The North Atlantic Oscillation

2017-10-16 17:43:15.000 – Taylor Regan, Weather Observer

 

Last night, during our live forecast discussion on Facebook, we were asked a question about the North Atlantic Oscillation, or NAO. So, what is the NAO, and why is relevant to our weather discussion?

To understand the NAO we need to first take a look at the atmosphere, particularly over the North Atlantic Ocean. Here, an area of permanent low pressure sits over Iceland (known as the Icelandic Low or Subpolar Low), and an area of high pressure sits over the Azores (known as the Azores High or Subtropical High). The location and intensity of these pressures overhead influence both the direction and strength of westerly winds which eventually make their way to Europe.

Figure 1. Graphic Depiction of the NAO. (Image from Wikipedia)

Variation in strength of these two pressures is known as the North Atlantic Oscillation and plays an important role in the climate of the Northern Hemisphere, particularly in winter. The NAO affects not only the strength/direction of westerly winds, but also the location of storm tracks across the North Atlantic.

The strength of the NAO can be determined or quantified by looking at the difference in seasonal average air pressures between stations located within each of the two stable pressure areas. If there is a large difference in pressures, it is considered a positive NAO (NAO+). This is associated with cooler summers and mild and wet winters in Central Europe. Conversely, when the North Atlantic Oscillation index is low, the westerly winds are suppressed, and as a result, northern Europe experiences cold and dry winters. Storm tracks instead move more southwards towards the Mediterranean Sea. As a result, southern Europe and North Africa see increased rainfall and storm activity.

Figure 2. Positive and negative NAO schematics. (Image from muchadoaboutclimate.wordpress.com)

So, how does this impact our forecasts in New England? Well, in the winter, when the NAO is very strong (NAO+), the low over Iceland dictates a strong south-westerly pattern, preventing Arctic air from being as influential in the region. As a result, the Northeast can see significantly warmer winters. Conversely, when the NAO index is low (NAO-), the eastern seaboard is more often subjected to blasts bitter cold Arctic air heavy snowstorms.

One way to better visualize the effects of the NAO is to imagine a baseball pitching machine. I know, it sounds a little crazy, but hear me out. The baseball pitching machine works by feeding a baseball through a gap between two quickly spinning wheels. This gap is just smaller than the ball, and the wheels grip the baseball and force it to move, eventually releasing the ball, sending it on its way to the batter waiting to hit a homerun. In baseball, you can change the speed the ball is released at by increasing the speed the wheels turn, and you can change the trajectory of the ball, or its path through the air, by adjusting the speed of the wheels relative to one another.

Figure 3. Baseball pitching machine. (Image from pitchingmachinesale.com)

Now, replace the top wheel of the pitching machine with the Icelandic Low, and the bottom wheel with the Azores High. In this case, the prevailing westerly wind and storm tracks are represented by the baseball. Changes in the pressures of the Subpolar Low and Subtropical High (both on their own and with respect to one another) influence the speed and also direction of the airflow between them.

 

Taylor Regan, Weather Observer