Long-term meteorological preconditioning of the North Adriatic coastal floods

Flooding of the North Adriatic coast is examined through 14 years of hourly sea-level data, recorded at Bakar. The threshold-exceeding sea levels are studied in respect to four major components: (i) tides, (ii) elevations generated by synoptic and smaller-scale meteorological disturbances (storm surges and seiches of the Adriatic), (iii) low-frequency oscillations (0.01<f<0.1 cpd) predominantly induced by planetary atmospheric waves and (iv) seasonal processes. Synoptic meteorological forcing, associated with the travelling cyclones, is the major contributor to the peak events and has been amply studied. The relative phases of the storm surge, the pre-existing fundamental Adriatic seiche and the astronomical tide are also decisive factors. However, the lower-frequency disturbances of air pressure and wind, associated with planetary atmospheric waves, account for one third of total sea-level variability. They can give rise to sea-level changes in the range of 70 cm, thus posing strong, days lasting preconditions for flooding. It is shown that in the absence of these long-period sea-level variations, the total number of events with sea level exceeding the 99.5% threshold value is reduced more than ten times; they are not the dominant, but are the ever-present component of floods in the North Adriatic. Under changed conditions of the expected global sea-level rise their impact may become ever more important. Consideration of the forecast of planetary-wave troughs, which are best visible at the 500 hPa surface, may enable an early warning of increased risk of flooding.