Wave height variations in the North Sea and on the Norwegian Continental Shelf, 1881-1999

Analyses of overlapping Norwegian Meteorological Institute (DNMI) hindcast squared monthly mean wind speeds and monthly mean significant wave heights show a linear relation. Based on available time series of wind speed, computed from sea level pressure (SLP), this enables us to establish long and consistent time series of monthly mean significant wave heights. Data sets of monthly mean 10 m wind speeds from nine locations in the North Sea and on the Norwegian Continental Shelf have been investigated. The data sets include World Meteorological Organization (WMO) data, 1881-1982, DNMI hindcast data, 1955-1999, and National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) data, 1948-1999. From these time series it is evident that mean wave heights are subject to significant variations within a variety of time scales. There is a positive trend starting in the 1960s, mainly in northern parts of the North Sea, with significant regional variation. The increase is, however, not more dramatic than the decrease which occurred from 1881 and towards the beginning of the 20th century. Analyses of annual maximum significant wave heights based on 6 h values of DNMI data, 1955-1999, strongly indicate increasing wave heights and rougher wave climate at the stations off the coast of mid-Norway. At the other stations the trends are only weakly positive or not apparent at all. (C) 2003 Elsevier Science Ltd. All rights reserved.