The North Atlantic Oscillation

I thought I would dedicate this second blog post to the North Atlantic Oscillation (NAO)– one of the leading modes of natural variability in the Northern Hemisphere (NH). It is important to get to know this phenomenon a bit better when looking into what is causing the extreme weather and climate around the North Atlantic basin, and that because the NAO has shown to exert a strong influence on the climate over large parts of the Northern Hemisphere.

The NAO is characterized by a fluctuation in sea level pressure (SLP) between the Arctic basin and the mid-latitudes. The differences in SLP drive a westerly flow that moves through the high- and low pressure systems, creating a zonal planetary-scale wave pattern across the Northern Hemisphere. Therefore the NAO is also characterized by an out-of-phase relation in the strength of this zonal flow along ~55° and 35°. The westerly flow is strongest during winter when the largest anomalies in SLP occur, as you can see in the figure below (from Hurrell et. al). Dark (light) shading indicate negative (positive) departures from the mean . So the characters of the NAO are most pronounced during the Northern Hemisphere winter months (boreal winter).

Where these high- and low-pressure systems are located, and how strong they are, determine the phase or the index of the NAO. All results are based on the winter months when the variability is largest. A high index polarity is defined as anomalous strong subpolar westerlies, with a deeper than normal low pressure over the polar region and a higher than normal subtropical high pressure . A low index polarity is defined as anomalous weak westerlies and pressure systems. A consequence of these contrasting polarities is that anomalies in climate on seasonal time scales typically occur over large geographical regions.  These anomalies in climate include surface air temperature, SST, changes in storminess and precipitation, ocean heat content, ocean currents and their related heat transport, and sea ice cover.

High index conditions are characterized by a northeastward shift and an increased intensity in the stormtrack across the North Atlantic, from northeastern North America to northern Europe, which tend to give wet and mild conditions in these areas. Stronger northerly winds over Greenland and northeastern Canada carry cold air southward and decrease land temperatures and SST over the northwest Atlantic. Warming over North America associated with the stronger clockwise flow around the subtropical Atlantic high-pressure center is also notable. Evaporation exceeds precipitation over much of Greenland and the Canadian Arctic during high NAO index winters. Drier conditions also occur over much of central and southern Europe, the Mediterranean and parts of the Middle East, whereas more precipitation than normal falls from Iceland through Scandinavia. During low index periods the weaker (and fewer) winter storms crossing the Atlantic on a more west-easterly path bring moist air into the areas surrounding the Mediterranean. Northern Europe and the eastern part of the US experiences cold air outbreaks and hence snowy weather conditions. Below you can see a (much generalized) picture of the climate patterns created by the NAO in its different phases (found on Google).

Phu, I know that was a lot in one post, but it covers the most essential information about the NAO, which is important to know in order to be able to look at how anthropogenic activity is influencing our weather versus these natural modes of variability. In my next post I will look into if/ how anthropogenic activity is influencing the NAO and what consequences that may have/are having on our weather and climate.

If you're hungry for more or want a more detailed overview of the NAO, I suggest you read the paper I already referenced to; An Owerview of the North Atlantic Oscillation.