CHANGING WAVES AND STORMS IN THE NORTHEAST ATLANTIC

The European project WASA (Waves and Storms in the North Atlantic) has been set up to verify or disprove hypotheses of a worsening storm and wave climate in the northeast Atlantic and its adjacent seas in the p resent century. Its main conclusion is that the storm and wave climate in most of the northeast Atlantic and in the North Sea has undergone significant variations on timescales of decades; it has indeed roughen ed in recent decades, but the present intensity of the storm and wave climate seems to be comparable with that at the beginning of this cent ury. Part of this variability is found to be related to the North Atla ntic oscillation. An analysis of a high-resolution climate change expe riment, mimicking global warming due to increased greenhouse gas conce ntrations, results in a weak increase of storm activity and (extreme) wave heights in the Bay of Biscay and in the North Sea, while storm ac tion and waves slightly decrease along the Norwegian coast and in most of the remaining North Atlantic area. A weak increase in storm surges in the southern and eastern part of the North Sea is expected. These projected anthropogenic changes at the time of CO2 doubling fall well within the limits of variability observed in the past. A major methodi cal obstacle for the assessment of changes in the intensity of storm a nd wave events are inhomogeneities in the observational record, both i n terms of local observations and of analyzed products (such as weathe r maps), which usually produce an artificial increase of extreme winds . This occurs because older analyses were based on fewer observations and with more limited conceptual and numerical models of the dynamical processes than more recent analyses. Therefore the assessment of chan ges in storminess is based on local observations of air pressure and h igh-frequency variance at tide gauges. Data of this sort is available for 100 yr and sometimes more. The assessment of changes in the wave c limate is achieved using a two-step procedure; first a state-of-the-ar t wave model is integrated with 40 yr of wind analysis; the results ar e assumed to be reasonably homogeneous in the area south of 70 degrees N and east of 20 degrees W; then a regression is built that relates m onthly mean air pressure distributions to intramonthly percentiles of wave heights at selected locations with the help of the 40-yr simulate d data; finally, observed monthly mean air pressure fields from the be ginning of this century are fed into the regression model to derive be st guesses of wave statistics throughout the century.