PAST AND FUTURES CHANGES IN THE NORTH SEA EXTREME WAVES Sofia Caires 1 , Jacco Groeneweg 2 and Andreas Sterl 3 Past and future changes in the North Sea extreme waves are investigated in this article. Estimates obtained from non-stationary extreme value analyses, expressing the extreme value distribution parameters as functions of time and wind speed related covariates, are given. The results show that there is a significant trend of about 9 mm/yr in the current climate extremes of significant wave height and a trend of 1 mm/yr in the projections from 2001 to 2100. The characteristics of the extremes of wave period depend on whether swell or wind-sea events are considered. If both types of events are considered, the extremes are dominated by swell events and no present or future changes are identified. Considering wind-sea events only, a trend of less than 0.01s/yr in the present climate wave periods and a trend an order of magnitude smaller in the projections from 2001 to 2001 were detected. 1. Introduction The current approach to obtain hydraulic boundary conditions for the Dutch water defences involves stationary extreme value analysis of wave conditions that are measured at offshore locations. In such approach the extreme wave climate is assumed to be stationary. However, it is believed today that climate is not stationary, as the detection of both decadal variability and long term time trends in different climate variables, reported by several authors, indicates. The past and future changes in the North Sea wave extremes have been investigated in this paper, trying to answer questions such as: Has the North Sea extreme wave climate changed in the last decades? How is it expected to change in the future? Previous studies on the influence of climate changes on wave extremes were based only on changes in significant wave height (see e.g. Wang and Swail, 2006 and Caires et al., 2006b). For the design of coastal defences the wave period is also an important parameter. Therefore, in this study we have not only analysed changes in extremes of significant wave height (Hs) but also changes in extremes of wave period. A problem involved in the estimation of future wave extremes is that no projections of future wave conditions are available in the global climate model computations (IPCC 2007). Using the available global climate model results, two approaches could in principle be used to quantify future changes in the wave extremes: - Dynamically, by using climate models wind speed projections to force a 1 Hydraulic Engineering, Deltares | Delft Hydraulics, P.O. Box 177, 2600 MH Delft, The Netherlands, sofia.caires@deltares.nl 2 Hydraulic Engineering, Deltares | Delft Hydraulics, P.O. Box 177, 2600 MH Delft, The Netherlands, jacco.groeneweg@deltares.nl 3 Global Climate, Royal Netherlands Meteorological Institute (KNMI), P.O. Box 201, 3730 AE De Bilt, The Netherlands, sterl@knmi.nl 1