The North Atlantic oscillation influence on the wave regime in Portugal: an extreme wave event analysis

Abstract

Waves in the North Atlantic are strongly seasonal, and peak in the winter season. The west coast of Portugal is exposed to winter swell, generated by wind associated with North Atlantic extratropical cyclones. The track of these storms, generated near the North America east coast, is strongly influenced by the North Atlantic Oscillation (NAO). When the NAO is in its positive phase they normally track northeast and reach Western Europe well north of the Iberian Peninsula, in the British Islands or Scandinavia. However, in the negative NAO situation, the track of the storms is more zonal and south than usual, due to a weakened NAO. The characteristics of wave regime in Portugal are shown to be strongly related to the NAO phase and corresponding storm tracking. Positive NAO storms, tracking northeast towards the north of Europe, drive longer period swell from the northwest, whereas negative NAO storms have associated shorter period swell arriving to Portugal from a more westerly direction. The relation between the NAO phase and the storm tracks and the characteristics of the wave regime is investigated with ten year observations from four directional waverider coastal buoys, located off the coast of Portugal