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Theory and Observation
The term North Atlantic Oscillation refers to three things:
The term oscillation in the name North Atlantic Oscillation may be misleading. There does not have to be a regular cycle involved. Instead the theory asserts, at least in the way it is usually presented and explained, that there are two meteorological states for the North Atlantic. These states are a bit persistent once in place but eventually the condition switches. The primary characteristics of the states are the sea surface temperature (SST) and the magnitude of the pressure difference between the Azores area and the Iceland area. These states will be referred to as warmer and cooler.
In the cooler phase the theory states that the winds from the west blow strongly directly across the Atlantic. There is a high pressure system located near the Azores Islands and a low pressure system over Iceland. Thus this state is one of cooler SST's and a higher differential pressure. It is often denoted as NAO+.
The low pressure system over Iceland has winds flowing counterclockwise around it. The high pressure system over the Azores has winds flowing clockwise. Thus the wind coming into the North Atlantic from the west gets accelerated by the pair of rollers situated over Iceland and the Azores.
The winds pick up heat even from the ocean in its cooler state and carry it across northern Europe and Asia. Continental temperatures to the east are raised as a consequence. This makes for milder, wetter winters there. To the west in northern Canada and Greenland the winters are colder and drier. To the south in the eastern United States the winters are mild and wet.
In the summers for the NAO+ the strong westerlies are cooled by their passage over the Atlantic and thus summers in northern Europe are cooler.
When SST's are warmer and the pressure is not so high in the south and/or not so low over Iceland the winds from the west are not constrained from continuing on their natural trajectory to the southeast. Their movement toward the equator results in their slowing down due to the Coriolis Effect. In this meteorological configuration the winds are not bringing heat and moisture to northern Europe and Asia. The winters are then colder and drier and the summers hotter. In their southeasterly trajectory the winds from the west then bring warmer temperatures and more rain to the Mediterranean Basin and the Middle East.
This pattern was identified by Sir Gilbert Walker in the 1920's. Walker identified a number of circulation patterns, some of which are disputed.
The effects of the North Atlantic Oscillation are often coupled to the Arctic Oscillation and the North Pacific Oscillation.
A North Atlantic Oscillation Index has been constructed based upon the pressure differences between the Azores and Iceland. The tabulated values by month from January 1950 to October 2009 were used to construct the histogram shown below. The data are from the National Oceanic and Atmospheric Administration (NOAA).
NOAA also compiles a similar index for the eastern Atlantic. The histogram for this index is shown below.
The notion that there are two states would not necessarily mean only exactly two values. There would generally be a distribution. A bimodal (two peaked) distribution could be something like the following.
The conjectured bimodality of the NAO index is just not there.
If there were some persistence of state then a scatter diagram of the index plotted against its value some time previously would show a structure such as this one.
The sizes of the circles are to denote the frequency of occurrence. Thus when in the lower state the system tends to stay in the lower state, but sometimes moves to the upper state. And likewise when the system is in the upper state it tends to stay in the upper state, but sometimes moves to the lower state.
The scatter diagrams below are attempts to find some correlation between the value of the index and what it was one month previously and what it was 12 months previously.
The conjectured persistence is not there. Nor is there any tendency to flip back and forth either. Basically there is only a slight correlation (0.14) between the state at one time and its state one month previously and no correlation with its state 12 months previously.
The NOAA data can be used to determine whether there is a spatial correlation of the North Atlantic index and the Eastern Atlantic index. The result is shown below.
The answer, by observation, is that there is no correlation between the values of these indices in the same month.
Although the North Atlantic Oscillation does not necessarily involve a fixed cycle it is worthwhile to look at the spectrum for the series to see if there is any tendency toward cyclicity.
As the spectrum graph shows there are peaks for 5-month, 8-month, 14-month, 30-month and 32-month cycles. What is plotted on the vertical axis is the absolute value of the Fourier coefficients for each cycle. The dominant cycle thus appears to be the one with a period of about 2½ years.
The notion that there are two discernible meteorological states for the North Atlantic is not born out, but that may not be essential to the theory. The two-state assertion may just be a simplification for purposes of explanation. The two-state assertion may really be a reference to the extremes of a continuum. The theory of the North Atlantic Oscillation is eminently plausible. The most important part of the concept would be the correlation of the state of the North Atlantic and the weather conditions over Europe. That aspect was not examined in the above.
(To be continued.)
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