Global changes and variability in extreme sea levels from 1846-2014

Abstract

Extreme sea levels exert a major control over the coastal zone, and many studies have found that they are changing at locations around the world. This thesis tests the assertion that these changes are predominantly caused by the global rise in mean sea level by investigating the importance of variability, over a range of timescales, in the other main components of sea level. The analysis is undertaken using a quasi-global dataset of 220 tide gauge records that range in length from 28-164 years.
For the first objective, secular (linear) trends in 15 different tidal levels were found to be significant (95% confidence) at between 34% and 63% of the study sites, depending on the tidal level analysed. Significant trends were distributed at sites around the world and at 37 sites the magnitude of the trends was over 1 mm/yr, comparable to the rise in global mean sea level over the 20th century. Spatial patterns were observed on local, regional and global scales. The global rise in mean sea level may be the cause of significantly more positive trends than negative trends occurring in high water levels, but other mechanisms appear more important at many locations.
The second objective assessed changes in the meteorological component of sea level. Significant tide-surge interaction was found at 59% or 81% of the 220 study sites, depending on the method used. At locations that have significant tide-surge interaction skew surge is the better parameter for the representation of the meteorological component. Only 13% of sites had significant secular trends in skew surge. There were significantly fewer negative trends in skew surge than the non-tidal residual, which may be because the influence of phase offsets is removed when using skew surge. Inter-annual variability in skew surge is large, but strong correlations between different regional skew surge and climate indices were not found to be significant in this thesis.
The third objective evaluated how the variability in the tidal and meteorological components of sea level - together with changes in mean sea level - influenced seasonal, inter-annual and secular changes in extreme sea levels. Variability in baseline extreme sea level (i.e. timescales greater than 6 months) was dominated by secular changes in mean sea level and the seasonal and inter-annual variability in mean sea level and skew surge, at most sites. The combined magnitude of the extracted signals reached 2.4 m in the North Sea, but was typically between 0.8 and 1.2 m. Changes in the relative phase of each signal, as well as the magnitude, may alter the magnitude of extreme sea levels. If independent signals in different components and timescales occurred in phase then the baseline extreme sea levels would increase at all sites in this study.
This thesis presents novel findings that show that while secular trends in mean sea level are important to changes in extreme sea levels, significant changes are occurring in all components and over many timescales. Shifts in both the magnitude and phase of signals in all components and over all timescales should be considered in extreme sea level projections. Accurate calculation of extreme sea levels has important implications for applications in the coastal zone, including flood defence, navigation, energy extraction and habitat protection.

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ORCID for Ivan Haigh: orcid.org/0000-0002-9722-3061

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