Decadal changes of the Adriatic sea level – exploring the combined effect of sea level rise and climate regime’s shift

In this study, an analysis of the observed Adriatic mean sea level time series has been carried out in order to determine the primary causes of the changes documented during the last 50 years.  Monthly sea level data were downloaded from the Permanent Service for Mean Sea Level for seven stations located along the northern and eastern Adriatic coast: Venice, Trieste, Rovinj, Bakar, Zadar, Split and Dubrovnik. Significant positive sea level trend, related to climate change, was detected at the majority of the stations. Further on, using Rodionov’s regime shift index algorithm, several regime shifts were detected. The first pronounced regime shift occurred in 1989 resulting with mean sea level lower than usual for an average of 4.37 cm; the second regime shift occurred in 1996 when mean sea level increased for an average of 2.07 cm; and the third regime shift, which is still on-going, started in 2009 when mean sea level abruptly increased to 5.3 cm above average.  A relationship between North Atlantic Oscillation (NAO) and sea level data has been explored, using both monthly and yearly data. High and significant correlation between the two was established for all data, and in particular for the winter season (December, January, February, March). All climate shifts were related to pronounced changes of NAO. The negative shift starting in 1989 was related to the positive phase of NAO, i.e. to weaker cyclonic activity over the Mediterranean and the Adriatic Sea. Oppositely, the two latter positive regime shifts were related to significant decrease and negative phases of NAO, with NAO reaching the most negative values of the entire observation period during the shift starting in 2009. Negative phase of NAO corresponds to stronger cyclonic activity over the Mediterranean and the Adriatic Sea. In conclusion, documented rise of the Adriatic sea level during the last 50 years, and in particular accelerated rise during the last 20 years represent a combination of mean sea level rise due to climate change and due to atmospherically induced shift of climate regimes.