The influence of the North Atlantic Oscillation on the sea-level around the northern European coasts reconsidered: the thermosteric effects.

The thermosteric contribution of the North Atlantic Oscillation (NAO) to the North Sea sea-level for the winter period is investigated. Satellite sea surface temperature as well as in situ measurements are used to define the sensitivity of winter water temperature to the NAO as well as to determine the trends in temperature. The sea surface temperature sensitivity to the NAO is about 0.85 degrees C per unit NAO, which results in thermosteric sea-level changes of about 1-2 cm per unit NAO. The sensitivity of sea surface temperatures to the NAO is strongly time-dependent. Model data from a two-dimensional hydrodynamic tide+surge model are used in combination with the estimated thermosteric anomalies to explain the observed sea-level changes and, in particular, the sensitivity of the datasets to the NAO variability. The agreement between the model and the observed data is improved by the inclusion of the thermosteric effect.

Modelling waves and surges during the 1953 storm.

Waves and sea levels have been modelled for the storm of 31 January-1 February 1953. Problems in modelling this event are associated with the difficulty of reconstructing wind fields and validating the model results with the limited data available from 50 years ago. The reconstruction of appropriate wind fields for surge and wave models is examined. The surges and waves are reproduced reasonably well on the basis of tide-gauge observations and the sparse observational information on wave heights. The maximum surge coincided closely in time with tidal high water, producing very high water levels along the coasts of the southern North Sea. The statistics of the 1953 event and the likelihood of recurrence are also discussed. Both surge and wave components were estimated to be approximately 1 in 50 year events. The maximum water level also occurred when the offshore waves were close to their maximum. The estimation of return period for the total water level is more problematic and is dependent on location. A scenario with the 1953 storm occurring in 2075, accounting for the effects of sea level rise and land movements, is also constructed, suggesting that sea level relative to the land could be 0.4-0.5m higher than in 1953 in the southern North Sea, assuming a rise in mean sea level of 0.4m.