Jun 24, 2005 - The Sea of the Hebrides is the body of water that lies between the ... 1º North Atlantic model, through a 7.5km Malin/Hebrides Shelf model.
Proceedings of The Fifteenth (2005) International Offshore and Polar Engineering Conference Seoul, Korea, June 19−24, 2005 Copyright © 2005 by The International Society of Offshore and Polar Engineers ISBN 1-880653-64-8 (Set); ISSN 1098-6189 (Set)
Waves and Climate Change in the Sea of the Hebrides Judith Wolf and David K. Woolf Proudman Oceanographic Laboratory, Liverpool, U.K.
National Oceanography Centre, Southampton, U.K.
ABSTRACT
INTRODUCTION
There is mounting evidence for the effects of climate change both globally and regionally. Global warming and sea level rise are now established but may appear insignificant locally, although the expected acceleration in rate may make this more noticeable. The most important issue for individuals and communities, who have to make decisions (based on existing evidence) about how to manage response to climate change, is the likely local impact. This may be in terms of secondary effects, e.g changes in rainfall, and may vary greatly from the global average. Wave height in the North Atlantic, as observed from in-situ and altimeter observations, has increased over the last quarter-century. The North Atlantic Oscillation (NAO) appears to be correlated with increasing wave height in the North Atlantic over recent decades. Prediction of future impacts requires understanding the role of such decadal oscillations and their likely future evolution as well as long-term trends in sea level and wave height due to global warming and possible rapid climate change scenarios. It is important to understand these effects on relatively small scales. Here we examine the impacts of changing wave climate on the rocky coast of NW Scotland, specifically the Sea of the Hebrides.
Monthly mean wave height in the North Atlantic, as observed from insitu and altimeter observations, has increased by about 0.6m over the latter part of the twentieth century (Woolf et al., 2002). The North Atlantic Oscillation (NAO) appears to be correlated with increasing wave height in the North Atlantic over recent decades. Prediction of future changes in wave height requires understanding the role of such decadal oscillations and their likely future evolution, as well as longterm trends in sea level and wave height due to global warming and possible rapid climate change scenarios. We need to know what the role of the NAO is now and whether it will be predictable in the future. It is also important to understand these effects on the relatively small spatial scales at which decisions need to be made but also the mechanisms behind the statistical results still need to be elucidated. Wave observations from in-situ buoys are localized in space. Altimeter wave measurements are localised in time although now providing a good wave climatology since observations are available since 1992. Wave models offer the capability of interpreting, interpolating and extrapolating the available observations and providing predictions for remote sea areas such as the Western Isles of Scotland. Here we present further results for the Sea of Hebrides, which is an area subjected to some of the most severe weather and wave conditions in the UK.
The Sea of the Hebrides is the body of water that lies between the Outer and Inner Hebrides island groups in NW Scotland. The impact of any increase in wave height in the North Atlantic at the coastline will be most significant in this area Crofting, fishing, fish farming and tourism are the most significant economic activities. Impacts of climate change in this area may include interference to ferries and fishing activity, changes in potential wave energy availability and changes in coastal erosion and habitats.
Various recent studies have aimed at understanding and quantifying changes in sea level, storm surge and wave climatology due to present and predicted climate change. Several studies (e.g. WASA and STOWASUS-2100) have examined the effects of climate change on sea level, tides and storm surges over long (several decades) simulations (Flather et al., 1998; Flather and Williams, 2000; Flather et al., 2001). Similarly, long runs of regional wave models have been made e.g. WASA project (Günther et al., 1998), ERA-40 reanalysis (Caires and Sterl, 2003). One objective is to investigate which parts of Britain's coastline may have experienced an increase in wave height similar to that observed in-situ and by satellites in the North Atlantic. The challenge is to combine expertise from satellite remote sensing, statistical analysis and wave modelling to provide a useful tool for those involved in coastal management. The JERICHO project (Cotton et al., 1999) examined trends in offshore wave climate from satellite and buoy data and used the SWAN wave model to transform offshore wave climate to the coast (Wolf et al., 2000, Hargreaves et al., 2001). The Tyndall project on coastal vulnerability has produced results on sea level and wave height due to climate change for three locations: the north Norfolk coast, Christchurch Bay and the Sea of Hebrides
Wave models provide a tool to study detailed impacts of various climate change scenarios. The model system used here comprises three nested models using both the PRO-WAM and SWAN models, from a 1º North Atlantic model, through a 7.5km Malin/Hebrides Shelf model to a 1.85km Sea of Hebrides model. This allows the effect of winds over the whole North Atlantic to be investigated while also studying the local coastal wave impact including refraction and shoaling around the Western Isles of Scotland and make the connection between the statistical results from altimeter data to the dynamics. KEYWORDS: Wave model; climate Oscillation; UK coast; Sea of Hebrides
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(Tsimplis et al., in press). Wang and Swail (2001) and Woolf et al (2002) among others have related North Atlantic wave height to the NAO, a measure of the North Atlantic meridional atmospheric pressure gradient and the strength of the prevailing westerly winds and Woolf et al (2004) discussed the effect of increasing wave heights on ferry services in the Sea of the Hebrides. Wang et al. (2004) show how the wave climate may be projected into the future using climate models with different forcing scenarios.
temperature difference. Reducing this gradient (as would occur under greenhouse warming) results in fewer and weaker storms. Some recent results from the Hadley Centre model suggest an increased frequency of storms (Jenkins et al., 2003) in a global warming scenario but with low statistical significance. It is difficult to predict likely changes in storminess over the next 100 years but further refinements in climate models will mean increasing confidence in the predictions of impacts in the future.
Two different wave models have been implemented: the PRO-WAM model (Monbaliu et al, 2000) for the larger extent of the Malin/Hebrides Shelf and SWAN (Booij et al, 1999) for the higher resolution model of the Sea of the Hebrides. These models share the same basic physics and methodology but differ in implementation. Nested models are more efficient computationally than running large area models for long periods of time at high resolution. First results of the models were presented in Wolf et al. (2002a). Some wave and wind data were available for model validation at locations near the shelf edge (RARH and SES). This paper presents further model validation results using satellite altimeter wave data, and in-situ data near South Uist and at K4 (56.9ºN, 13ºW) and tests of various hypotheses with the models. Some results are presented for different NAO scenarios and some local impacts are discussed. We extend the impacts to examine potential implications for wave power generation and examine the local evidence for an increase in storminess versus a strengthening of the mean winds.
The winds at Stornoway airport (Isle of Lewis) from 1983-1999 are plotted in Fig. 1. Winds for the winter months December-March are included. It may be seen that the mean winter wind speed has an upward trend, although this may not be statistically significant. The winter of 1995-1996 is anomalous in having a negative NAO index and the mean wind direction moves north of west. The storminess can be characterised by the number of hours of wind speed in excess of 32 knots. Although this exhibits a large inter-annual variability it may be seen that the winters of 1988-89 and 1989-90 were particularly stormy. January 1993 was also very prone to gales and was selected for particular study since the largest waves were observed by the TOPEX altimeter during that month. Mean winter wind speed
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SEA OF HEBRIDES The Sea of Hebrides is the body of water that lies between the Outer and Inner Hebrides island groups, separating the islands of Barra, North and South Uist, Benbecula, Harris and Lewis from Skye, Canna, Rum, Coll, Tiree and Mull and is exposed to the most severe wave climate in the UK. The impact of any increase in wave height in the North Atlantic will be most significant in this area. Relative sea level rise in this area has been negligible to date, the local uplift due to glacial rebound approximately cancelling the global rise of about 12mm/year, which is the present consensus, given in the Intergovernmental Panel on Climate Change (IPCC) Third Assessment Report (Church et al., 2001). This may not be true in the future if sea level rise accelerates and rising coastal sea levels may also increase wave height at the coast since shallow water waves are depth-limited. Although the melting of Arctic Sea ice will not contribute to sea level, the increase in fetch for winds from the north could have an impact in increasing wave heights from that direction. Recent trends have seen sea ice in the Arctic reducing by about 3 percent per decade (Cavalieri et al., 2003), a record minimum being reached in September 2002 (Serreze et al., 2003). Conversely any increase in sea ice due to a rapid cooling, possibly over decades (‘The Day After Tomorrow’ effect although not on such a rapid time scale) could much reduce wave heights. In the present-day climate the Norwegian Sea is kept largely ice-free by the Gulf Stream/North Atlantic Drift. At the last glacial maximum it is estimated that the winter sea ice could have extended to the south of Iceland and the Faeroes (Pflaumann et al, 2003), reducing the fetch from the north from thousands of kilometres to a few hundred. Fetch from the west would also have been greatly restricted. Two possible explanations for the increase in wave height with NAO are (i) an increase in the mean zonal wind increasing the build-up of waves over several days or (ii) an increase in storminess i.e. the frequency and intensity of storms (Woolf et al., 2002). The incidence of depressions on the polar front may change due to global warming. Sinclair and Watterson (1999) used the CSIRO General Circulation Model (GCM) of the atmosphere to examine changes in the frequency and strength of mid-latitude storms under a doubled CO2 scenario. The polar regions were found to warm more than the tropics, reducing the equator to pole
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WAVE MODELS The basic model setup was described in Wolf et al (2002). The PROWAM model (Monbaliu et al., 2000) was set up on a NE Atlantic grid
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1.85km grid for the Sea of the Hebrides (SOH) was added. The NE Atlantic model used here (NEA) was extended further north to 70ºN to include more of the wave generation area for northerly wind conditions. As discussed later this model was eventually extended to cover the whole North Atlantic at 1º resolution.
with a nested Malin/Hebrides shelf 7.5km model (see Fig. 2). For the present work a SWAN model (Booij et al., 1999; Ris et al., 1999) on a
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MALIN/HEBRIDES SHELF MODEL Digitised bathymetric data were supplied by the British Oceanographic Data Centre on a grid of approximately 1.85km resolution (1/60º latitude by 1/40º longitude, approximately 1 nautical mile), with 401 by 421 grid-points. A sub-set of this data, extending from 10ºW to 0ºE and 55ºN to 62ºN was chosen to correspond to the area chosen for study of satellite data. An intermediate grid (1/15º latitude by 1/10º longitude) was derived from these data at one-quarter the resolution i.e. about 7.5km, generating just over 10,000 grid-points.
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Within the 7.5km model a nested 1.85km model was set up for the Sea of Hebrides (SOH), extending from 55-58ºN and 5-8ºW. Fig. 3 shows the bathymetry of the SOH model. The SWAN model was implemented on this model grid in stationary mode. The location of the LIMPET wave power installation on Islay is indicated as well as the locations of the wave observations west of South Uist and the TOPEX tracks 44 and 189.
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Fig. 2: NE Atlantic model area showing extent of Malin/Hebrides model (green), Sea of Hebrides nested model (blue) and parts of relevant TOPEX tracks (red)
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Fig. 3: Sea of Hebrides model area showing depth contours, TOPEX tracks (dashed lines) and in-situ measurements (+) near South Uist as well as location of LIMPET wave power installation at SW corner of Islay (o)
MODEL VALIDATION Two months were selected previously for closer study. Periods when the NAO index is positive are designated NAO+ and when it is negative are NAO-. These periods approximately correspond to strong and weak prevailing westerly winds. The NAO+ month selected was February 1997 and the NAO- month was December 1995. Examination of the winds for these periods showed that, during February 1997 the winds were consistently westerly, whereas in December 1995 the winds were mostly easterly.
WAVE POWER Energy generation from waves is being carried out by Wavegen at Islay, using a LIMPET device (www.wavegen.co.uk), the location is shown on Fig. 3. The operation of wave power devices depends on the availability of the resource. Maps of wave energy are given (www.dti.gov.uk/energy/renewables/technologies/atlas.shtml) at a rather coarse scale. Also the installation and maintenance requires suitable weather windows with low wave conditions. The use of local area models can identify more accurately suitable areas for installations. As used here, a local area model, nested within larger forecast models, can assist in detailed planning of operations. Further refinement of the bathymetry and inclusion of a current model (e.g. Wolf et al., 2002b) would be beneficial to resolve the detailed effects of local refraction and shoaling. If the projected increase in both mean and extreme wave height over the twenty-first century occurs as suggested in Wang et al. (2004) then extreme events will become more frequent and the mean wave height will increase, probably implying a reduced life span of the wave power installation but increased output.
The winter of 1995-1996 was previously observed to be anomalous in having a preponderance of easterly winds at Holderness (Wolf, 1998), whereas the winters of 1994-1995 and 1996-1997 were more typical of recent climatic conditions, with prevailing westerly winds. Some wave and wind data were available from the Met Office buoy at RARH (57ºN 9ºW) for December 1995. Some wave data were also acquired from the Shelf Edge Study (SES) for September-October 1995. The location of the SES wind and wave data was 56.46ºN 9ºW. Two events from this period (21-29 September and 8-9 October) were therefore also modelled to test the Malin/Hebrides shelf model.
RESULTS
The wind forcing for the Malin/Hebrides shelf WAM wave model was derived from the ECMWF 1º resolution global model output at 6hourly time intervals. Agreement between model and observations was reasonable with discrepancies attributable to the coarse spatial and temporal resolution of the wind data. Also the data were rather close to the boundary of the model.
The results of the model validation are shown in Figs. 5 and 6. Fig. 5(a) shows the Waverider observations for South Uist for 5-15 February 1982. Fig. 5(b) shows the equivalent period from the Malin/Hebrides shelf model Deep
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This paper presents further model validation results, using satellite altimeter wave data for January 1993 together with buoy data at K4 and in-situ data near South Uist in February 1982. The wind data were obtained from the ERA-40 global reanalysis done by ECMWF on 1º resolution and 6-hourly time intervals.
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TOPEX/Poseidon tracks 189 and 44 are most useful as they penetrate into the Sea of the Hebrides. Large waves were observed in January 1993. Model simulations were carried out for 11, 17 and 21 January to coincide with altimeter passes. Further altimeter tracks 37 and 146 were also used for comparison with the K4 buoy and model results.
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Three Waverider buoys were deployed between 1976 and 1985 at locations west of South Uist, as shown in Fig. 3, referred to as Deep, Offshore and Nearshore. The best year of concurrent data was from July 1981-June 1982, shown in Fig. 4. The largest waves during this period were during February 1982 so this was selected for simulation. 12
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The South Uist data are reasonably well-reproduced reaching a wave height of 8m at the Offshore and Nearshore stations however the Malin model does not distinguish sufficiently between the three stations and does not reach the maximum wave height of over 10m observed at the Deep station. The observed wave heights seem somewhat anomalous at this time since the Nearshore waves are higher than those Offshore. Output from runs of the SOH model are shown on Fig. 5(a). These agree better with the observations due to better resolution of the
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(b) Fig. 5 Wave height at South Uist for February 1982 (a) Observed wave height (m) with SOH model results (b) Malin/Hebrides shelf model wave height (m)
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Fig. 4: One year of wave heights at South Uist
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communication). The former is a likely scenario (Caires and Sterl, 2003) and there will certainly be limitations in the model wind accuracy due to the rather coarse spatial and temporal resolution especially near the peak of an intense depression. In fact the ERA-40 winds along the altimeter tracks were well-modelled for 11 and 17 January although under-predicted (18m/s instead of 21 m/s) for 21 January. Further buoy data were acquired at K4 (56.9ºN 13ºW) for January 1993. This confirms a general underestimate of the model wave height although the timing of events is good (Fig. 7) and the altimeter is in good agreement with the buoy although not coinciding with the largest events. Wang and Swail (2002) produced better kinematically-reanalyzed wind fields using a very labour-intensive method, paying particular attention to the modelling of extratropical storms. This reiterates a well-known conclusion that the accuracy of wave models is limited by the accuracy of the atmospheric model used to provide the forcing, the solution to which is for better highresolution wind reanalyses to become available.
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Typical wind systems for extreme events as experienced here are strong SW winds as a depression develops, passing north of Scotland; winds back southerly and then veer westerly again. Usually the NE Atlantic experiences a rather separate weather system and modelling this in isolation may be acceptable. Fig. 8 shows the wind field over the North Atlantic just before the maximum wave event observed on 21 January 1993.
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(b) Fig. 6: Along-track TOPEX altimeters wave height compared with model results (a) Track 44, (b) Track 189. Horizontal axis is horizontal along-track distance in metres. Vertical axis is wave height in metres. Black line is TOPEX, red line is Malin model driven by ERA-40 winds, blue line is SOH model forced by TOPEX data at boundary The model significantly underestimates the altimeter waves in January 1993 (Fig. 6), although the general along-track trend is similar. There is more variability in the altimeter wave heights compared to the much smoother model results. The marked reduction in wave height in the lee of the islands is evidenced in Fig. 6(b) at the eastern end of track 189. One possibility for the underestimation of wave height is that the fetch was constrained in the NE Atlantic model so a full North Atlantic model was implemented to provide improved boundary conditions. This only increased the wave heights slightly, by about 5% on average in the strong wind events, with a maximum increase of 16% further offshore near the storm centre (but not coincident to the maximum wave heights experienced by the altimeter). Two other possibilities are (a) the ERA-40 winds are underestimated especially in extreme winds and (b) the altimeter waves are overestimated (the altimeter algorithms are less well-validated for waves over 10m – Woolf, private
Fig. 8: Winds over North Atlantic from ERA-40 at 18:00 21 January 1993. Wind-speed in m/s The strongest winds are confined to the NE quadrant although in this case westerlies extend all the way across the North Atlantic and
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Exploring the range of possible present-day scenarios can be done by forcing the SOH model with wave climate statistics (e.g. 90th percentile etc.) derived from altimeter data as shown in Fig. 11 for a location near Lewis, for the months of January to March. The extreme waves are most sparse so the distribution is less well-defined at the upper end. Examining the statistics of extremes is complex e.g. requiring a nonstationary generalized extreme value analysis (Wang et al., 2004).
contribute to the wave height due to their extensive fetch. If the westerly winds were underpredicted further offshore it could explain part of the underprediction in wave height. It is interesting to note that the local maximum wind speed of just over 20m/s is very similar on 11 and 21 January but the waves are significantly higher on 21 January so the difference must be due to the different evolution of the wind field in the Atlantic. Higher wind-speeds require a longer duration for the waves to reach full development. This shows how models are required to help understand observational results by including non-local effects in space and time.
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Results for 21 January 1993 from the Malin/Hebrides shelf model and the SOH model are shown in Figs. 9 and 10. The former is driven by the ERA-40 winds and so underestimates the wave height as discussed above. The SOH model has been forced by wave and winds observed at the boundary of the model area by the altimeter (assuming constant winds over the area) and this allows a hindcast to be done which gives good agreement along the altimeter track as shown in Fig. 6(b). Malin/Hebrides Shelf Model 20:00 21 Jan 1993 62 12
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Further results from the SOH model are shown in Fig. 12. These illustrate possible typical (rather than extreme) wave conditions in NAO+ (10m/s SW winds) and NAO- (5m/s NE winds) winter months as exemplified by February 1997 and December 1995. In the NAO+ scenario (most typical in recent winters) large waves are generated approaching from the SW. This may be due to the passage of a depression possibly preceded by a period of intense westerlies. Waves in the Sea of the Hebrides will vary greatly depending on location and the amount of shelter available locally; the SW coasts of Islay, Mull, Skye and South Uist are exposed to the full force of the waves. Much lower waves may be expected in the lee of the Outer Hebrides e.g. in the Little Minch. In the rather anomalous NAO- scenario the winds are from NE and not as strong. The west coast of Scotland is much less exposed from this direction. Some waves penetrate from the north and short steep seas can be generated locally in fetch-limited growth even with offshore-directed winds, crossing seas are suggested in the lee of islands and the effect of local strong tidal currents (not considered here) could also be important. In future climate change scenarios we need to know whether the NAO relation with wave height will continue and if so what the NAO index will do. Wang et al. (2004) assume the relationship will continue to hold and suggest the occurrence of the positive phase of the NAO will be more frequent under global warming. The problem is that for waves our observations only extend over the last few decades. Jevrejeva et al (2005) study the relation between atmospheric circulation patterns such as the NAO and sea level (for which records extend much further into the past) and show that relationship changes over time. This may also be true for waves.
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depressions in generating severe wave conditions. The local area model can also be forced by wave climatology to provide inshore climatology in specific locations.
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ACKNOWLEDGEMENTS
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The work described here was (partly) supported by the Tyndall Centre for Climate Research Project "Towards a vulnerability assessment of the UK coast". Additional support from the United Kingdom Natural Environment Research Council was through the POL Programme 1 and “Ocean Variability and Climate” core strategic programmes and the Centre for observation of Air-Sea Interaction and fluXes (CASIX). Digitised bathymetric data were supplied by BODC. Winds at Stornoway were obtained from the BADC web site. Model winds were obtained from the ECMWF ERA-40 reanalysis. Wave data at K4 were supplied by the Met Office.
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REFERENCES −7.5
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Booij, N., R.C. Ris & L.H. Holthuijsen 1999. A third-generation wave model for coastal regions, Part I, Model description and Validation. Journal of Geophysical Research, 104, C4, 7649-7666. Cavalieri, D. J.; Parkinson, C. L.; Vinnikov, K. Y. 2003 30-Year satellite record reveals contrasting Arctic and Antarctic decadal sea ice variability. Geophysical Research Letters, 30, 18, doi:10.1029/2003GL018031 Caires, S. and Sterl, A. 2003 Validation of ocean wind and wave data using triple collocation. Journal of Geophysical Research, 108, C3, 3098, doi:10.1029/2002JC001491. Church, J.A., Gregory, J.M., Huybrechts, P., Kuhn, M., Lambeck, K., Nhuan, M.T., Qin, D. & Woodworth, P.L. 2001 Changes in sea level. In Climate Change 2001: The Scientific Basis. Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, J.T. Houghton, Y. Ding, D.J. Griggs, M. Noguer, P.J. van der Linden, X. Dai, K. Maskell and C.A. Johnson, eds. Cambridge University Press, Cambridge, 881pp. Cotton, P.D., Carter, D.J.T., Allan, T.D., Challenor, P.G., Woolf, D. Wolf, J. Hargreaves, J.C., Flather, R.A., Bin Li, Holden, N. & Palmer, D. 1999 JERICHO Project final report. Satellite Observing Systems. Flather, R.A., Smith, J.A., Richards, J.D., Bell, C. & Blackman, D.L. 1998. Direct estimates of extreme storm surge elevations from a 40-year numerical model simulation and from observations. The Global Atmosphere and Ocean System, 6, 165-176. Flather, R.A. & Williams, J.A. 2000. Climate change effects on storm surges: methodologies and results. pp. 66-78 in Beersma, J., Agnew, M., Viner, D. and Hulme. M. (eds.) Climate scenarios for water-related and coastal impact. ECLAT-2 Workshop Report No. 3, KNMI, the Netherlands, 10-12 May 2000. CRU, Norwich, UK. 144pp. Flather, R.A., Baker, T.F., Woodworth, P.L., Vassie, J.M. & Blackman, D.L. 2001 integrated effects of climate change on coastal extreme sea levels. 36th Conf of River and Coastal Engineers, Keele University, June 20-22 2001. Günther, H., Rosenthal, W. Stawarz, M, Carretero, J.C., Gomez, M., Lozano, I., Serrano, O. and Reistad, M. (1998) The Wave Climate of the North-East Atlantic over the period 1955-1994: the WASA Wave Hindcast. The Global Atmosphere and Ocean System, 6, 121-163. Hargreaves, J.C., Carter, D.J.T., Cotton, P.D. & Wolf, J. 2001 Using the SWAN wave model and satellite altimeter data to study the influence of climate change at the coast. The Global Atmosphere and Ocean System, 8, 1, 41-66.
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CONCLUSIONS The PRO-WAM and SWAN models have been implemented on several grids for the purpose of studying wave transformation in the Sea of the Hebrides. The ultimate aim is to assess the vulnerability of different coastal communities to climate change by providing detailed local predictions of wave conditions. Initial results on validation of the wave models appear satisfactory. Discrepancies between the model and observations can mostly be ascribed to the limitations in spatial and temporal resolution of the model winds. The possible effects of intensification of westerly winds versus increased storminess have been discussed briefly. There is some evidence from wind record at Stornoway and analysis of the weather patterns in the Atlantic for 21 January 1993 to suggest that the intensification of westerlies may be significant. Use of the Met Office mesoscale model data (at 12km and hourly resolution) would no doubt remove one source of uncertainty but these data were not available until very recently. Further runs with this model system can be carried out to further elucidate the relative importance of sustained westerlies versus intense
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