Decadal changes in the wind forcing over the North Sea

Using data from the NCEP/NCAR reanalysis for the 40 yr period from 1958 to 1997, a wind statistic for the North Sea is derived. This analysis includes consideration of the monthly mean wind speed and decadal variability in angular distribution. The wind density function is introduced, which combines frequency of occurence and mean wind speed for a given direction, The North Sea is subdivided into 3 sectors. Changes in the wind density function for the 3 regions are analysed for 2 seasons, October to January and February to March. The annual mean wind speed for the North Sea shows a rising trend of similar to 10 % during the last 40 yr, mainly restricted to the period from October to March. For October to January, west-southwesterly wind directions are enhanced in the last 3 decades compared to the period from 1958 to 1967 for the whole North Sea area. The last decade from 1988 to 1997 is outstanding: The duration of typical winter wind conditions, with high wind speeds and the prevalence of west-southwesterly winds, is extended from October to January in the first 3 decades towards February and March in the last one. The wind density functions for both seasons investigated and all of the 3 sectors resemble each other in the last decade. In the first 3 decades an enhancement of southerly wind directions is found in the northern part of the North Sea, northward of 59 degreesN. This trend is broken in the last decade when west-southwesterly directions for the whole North Sea area predominate.