Andrew G. P. Shaw

The influence of the North Atlantic Oscillation on sea-level variability in the North Atlantic region

Satellite altimeter (Topex/Poseidon, 1992–2001) and tide-gauge measurements are used to explore the relationship of the sea level of the North Atlantic and neighbouring seas and coastlines to the North Atlantic Oscillation (NAO). Altimeter measurements suggest significant gyre-scale influence of the NAO in the North Atlantic, but also stronger influences on the continental shelf and inland seas of Europe. A north–south dipole in sea-level anomaly consistent with a hydrostatic response to the NAO sea-level pres- sure dipole is evident, but there are also large non-hydrostatic effects. The strongest response on the European Shelf is in the southeastern part of the North Sea where sea level is positively correlated to NAO Index. The sea level in two semi-enclosed seas, the Baltic Sea positively and the Mediterranean Sea negatively, is also strongly influenced by the NAO. A weak negative correlation is apparent around the northeastern coastline of North America. These features are confirmed by contemporary coastal tide-gauge data, but the tide-gauge data also show that the influence of the NAO was weaker early in the Twentieth Century (20C) on parts of the Northwest European coastline. Inter-annual sea-level variability associated with fluctuations in the NAO are generally much larger than those associated with secular trends. Inferred multi-decadal fluctuations associated with the NAO are very substantial compared to the 15(35) cm estimated for 20C global sea-level rise (Church, J.A., Gregory, J.M., Huybrechts, P., Kuhn, M., Lambeck, K., Nhuan, M.T., Qin, D. and Woodworth, P.L. (2001). Changes in sea level. Chapter 11 of the Intergovernmental Panel on Climate Change Third Assessment Report, pp. 639–694. Cambridge University Press, Cambridge.) and scenario forecasts for the 21C (350 cm). Therefore, the behaviour of the NAO in the next few decades will be a major regional factor in sea-level rise and coastal vulnerability in some European regions.

Sea level extremes in southern Europe

Knowledge of sea level extremes is important for coastal planning purposes. Temporal changes in the extremes may indicate changes in the forcing parameters, most probably the storm surges. Sea level extremes and their spatial and temporal variability in southern Europe are explored on the basis of 73 tide gauge records from 1940. This study uses all data available to infer risks at the coast caused by extreme sea levels. Extreme values of 250 cm are observed at the Atlantic coasts with smaller values in the Mediterranean where, with the exception of the Strait of Gibraltar and the Adriatic Sea, the extreme values are less than 60 cm. At the Adriatic Sea values of up to 200 cm are found. When the tidal contribution is removed the differences between the various areas reduce. The spatial distribution of the extremes of the tidal residuals is well represented by the hindcast of a two-dimensional hydrodynamic model forced by the atmospheric pressure and the wind, although the model underestimates the extremes. Higher return levels (200–300 cm for the 50-year return level) are observed in the Atlantic stations due to the larger tides. In the Mediterranean, higher values are found in the northern Adriatic (between 150 and 200 cm) while in the rest of the domain they vary between 20 and 60 cm. The nonlinear interaction between tides and surges is negligible in the Mediterranean, thus the joint tides-surges distribution can be applied. The interannual and decadal variability in time of extremes is caused by mean sea level changes.

Past and Current Climate Changes in the Mediterranean Region

Mediterranean climate change during the last 60 years is based on homogenized daily temperature and quality controlled precipitation observational data and gridded products. The estimated changes indicate statistically significant Mediterranean summer temperature increase and a reduction in winter precipitation in specific areas. Reconstructions of Mediterranean sea level suggest a rise of some 150 mm since the beginning of the nineteenth century. A 20 years long reanalysis (1985–2007) was produced, showing long term temperature variability and a positive salinity trend in the ocean layers from the surface to 1,500 m depth. A prominent increase in summer temperature extremes is found in the whole Mediterranean region, while warm bias in the mid twentieth century station data is removed by homogenization. No basin-wide trends in precipitation and droughts are found for the second half of the twentieth century, while trends in extreme winds are largely negative, as are those of the related cyclones and cut-off-lows. The role of large scale pressure patterns like the NAO for variabilities and trends is discussed for the different parameters considered.