The NAO was discovered in the 1920s by Sir Gilbert Walker. Similar to the El Nino phenomenon in the Pacific Ocean, the NAO is one of the most important drivers of climate fluctuations in the North Atlantic, Europe, the Mediterranean Sea and as far east as northern sections of central Asia.
Especially during the months of November to April it explains a good deal of the variability of atmospheric disturbances in the North Atlantic region, and consequently wind speed and wind direction changes, changes in temperature and moisture distribution (within the region) and the intensity, number and track of storms. It is a key driver e.g. of whether winter storms track northward toward northern Europe, or more southerly toward the Mediterranean Sea.
A great deal however is not known, for example why the phase of the NAO fluctuations has reversed since the early 1980's with consequent widespread effects on North Atlantic and European weather that are difficult to disentangle from anthropogenic effects (the effects of man and the global economy).