Summary of changes and trends

• Wave data from ships and buoys indicate that the mean winter wave height in the northeast Atlantic increased significantly between the 1960s and 1980s. Satellite data confirm that this increase continued into the early 1990s.
• In the northern North Sea, there was an upward trend of about 5-10% (0.2-0.3m) in mean significant wave height (Hs) for January–March for the period 1973-1995, but a decrease thereafter.
• In the central North Sea, the trend for January–March was upwards until 1993/94, with a decrease thereafter. The October–December Hs peaked around 1982/83 and 1983/84, with a similar high value in 1999/2000.
• In the southern North Sea, there is no discernible trend in Hs for January–March and only a slight indication of a downward trend in Hs for October–December from 1980/81.
• At Sevenstones LV, off land’s End, the acceptable value is an increase of 0.02 m/yr in mean wave height over a period of about 25 years. This trend seems to have persisted into the early 1990s at least, although recent winters have suggested a levelling off.

1. Introduction

The wave climate can be considered as consisting of three parts: the long term mean climate, the annual or seasonal cycle and non-seasonal variability on both the short term (within year) and long term (year to year, or interannual). In UK waters, wave climate is strongly seasonal with mean wave heights peaking around January, but with a high risk of both high monthly mean wave heights and extreme wave heights throughout autumn and winter (October to March). There is also high inter-annual variability in monthly mean wave heights, particularly in an ‘extended winter period’ from December to March and these months are those primarily associated with the North Atlantic Oscillation. (The NAO index is a measure of the mean atmospheric pressure difference across the North Atlantic, from north to south; see for example Jones et al., 1997.)

The height of offshore waves depends on the strength of the wind and the distance and length of time over which the wind has acted on the ocean surface. Waves approaching the UK coastline could have been generated not only locally and in the north-east Atlantic Ocean, but also from the north-west Atlantic and even from the south Atlantic. Coastal waves are influenced by local water depth and by the nature of the seabed.

High waves can cause risk to platforms and pipelines and disruption to routine marine operations. Estimates of likely extreme waves are essential for the design of ships and offshore structures such as oilrigs. At the coastline, waves can affect coastal development - larger waves can damage seawalls, cause coastal flooding and lead to increased rates of erosion of soft coastlines such as the glacial till cliffs in East Anglia and Yorkshire. The most serious coastal flooding events are often caused by a combination of high tides, storm-surges and waves.

2. Wave measurements

The measurement of waves is a relatively recent development, with only very crude instruments available prior to about 1955. In the 1960s and 1970s, the National Institute of Oceanography equipped a number of lightships around the coastline with ship-borne wave-recorders that used acceleration and pressure fluctuations to provide information on wave heights and periods (but not directions). The recorders were typically only deployed at each site for 1-2 years, the main exception being at Sevenstones LV, which eventually provided one of the longest wave records from UK waters. Wave-following buoys using accelerometers replaced pressure-type wave recorders and by the late 1970s most wave recording was being carried out using these instruments. A wide range of instruments for measuring waves has been developed in recent years, including directional wave buoys, downward looking lasers and HF radar; the satellite altimeter has proved particularly successful for climate studies, providing global coverage. See Tucker and Pitt (2001) for a description of wave-measuring instruments.

Descriptions of the monitoring networks that regularly measure waves are given in Chapter 1, including details of details of how to access near real-time data.

Click here for a list of links to monitoring networks and data sets.

The longest periods of wave measurements, at a consistent location around the UK coastline, are believed to be as follows (Law et al., 2003):

Coastal wave data

• Off the North Kent coast (1979-1998 off Whitstable, 1996 to present off Herne Bay)
• Tees Bay (1988-present)
• Perranporth (1975 – 1986)

Offshore wave data

• Sevenstones Light Vessel (1962-1988)
• Forties Field (1974-present)
• Frigg QP (1979-present)
• Ekofisk Field (1980-present)

Figures 1 to 7 show wave data for 2000 – 2002 at selected stations of the Met Office’s Marine Automatic Weather Station (MAWS) Network.

3. Long-term mean wave climate and annual seasonal cycle

Figure 8 shows monthly means of significant wave heights (and wind speed at 10m above the sea surface) derived using data from altimeters in the satellites Geosat, ERS-1, ERS-2, TOPEX-Poseidon and Jason from 1985 onwards. These fitted sine curves indicate a maximum ranging from late December in the southeast to mid-January at the northwest location where the February mean was 4.9m compared to 4.5m in January; but the maximum individual, recorded wave height of 11.8m and the maximum individual monthly mean of 7.3m were both in January 1993 (Carter, personal communication).

In British waters, the west coast of Ireland and the Outer Hebrides experience the highest wave heights (long term mean significant wave height (Hs) of 3.0 m. Off the English and Welsh coastline, southwest Wales and western Cornwall experience the highest mean significant wave heights (2.0 - 2.5 m), whilst the English Channel and Eastern English coastline are the most sheltered, with a long term mean Hs of 1.5m or less (Cotton et al., 1999).

The annual range in Hs (i.e. the difference between winter and summer) follows a similar pattern to the long-term mean. The winter to summer range is greatest in the north and west and lowest in the south and east. It decreases eastwards into the English Channel and southwards into the North Sea, from > 3 m at 20ºW, to 1 m or less at the southeast tip of Kent (Cotton et al., 1999).

4. Short- and long-term non-seasonal variability

The seasonal cycle explains most of the variability in the monthly data of the northeast Atlantic (~70% at 15-20ºN), but less than half (30-50%) of the variance in the North Sea and English Channel (Cotton et al., 1999). Therefore, inter-annual variability is also important, with some winters much stormier than others.

Reliable long-term measurements of wave height in the North-east Atlantic, including UK waters, are available only since the 1960s (see section 2). Analyses of these wave data from ships and buoys give varying estimates of the change in wave height in the long term, e.g.:

A review of the estimates is given by Carter (1999). The conclusion of a major review and study of the wave climate around the British Isles, carried out as part of the JERICHO project (Cotton et al., 1999), was that analysis of the wave data from ships and buoys shows that the mean winter wave height in the northeast Atlantic increased significantly between the 1960’s and 1980’s; and that the more recent analyses of satellite data from October 1992 to December 1997 confirmed that this increase continued into the early 1990’s, with an acceptable value of 0.03 m/yr increase in the mean winter Hs in the northeast Atlantic.

Click here to see the percentage increase in mean winter significant wave height in the Northeast Atlantic, 1985-89 to 1991-96.
Link to figure 1 in http://www.satobsys.co.uk/Jericho/webpages/jeriview.html .

At Sevenstones LV, Cotton et al. (1999) state that the acceptable value is an increase of 0.02 m/yr in mean wave height over a period of about 25 years. This trend seems to have persisted into the early 1990s at least, although recent winters have suggested a levelling off, perhaps the beginning of a decreasing trend.

Mean significant wave height (Hs) has been derived from wave measurements over the last 30 years at Shell UK platforms in the northern, central and southern North Sea, as part Shell’s METNET network (figure 9). In the northern North Sea there appears to be an upward trend in Hs for January–March until 1994-1995; analysis of Hs for the period 1973-1995 showed that it appeared to have increased by around 5-10% (0.2-0.3m) (Leggett et al., 1996). Thereafter, Hs for January-March appears to have decreased. For the central North Sea, there is a suggestion that the trend for January–March is upwards until 1993/94, with a decrease thereafter. The October–December means peak around 1982/83 and 1983/84, with a similar high value in 1999/2000. For the southern North Sea, there is no discernible trend in Hs for January–March and only a slight indication of a downward trend from 1980/81 for October–December. Cotton et al. (1999) state that although the trend apparent at Sevenstones LV may have extended as far east as the northern North Sea, there is no evidence to suggest any similar increases in the central and southern North Sea.

5. Wave climate and the North Atlantic Oscillation

Waves are strongly related to wind conditions, particularly their strength and persistence, so a link to the north-south atmospheric pressure gradient over the North Atlantic could be expected. The increase in wave heights from 1962 to 1985 off Land's End (Carter and Draper, 1988) has been correlated with air pressure gradients (Bacon and Carter, 1993).

Kushnir et al. (1997) have tied the increase in wave heights to the increase in wintertime storminess and mean wind speeds in the North Atlantic during the last 30 years or so. The WASA Group (1998) investigated evidence for increasing storminess during the 20th century using meteorological data. They concluded that the storm climate in the NE Atlantic and North Sea had undergone variations on decadal time scales and had indeed worsened in recent decades (1980 onwards). However, they found that the recent intensity was not unprecedented, being comparable to that at the start of the 20th century, with lower intensity in the intervening period. Also, there was no evidence for an increase in the number of storms or a tendency for storms to increase in intensity in recent decades. Since the NAO has increased in intensity during recent decades and with it the westerly mean wind flow, the WASA group considered that any noticeable increase in Hs since the 1960s could be positively correlated with this, rather than with storm intensification; with a high or positive NAO index associated with increased wave height compared to a low or negative index.

The influence of the NAO on the winter wave climate in the northeast Atlantic and UK waters has been studied in detail, primarily using satellite altimeter measurements of significant wave height (Cotton et al., 1999; Woolf et al., 2002 and 2003). As an example, figure 10 shows the relationship between monthly mean wave heights and the NAO over a 15-year period since 1985, generated from altimeter data.

To the west of Scotland, the relationship is particularly strong - describing about 70% of the variance and implying monthly mean wave heights varying from 3 metres to 7 metres for extreme negative winter NAO Index and positive winter NAO Index respectively. The relationship is weaker elsewhere - vanishing on the East Coast of Britain - but is a major feature of the region as a whole. In terms of the sensitivity of the winter mean Hs to changes in the NAO, the wave climate off the north-west of Scotland (the Outer Hebrides) is highly sensitive, such that a unit change in the NAO will induce a 0.42m increase in the mean winter Hs, and a 1.28m change in the 100 year return value (Cotton et al., 1999; Woolf et al., 2002 and 2003).

The wave climate in the Celtic Sea/Irish Sea and Lyme Bay is also sensitive to the NAO, (54% of the variance in Carmarthen Bay with a 0.2m change in mean Hs and 0.69 m change in 100 yr Hs per unit NAO change and 13% of the variance at Lyme Bay). The relationship in the northern North Sea is strong during December to March, but the correlation between the NAO and the waves for a region offshore of Holderness (NE England) is insignificant Cotton et al., 1999; Woolf et al., 2002 and 2003).

6. References

Bacon, S. and D.J.T. Carter (1991). Wave climate changes in the North Atlantic and North Sea. International Journal of Climatology, 11: 545-558.

Bacon, S., and D.J.T. Carter (1993). A connection between mean wave height and atmospheric pressure gradient in the North Atlantic, International Journal of Climatology, 13: 423–436.

Barratt, M.J., 1991. “Waves in the NE Atlantic.” Offshore Technology Report OTI 90545, HMSO.

Carter, D.J.T. (1999). Variability and trends in the wave climate of the North Atlantic: a review. In Proceedings of the 9th ISOPE Conference, Cupertino, CA, USA, vol. III, pp. 12 -18. International Society of Offshore and Polar Engineers, Cupertino, CA, USA.

Carter, D.J.T. and L. Draper (1988). Has the Northeast Atlantic become rougher? Nature, 332: 494.

Cotton, P.D., D.J.T. Carter, T.D. Allan, P.G. Challenor, D. Woolf, J. Wolf, J.C. Hargreaves, R.A. Flather, Li Bin, N. Holden and D. Palmer (1999). Joint Evaluation of Remote Sensing Information for Coastal And Harbour Organisations (JERICHO). Final Report to the British National Space Centre, project no: R3/003. 38 pages. Retrieved 10th September 2003 from the World Wide Web: http://www.satobsys.co.uk/Jericho/webpages/jeripdf.html

Jones, P.D., T. Jónsson, T. and D. Wheeler (1997). Extension to the North Atlantic Oscillation using early instrumental pressure observations from Gibraltar and South-West Iceland. International Journal of Climatology, 17: 1433-1450.

Kushnir, Y., V. J. Cardone, J. G. Greenwood, and M. Cane (1997). On the recent increase in North Atlantic wave heights. Journal of Climate, 10: 2107–2113.

Law, F.M., F. Farquharson, A. Brampton, M. Dale and R.A. Flather (2003). Environmental change indicators (including those related to climate change) relevant to flood management and coastal defence. Defra/EA R & D technical report FD2311-TR.

Leggett, I., F.L. Beiboer, M.J. Osborne and I. Bellamy (1996). Long-term metocean measurements in the northern North Sea. Pp 1 to 9 in Climate change offshore N.W. Europe, SUT, London.

Tucker, M.J. and E.G. Pitt, E.G. (2001). Waves in ocean engineering. Elsevier Ocean Engineering Book Series (v.5), 521pp. Amsterdam: Elsevier.

WASA Group (1998). Changing waves and storms in the northeast Atlantic. Bulletin of the American Meteorological Society, 79: 741-760.

Woolf, D.K., P.G. Challenor & P.D. Cotton. 2002. The variability and predictability of North Atlantic wave climate. Journal of Geophysical Research, 107(C10), 3145.

Woolf, D.K., P.D. Cotton & P.G. Challenor. (2003). Measurements of the offshore wave climate around the British Isles by satellite altimeter. Philosophical Transactions: Mathematical, Physical & Engineering Sciences, 361(1802), 27-31.