One of the most remarkable recurring atmospheric phenomena over the Atlantic 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, with which the ocean interacts.

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.
We have observed weather variations in this way since the 17^th 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 drive our weather. Improving our understanding of these variations over periods of ten years and more is key to reliable climate forecasting. In this respect, the permanent ocean-observing capability afforded by altimetric satellites, in combination with other satellites and in-situ measurements, is a vital aid.