Evolution of the North Atlantic Oscillation

The year-to-year variations of winter climate of the North Atlantic region including Northern Europe, northwest Africa and North America are largely influenced by the phases of a meridional pressure gradient in the ocean-the North Atlantic Oscillation (NAO). The NAO has been examined using winter sea-level pressure data and an index has been formulated that captures the variation of its intensity. The oscillation is shown to be detected in a coupled ocean-atmosphere general circulation model. The simulated teleconnections in the peripheral land masses of the North Atlantic Ocean are compared with observations. It is suggested that explanations of these climatic variations may be found through diagnoses of such model simulations. The evolution of the NAO has been investigated by inspecting the progression of sea-surface temperature (SST) and sea-level pressure (SLP) anomalies for the high and low phases of the oscillation. The progression of the anomalies is found to be generally similar but with opposite signs in the two phases. The high phase is foreshadowed by a major reversal of sea-level pressure conditions in the Icelandic Low region in October. Analyses indicate that the high phase is characterized by a distribution of warm water in the central portion of the North Atlantic. This warm water is flanked by anomalously cold water in the north and in the tropics and subtropics. This distribution is explained as a consequence of the intensification of currents in the Atlantic gyres which is related to the strengthening of the Azores High and Icelandic Low. This explanation is supported by the OSU GCM simulation.