Meteorological conditions associated with sea surges in Venice: a 40 year climatology

The frequency of sea surges in Venice has increased during the 20th century, and the trend has been particularly pronounced in the last four decades. However, the time series of independent surge events (i.e. events separated by at least 1 week) has remained nearly stationary during that period. This suggests that, although the sea level rise (due to global warming and human activity in the region) is leading to more multiple events, the frequency of meteorological conditions that trigger independent events seems to be nearly balancing the effects of sea level change. Such meteorological conditions are identified by compositing sea level pressure (SLP) and 995 hPa wind during and before independent sea surge events in Venice. The composite analysis shows that these correspond to storms orographically induced over the western Mediterranean basin, when an Atlantic synoptic system is perturbed by the Pyrenees and/or the Alps. It is, however, the persistence, intensity and relative position of such storms to the Adriatic Sea that contribute to the optimum conditions for the occurrence of floodings in its northern embayment. It is shown that the synoptic picture is translated into persistent low SLP over the Venice region, negative north–south SLP gradient over the Adriatic, and south‐southeasterly to southeasterly wind over the central and northern parts of the sea. During the 40 year period under analysis, the persistence and intensity of the most adverse scenarios for the occurrence of sea surges in Venice have been generally decreasing; significant trends have been found in the tails of the distributions of Venice SLP, SLP north–south gradient, and of surface wind over the northern Adriatic. It is the balance between these trends and the continuing sea level rise that may account for the near‐stationarity of independent sea surge events during the last four decades. Copyright © 2002 Royal Meteorological Society