[2] The term used to denote oscillations that appear to follow a regular pattern but which do not have a fixed period.
Climate oscillations that appear to follow a regular pattern but which do not have a fixed period are called quasiperiodic.
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 to be phase-locked to, the "period" is likely to be irregular.
[5] Models to predict and thereby assist adaptation to ENSO have a large potential benefit to Australian wheat farmers.