Northern Hemisphere Climate Variability: Character, Forcing Mechanisms, and Significance of the North Atlantic/Arctic Oscillation

The North Atlantic Oscillation (NAO) has long been recognized as an important source of climate variability over the north Atlantic region. The NAO is often referred to as the regional expression of the Arctic Oscillation that reflects a redistribution of atmospheric mass between the Arctic and the middle latitudes of the Northern Hemisphere (NH). This review highlights our current understanding of these prominent teleconnection patterns and discusses outstanding questions and challenges that scientists face as they continue to build our knowledge of climate variability and change across the NH. The work discusses the strengths and limitations of the multitude of methods by which these phenomena have been defined, while highlighting recent changes in their character. Addressed also are the various internal and external forcing mechanisms that shape these phenomena and ultimately help define their predictive nature.