There is strong evidence for increased wave heightsThe distance between the trough and peak of a wave. 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 & 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 & Hasse, 1999; Gulev & 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. Most of the increase in wave heights is attributed to “swellLong period sea surface waves that are not locally generated.” responding to changes in the persistence of westerly winds over the North Atlantic rather than “wind sea” responding to local winds.

Changes   in  winds   and  waves   can   be   better  understood   by   considering    their   relationship to atmospheric pressure gradientsThe difference in atmospheric pressure between two locations. The larger this is the greater the tendency for increased wind speeds. (Bacon & Carter, 1993) and particularly to large-scale atmospheric variability such as the North Atlantic Oscillation (NAO)A pattern in the atmospheric pressure of the North Atlantic that is particularly important during the winter. The pattern is manifest by opposing systems of low pressure centred near Iceland and high pressure across the Azores and Iberia. The measurement is primarily used to distinguish between periods of generally stormy winters in the North Atlantic (when the pressure difference is large) and periods of reduced storminess (when the difference is low).. The characteristics of fluctuations in the westerly winds in the temperate northern hemisphere may also be described as an ‘annular mode’, the Northern Annular Mode (NAM)A pattern of climate variability covering the extra-tropical northern hemisphere. In the North Atlantic this is seen as the North Atlantic Oscillation but the NAM itself is a larger pattern that covers the Arctic, Pacific and land areas of the northern hemisphere. (Solomon et al., 2007).  Thus the increased mid-latitude westerlies in the North Atlantic can largely be viewed as reflecting either NAO or NAM changes (Solomon et al., 2007).  

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 daysUsed to assess the frequency of strong winds a gale day is a day on which the wind speed at a height of 10m has a mean speed of greater than 34 knots averaged over 10-minute intervals (approximately Beaufort Force 8).” 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/NAM in the 21st century (e.g. Terray et al., 2004; Miller et al., 2006). However,  alternative analyses  primarily based on  RCMs suggest  different and  mostly weaker  changes in  winds and  storminess (e.g. Hulme et al., 2002; Barnett et al., 2006). Typically,  climate   models   predict  a  decrease  in  the total number of extra-tropicalReferring, geographically, to any region of Earth outside of the tropics  i.e. north of the latitude 23.5°N or south of the latitude 23.5°S.  cyclones but an increase in the number of intense events (Lambert & Fyfe, 2006). Either a strengthening of the storm track or an increase in intense cyclones will result in a deterioration of wave conditions (Wolf & Woolf, 2006). This is a likely outcome in the wintertime in western and northern UK waters (Tsimplis et al., 2005) but there can be only low confidence in this prediction.

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What is already happening - High (that 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)

What could happen in the future - Low

There is a moderate amount of model output, but the agreement is low

See review.

Not stated.

David Woolf and John Coll
Environmental Research Institute, North Highland College, UHI Millennium Institute, Castle Street, Thurso, KW14 7JD