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S U M M A R Y Time series of annual Mean Tidal Range (MTR) have been assembled from 13 ports around the British Isles and secular trends in MTR computed at each site. Trends vary between -1.8 and 1.3 mm yr-' depending on location. At many sites the values are significantly non-zero implying different trends in Mean High Waters (MHWs) and Mean Low Waters (MLWs). Such tidal behaviour has also been observed previously at stations along the adjacent European coastline, but is not well understood. At several places, the trends are sufficiently large that they should be taken into account in investigations of impacts of sea level change and in extreme level engineering studies. They also suggest that, in general, time series of MWH should not be used as proxies for series of Mean Sea Level (MSL). For most of the British Isles data, MTR secular trend is larger (more positive) for larger trend in local MSL, or water depth. Lerwick and Newlyn hourly heights have been used to show that the observed MTR trends at most locations must be due primarly to changes in the dominant M2 tidal constituent. A comparison is given of the British Isles findings to those from neighbouring countries; British, Irish, French, Belgian and perhaps southern Dutch MTR trends are found to be considerably less than those reported from the northern Netherlands and the German Bight.

Changes in values of annual maximum high water (AMxHW), annual maximum surge-at-high-water (AMxSHW) and surge at annual maximum high water (SAMxHW) have been investigated using tide gauge data from Liverpool for the period 1768-1999. AMxHW and SAMxHW (measured with respect to mean high water) were found to vary considerably from year to year, but to exhibit no long-term change over the 232 years. On the other hand, values of AMxSHW were found to be larger in the late-18th, late-19th and late-20th centuries than for most of the 20th century, qualitatively consistent with knowledge of temporal variations in storminess in the region based on meteorological data and anecdotal information. The generalized extreme value method was used to present the available data on AMxHW and other annual extreme parameters in the 'return period' form most often employed by coastal engineers, with conclusions on the differences between each set of parameters in each epoch consistent with those obtained from the original time series. Finally, changes in the statistical distribution of surge-at-high-water (SHW), demonstrated by investigation of variations of percentile levels of SHW values, provided additional information on the temporal variations in extreme surges to that provided by AMxSHW values, pointing in particular to increased storminess during the late-18th and late-20th centuries, with a suggested secular trend in distribution shape from the late-18th century until recent decades.

Within the framework of a Tyndall Centre research project, sea level and wave changes around the UK and in the North Sea have been analysed. This paper integrates the results of this project. Many aspects of the contribution of the North Atlantic Oscillation (NAO) to sea level and wave height have been resolved. The NAO is a major forcing parameter for sea-level variability. Strong positive response to increasing NAO was observed in the shallow parts of the North Sea, while slightly negative response was found in the southwest part of the UK. The cause of the strong positive response is mainly the increased westerly winds. The NAO increase during the last decades has affected both the mean sea level and the extreme sea levels in the North Sea. The derived spatial distribution of the NAO-related variability of sea level allows the development of scenarios for future sea level and wave height in the region. Because the response of sea level to the NAO is found to be variable in time across all frequency bands, there is some inherent uncertainty in the use of the empirical relationships to develop scenarios of future sea level. Nevertheless, as it remains uncertain whether the multi-decadal NAO variability is related to climate change, the use of the empirical relationships in developing scenarios is justified. The resulting scenarios demonstrate: (i) that the use of regional estimates of sea level increase the projected range of sea-level change by 50% and (ii) that the contribution of the NAO to winter sea-level variability increases the range of uncertainty by a further 10-20cm. On the assumption that the general circulation models have some skill in simulating the future NAO change, then the NAO contribution to sea-level change around the UK is expected to be very small (<4cm) by 2080. Wave heights are also sensitive to the NAO changes, especially in the western coasts of the UK. Under the same scenarios for future NAO changes, the projected significant wave-height changes in the northeast Atlantic will exceed 0.4m. In addition, wave-direction changes of around 20 degrees per unit NAO index have been documented for one location. Such changes raise the possibility of consequential alteration of coastal erosion.