The North Atlantic Oscillation (NAO)

The North Atlantic Oscillation (NAO): ever heard of it? It is a hot new topic among weather folk and it has even grabbed a headline or two. It is a newly understood mechanism for variability in precipitation in Europe and the Mediterranean as far east as the Middle East.

The NAO accounts for fully one third to one half of the winter climate variability over Greenland, Europe, and the Mediterranean, particularly in terms of precipitation and temperature.

We’ve all heard of the Southern Oscillation and its relationship to the El Niño/La Niña cycle. The two are so closely tied, they are commonly lumped together as the El Niño/Southern Oscillation (ENSO) cycle.

The NAO works in a similar manner to the Southern Oscillation. It is a large-scale meridional seesaw of atmospheric mass between the Atlantic subtropical high (most of us know it as the Bermuda or Azores high) and the Atlantic sub-polar low near Iceland (the Icelandic low).

Translated, it means that the greater the pressure gradient between the two synoptic scale systems, the more meridional the flow of the mid-Atlantic westerlies. That means the winds flow in great north-south loops much the way an old river flows over level terrain. The weaker the gradient gets (the smaller the pressure difference), the more zonal (straight-line) the flow becomes.

The NAO changes over a both short-term period and a long-term period. The NAO alternately weakens and strengthens roughly every 6-10 years (shades of ENSO!) and has significant long-term variability over periods of greater than 50 years. During positive (high gradient) phases, the mid-Atlantic westerlies become more meridional and advect warm, moist Atlantic air northeastward. This results in warmer, wetter conditions in Scandinavia and northern Europe and cooler, drier conditions in Greenland and the Mediterranean. Conversely, when the NAO is in a negative (low gradient) phase, the mid-Atlantic westerlies are more zonal. This results in cooler, drier conditions in Scandinavia and northern Europe and warmer, wetter conditions to Greenland and the Mediterranean.

No one has a definitive theory of how the NAO sets up or continues, but there are indications it is a dynamic, interactive combination of both atmospheric and oceanic conditions. The oceans, because of their great capacity for storing heat, effect the long-term periodicity of the NAO more, whereas the atmosphere effects the short-term periodicity more. Signature elements of a positive NAO phase include colder than normal winter sea surface temperatures (SSTs) in the eastern Labrador Sea, eastern Mediterranean, and subtropical Atlantic Ocean, and warmer than normal SSTs in the North Sea and Sargasso Sea. The reverse is true for negative NAO phases.

Okay, so why does this matter? It matters for two reasons. First, a reliable water supply is a vital necessity to a burgeoning population, especially in areas where precipitation has always varied widely from year to year. An early warning of a precipitation deficit can help countries plan for water shortages just as an early warning of excess precipitation can help them plan for and control flood damage. Wars have been fought over water rights for thousands of years and floods have toppled whole civilizations.

As the world population continues to grow and puts more and more demands on water sources, these issues become ever more significant. Second, understanding the NAO cycle will help forecasters in the affected areas predict weather conditions more accurately. A positive NAO brings one type of winter weather and a negative cycle brings another. That can have a tremendous positive or negative impact on forecast products and on the customers they support. Do you know what the NAO is doing today? Maybe you should find out!

Written by Melody L. Higdon, 14 WS/DOPA