North Atlantic Oscillation

The North Atlantic Oscillation (NAO) is the name scientists have given to the see-saw-like climate pattern long observed over the North Atlantic Ocean. This pattern, or 'climate mode', most notably affects the winter weather in Europe and parts of North America, as well as the fish stocks of the North Atlantic. Technically, the NAO is characterized by an oscillating pressure variability between the normally sub-tropical high pressure area located over the vicinity of the Azores (Portugal) and a sub-polar low pressure area over the vicinity of Iceland. In general terms, the high over the Azores and the low over Iceland are regular climatic features, however during winter they experience an intensification, and it is this intensification that denotes an NAO event. Depending on the direction of the pressure intensification, an NAO event is considered either positive or negative. During the positive phase of the NAO, winters in northern Europe tend to be warmer and wetter, while the Mediterranean countries experience less precipitation; conversely, a negative phase of the NAO is generally associated with a colder winter across northern Europe and a wetter one in the Mediterranean.

Scientists are uncertain of the mechanism that drives the North Atlantic Oscillation, however they do know that it is not a purely atmospheric phenomenon, rather it is a result of ocean-atmospheric interactions. An average of 2 or 3 NAO events occur per winter. However, in the last 30 years, the occurrance of positive NAO events has markedly increased. Whether this increase is a natural part of the climate mode, or a result of anthropogenically induced changes to the chemical make-up of the atmosphere (ozone depletion, dramatically increasing levels of carbon dioxide) is currently one of the major questions researchers are trying to answer.

A climate index is a tool scientists develop to map basic facts of climate modes in a manner that reveals the basic pattern of the climate mode. The NAO index is derived from the steepness of the pressure gradient between the high and the low pressure systems mentioned above. North Atlantic Oscillation Index depicting postive and negative NAO events. Graphic and source material courtesy of Martin Visbeck of the Lamont-Doherty Earth Observatory