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quasiperiodicity
Quasiperiodicity is the property of a system that displays irregular periodicity. Periodic behavior is defined as recurring at regular intervals, such as "every 24 hours".〔(Periodicity - Definition and More from the Free Merriam-Webster Dictionary )〕 Quasiperiodic behavior is a pattern of recurrence with a component of unpredictability that does not lend itself to precise measurement.〔(Quasiperiodic - Definition and More from the Free Merriam-Webster Dictionary )〕 It is different from the mathematical concept of an almost periodic function, which has increasing regularity over multiple periods. The mathematical definition of quasiperiodic function is a completely different concept; the two should not be confused.
==Climatology==
In climatology, oscillations that appear to follow a regular pattern but which do not have a fixed period are called ''quasiperiodic''.〔(Not Found - UNISDR )〕〔''The meteorological glossary: 2d ed.'' 1930. Meteorological Office, Great Britain. "Certain phenomena which recur more or less regularly but without the exactness of truly periodic phenomena are termed quasi-periodic."〕
Within a dynamical system such as the ocean-atmosphere oscillations may occur regularly, when they are forced by a regular external forcing: for example, the familiar winter-summer cycle is forced by variations in sunlight from the (very close to perfectly) periodic motion of the earth around the sun. Or, like the recent ice age cycles, they may be less regular but still locked by external forcing. However, when the system contains the potential for an oscillation, but there is no strong external forcing it to be phase-locked to it, the "period" is likely to be irregular.
The canonical example of quasiperiodicity in climatology is El Niño-Southern Oscillation. In the modern era, it has a "period" somewhere between four to twelve years and a peak spectral density around five years.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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