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.
