85th AMS Annual Meeting

Tuesday, 11 January 2005
An Extreme Event Analysis for the Portuguese Coast - Coupling of Atmospheric and Oceanographic Features
Alvaro Semedo, NPS, Monterey, CA; and W. A. Nuss and T. H. C. Herbers
The coast of Portugal is exposed to swell, generated thousands of kilometers West by the wind associated with North Atlantic extratropical cyclones. Under the appropriate meteorological conditions, mostly between late fall and early spring, the significant wave height can reach values greater than 7 meters. The North Atlantic extratropical cyclones, generated near the North America East coast, normally move from west-southwest to east-northeast, and reach Western Europe well North of the Iberian Peninsula, in the British Islands. Occasionally, however, the semi-permanent Azores high migrates equatorward, and no longer constitutes a barrier to the eastward movement of these depressions. In these situations, the track of the North Atlantic extratropical cyclones is more zonal and south than usual. The storm and the swell are coupled and propagate at the same speed and nearly in the same direction, towards the Iberian coast. It can happen that a train of two or three extratropical cyclones, separated by no more than 1,500 kilometers and propagating at more than 50 knots, accelerates through the North Atlantic and hits the coast of Portugal and Galicia. With such a large fetch and strong wind blowing steadily in the same direction for several days, extremely high swell and sea waves are then registered near the coast, with severe impact on harbors and other coastal structures. The purpose of this study is to investigate the correlation between the North Atlantic storms and the wave heights off the coast of Portugal. Analysis of data from two directional waverider coastal buoys, located off the coast of Portugal, in Leixões and Sines, are compared with model simulations using the US Navy’s Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS) and the NOAA’s, Wavewatch III (WW3) wave prediction model. The relationship between wave forcing by these storms, their track and propagation speed, relative to wave packets that combine to produce extreme coastal wave heights will be examined.

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