Sea level variability: examples from the Atlantic coast of Europe

Abstract

This thesis investigates the statistics of the long-term variability in sea level records and trends along the Atlantic coast of Europe. This is undertaken for a range of regionally-representative open-sea sites. Tidal variability on a shorter time-scale is also investigated, for a local area subjected to intense natural and anthropogenic activity.
The longest available records of hourly sea level data have been analysed for (6) ports around the English Channel, and (5) around the Iberian Peninsula. A separate statistical analysis is used for the components of sea level variability: mean sea level, tides and meteorological residuals (surges). Extremes and percentiles of some of these components have also been studied. This shows that mean sea levels are increasing at between 1 and 3 mm yr-1. Strong influences are found in the standard deviation of the total observed sea level. The various tidal constituents show interesting, localised short-term variations in amplitude and phase. Some long-term trends exist and are mainly associated to local effects. There is no evidence of an increase in weather effects on sea levels, over the period analysed. Some trends are identified in extreme maximum and minimum levels; however, in most cases, they are influenced only by mean sea level. Sea level response to the NAO has been analysed. The NAO was found to influence extreme levels in the meteorological residual (standard deviation), at stations where the NAO´s centre of action are known to be strongest. Mean sea levels, detrended sea levels and non-tidal residual sea level standard deviations around the western European coastline are generally correlated negatively with the NAO index; the only exception to this is Dover, which is correlated positively to the NAO. This observation is consistent with the known positive correlation in the south eastern region of the North Sea. Sensitivity to the NAO varies, with the highest values found along the northern Spanish coastline. Overall, Coruna shows the greatest influence of the NAO, and, in the English Channel, this is found at Newlyn. Influences on the non-tidal meteorological residuals are also found within the northern region of Spain, suggesting that variance in this component is influenced considerably by the NAO.
More locally, the responses of coastal lagoons, to long-term sea level trends, are crucial for coastal management. Pressure measurements collected in the Ria de Aveiro Lagoon, Portugal, show a general increase in amplitude and decrease in phase, over the past 16 years. The causes of these changes are investigated using: (a) an analytical model of a wave propagating through a narrow inlet, into a Lagoon (analogy to RLC-circuit); and (b) a Bi-Dimensional Horizontal (2DH) hydrodynamic (vertically-integrated) model of the Ria de Aveiro Lagoon. Results suggest that changes in the bathymetry are the most significant contribution to the changes identified in the tides.

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