Trends in U.K. mean sea level

Secular trends in mean sea level have been measured at a number of tide gauge stations around the U.K. coast. Four of the longest time series (from Aberdeen, North Shields, Sheerness and Newlyn) span the common epoch 1916–82, during which sea level has been rising at all four stations at rates of the order of 1–2 mm/year. Over this timescale, sea level at Aberdeen has risen at a rate of 1.29 ± 0.22 mm/year slower than at Newlyn, which, assuming no oceanographic change in level between the two sites, infers a north‐south tilting of the U.K. mainland similar to that obtained by other authors. There are, however, important quantitative differences between the results of this and previous analyses. Sea level at Sheerness, meanwhile, is found to have been rising even faster than at Newlyn (by 0.62 ± 0.20 mm/ year), supporting the idea of greater land submergence in the south‐east of the country. The long sea level record available from Brest, France has enabled the computation of relative sea‐level secular trends between that station and the four longest‐established U.K. stations, using all their available data. The most accurate of these relative trends is that for Aberdeen over the period 1862–1982, during which time Aberdeen sea level has risen on average 0.78 ± 0.09 mm/year slower than at Brest, and at approximately the same relative rate throughout the period, as might have been expected if most of the change in sea‐level difference had been due to slow land movements. Rather interestingly, during 1916–82 Newlyn sea level has been rising 0.70 ± 0.13 mm/ year faster than at Brest, just across the English Channel. While rates of change of sea‐level difference between stations are capable of relatively straightforward interpretation, the complete understanding of the observed sea level secular trends at each station presents a more difficult task. Local winds and air pressure are found to have little spatially consistent influence on calculations of U.K. sea‐level secular change, although they are responsible for a large fraction of the higher‐frequency sea level interannual variability. There is no measurable secular change of the mean sea level seasonal cycle; similar sea level secular trends are observed in each part of the year. The large rates of rise of U.K. sea level, observed for the first part of this century, are found to have been significantly reduced at most stations in recent years. The primary cause of this deceleration has been a large drop in sea levels during the mid‐1970s, a feature which is also observed in records from stations along most of the European Atlantic coastline. Possible mechanisms for this are discussed. U.K. sea level secular trends for this century are consistent with containing a contribution of the order of the ‘global’ estimated rise (1–2mm/year), although the inability of the tide gauge records to distinguish between long‐term sea level changes and absolute land movements presents a major analysis difficulty. The two main requirements for future sea‐level secular change studies are shown to be (1) the decoupling of vertical, land and ocean changes by means of the connection of U.K. tide gauges to accurate, modern geodetic networks and through improved geodynamic modelling, and (2) the continued study of sea level variability at all time scales, in order to provide more accurate estimates of sea level secular trends.