From Glossary of Meteorology
(Also called climatic oscillation.) A slowly varying change of climate about a mean that recurs with some regularity but is not necessarily periodic.
The term "cycle" is used in climatology more loosely than in other physical sciences. It implies quasiperiodic behavior without any real precision in the recurrence of events. It usually indicates only that the event is more probable at the peak of the cycle than at the trough. The existence of numerous cycles has been postulated, ranging from those with timescales of geological length down to a few years.