances have a winter effect in the western part of the basin, while the Pyrenean sub-basins are affected during autumn.

The major finding is that similar flood behavior is detected since 1600. Results from our study help to improve our understanding of the past, present and future climates, as well as their impacts, thereby enhancing the knowledge base for addressing some aspects and impacts of climate change in order to reduce uncertainty about future outcomes. Furthermore, future investigations seeking to detect and prevent extreme events will find it particularly useful to establish relationships between modes of low-frequency atmospheric variability, synoptic types and flooding.