1.4.2. North Atlantic Oscillation (NAO)
One of the Atlantic's most remarkable recurring atmospheric phenomena makes its presence felt in winter. Called the North Atlantic Oscillation (NAO), it is driven by pressure differences between a high-pressure system over the Azores and a low-pressure system over Iceland, which interacts with the ocean.
Wave heights in winter (in metres per NAO index unit) are very sensitive to pressure variations over the North Atlantic. To the west of Ireland, for example, waves are significantly higher when the NAO index is positive.
The weather over the Atlantic exhibits trends that recur over the years in close step with the ocean. From North America to Siberia, regions bordering the Atlantic are exposed in turn to rain or drought, cold or mild temperatures, and strong winds or dead calm. In winter, two air masses influence atmospheric circulation over the North Atlantic: a depression to the north around Iceland, and an anticyclone to the south around the Azores. Their intensity shapes weather conditions on the shores of the ocean. Pressure variations in these air masses, and the associated ocean variations that interact with them, are the driving forces behind the North Atlantic Oscillation (NAO). There are two extremes or 'phases' of the NAO: a positive phase, when the 'Azores high' is especially high and the 'Icelandic low' is lower than normal, and a negative phase, when both are weaker. We have been observing these particular weather variations since the 17th century. But it was only when satellites arrived on the scene that we could begin continuous, long-term monitoring of the oceans and atmosphere to unlock the secrets of the mechanisms that control our weather. Improving our understanding of these variations over periods of ten or more years is essential to reliable climate forecasting. In this respect, the permanent ocean-observing capability afforded by altimetry satellites, in combination with other satellites and in-situ measurements, is a vital aid.
Sea level variation over the North Atlantic (seasonal and turbulent variations removed) computed from Topex/Poseidon. The NAO index variation is also shown.
Fluctuations in the North Atlantic Oscillation and the temperature variations that go with it lead to changes in sea level. The ocean reacts to shifts in the prevailing winds, which drive the currents, waves, sea surface temperature, etc. Temperature swings also cause sea surface height to vary. We can observe these variations using altimetry satellites. All these measurements can thus serve as indicators of NAO phases and can be used in climate prediction models.
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