Climatology and trends of the Adriatic Sea wind waves: analysis of a 37-year long instrumental data set

This study concerns the analysis of a 37-year long directional wave time series recorded since 1979 at the CNR-ISMAR oceanographic research tower, located in the Northern Adriatic Sea. The length of the time series allows to describe the wave climate in the North Adriatic and to identify trends and links with large-scale climate patterns. The northern part of the Adriatic Sea is characterized by two main wind and correspondingly wave regimes, strongly forced by the regional orography: the long-fetch south-easterly ‘sirocco’ and the short-fetch strong north-easterly ‘bora’ wind generated waves, blowing along the major and minor axis of the basin, respectively. Bora is the most frequent regime, but high waves are mostly associated to sirocco. A clear decrease of the significant wave height 99th percentile is evident, paralleled by a smaller, but distributed along the annual cycle, increase of the 50th and 75th ones. The estimated trend of the frequency of events above a certain threshold confirms an increase of the average storm frequency, with a shift from both the lower and the higher to the central part of the distribution, paralleled by a decrease of the maximum Hs values. In particular, a distributed decrease of the bora significant wave height is recognized. For sirocco the tendency is less pronounced, but with an evident increase of the frequency of the mean values. The high sensitivity of this particular area to even small variations of large-scale climate allows to explore possible links of the local wave, hence wind, activity with large-scale Northern Hemisphere circulation patterns or weather regimes. Our main interest is on the storm tracks and jet stream location/intensity over vast areas, which led us to choose four reference patterns: the North Atlantic Oscillation and the East Atlantic (EA), EA/Western Russia and Scandinavian patterns.