IMPACTS OF CLIMATE CHANGE ON STORMS AND WAVES
David Woolf 1,2 & John Coll 1
1 Environmental Research Institute, North Highland College, Thurso
2 National Oceanography Centre, Southampton
Executive Summary
There is strong evidence for increased wave heights in western and northern UK territorial waters and for increased occurrence of strong winds over the UK from the 1960s to the present. It is unclear whether recent behaviour is driven by “global climate change” or is simply natural variation and whether substantial changes in storminess are likely in the 21st century. Bacon and Carter (1991) inferred an increase in mean wave height of about 2% per year “over the whole of the North Atlantic in recent years, possibly since 1950” from observational data notably from Seven Stones Light Vessel (1962-1986). Recent analyses of a more extensive data set confirm a significant upward trend in wave heights in the North Atlantic, but only for the last 50 years and embedded within a pattern of multi-decadal variability over more than a century (Gulev and Hasse, 1999; Gulev and Grigorieva, 2004). There have also been significantly more severe storms over the UK since the 1950s (Alexander et al., 2005). However, trends in winds around the UK are much weaker than for wave heights, and therefore most of the increase in wave heights is attributed to “swell” rather than “wind sea”.
Changes in winds and waves can be better understood by considering their relationship to atmospheric pressure gradients (Bacon and Carter, 1993) and particularly to large-scale atmospheric variability such as the North Atlantic Oscillation (NAO). Wave heights in the North-East Atlantic and northern North Sea are known (from analysis of in situ data, satellite data and model reconstructions) to respond strongly and systematically to the NAO (e.g. Woolf et al., 2002 and 2003). Other parameters - such as cyclone activity (Gulev et al., 2001) and the number of “gale days” at coastal sites in Scotland - show a weaker, but still significant response to NAO. Thus, many of the changes over the last 50 years can be understood in terms of the behaviour of the NAO. The recent strong trend in the NAO (towards stormier conditions) is apparently unique in its history, but it is controversial whether this is a response to greenhouse gas forcing (Osborn, 2004). Many Global Climate Models suggest a general trend towards the stormier tendency of NAO in the 21st century (e.g. Terray et al., 2004). If so, worsening wind and wave conditions in the wintertime in western and northern UK waters are inevitable (Tsimplis et al., 2005; Wolf and Woolf, 2006). However, alternative analyses primarily based on Regional Climate Models suggest different and mostly weaker changes in winds and storminess (e.g. Hulme et al., 2002; Barnett et al., 2006)
Level of Confidence
High
(changes have occurred over the last 50 years).
There is a lot of data, and the agreement between data sources is high (noting that some parameters are more sensitive than others to change)
Low
(changes will occur in the future).
There is a moderate amount of model output, but the agreement is low
Key sources of Information
Please acknowledge this document as: Woolf, D. and Coll, J. (2006). Impacts of Climate Change on Storms and Waves in Marine Climate Change Impacts Annual Report Card 2006 (Eds. Buckley, P.J, Dye, S.R. and Baxter, J.M), Online Summary Reports, MCCIP, Lowestoft, www.mccip.org.uk