Are the Pacific Decadal Oscillation The El Niño Southern Oscillation And Other Atmospheric Circulation Features Actually “Oscillations”?

Bill Drissel asked me a question in late July regarding the use of the term “oscillation” when referring to the Pacific Decadal Oscillation (PDO), El Niño Southern Oscillation (ENSO), etc.

He provided a very insightful comment and question:

“My recent background is radio receivers.  When we use the word “oscillation”, we generally know (or suspect) the source.  We always refer to a voltage that is periodic in nature with a predominant frequency and more or less prevalent harmonics…..When one speaks of the El Niño Southern Oscillation or the Pacific Decadal Oscillation, do we know enough about them to call them “oscillations” (in the sense that my co-workers use the word)?  In particular, do the ENSO and PDO exhibit the same sort of regularity?”

In order to answer this excellent question, I have listed below the definitions of an “oscillation”.

1. From  http://physics.about.com/od/glossary/g/oscillation.htm

“Definition: Oscillation is a motion that repeats itself in a regular cycle, such as a sine wave or pendulum. Also Known As: periodic motion.”

2, From en.wikipedia.org/wiki/Oscillations

“Oscillation is the repetitive variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states”.

 Wikipedia lists the El Niño Southern Oscillation as an example of an oscillation.

Clearly Bill Drissel is correct.  The Pacific Decadal Oscillation (PDO), El Niño Southern Oscillation and other such atmospheric circulation features are not oscillations as defined above.

Although it has become conventional to refer to these atmospheric circulation as oscillations, in reality they are part of a chaotic system (the climate) which often have periods of time when a signal appears quasi-periodic, when this behavior actually is just part of its nonlinear character (e.g. see).  While we will not be able to change their names (they are so entrenched in the climate jargon), it should be recognized that the PDO, ENSO, and other such features do not “repeat….in a regular cycle” nor vary around  “about a central value”. 

The term “oscillation” implies more knowledge regarding these temporally variable climate features than actually exists.