Weather regimes: the challenge in extended-range forecasting.

A hypothesis to explain the low-frequency (10-to 90-day) variance of the mid-latitude atmosphere is presented. In this hypothesis it is proposed that the planetary-scale waves forced by topography or other zonal inhomogeneities and the day-to-day weather disturbances (synoptic scales) influence each other to generate weather regimes. These quasi-stable flow configurations are responsible for short-range climate anomalies such as droughts, heat waves, deep freezes, and excessive precipitation, as the weather-producing disturbances are organized into storm tracks. Onset and disruption of the weather regimes may be induced by the anomalous development of perhaps a single cyclonic disturbance, which can throw the quasi equilibrium out of balance. Thus, rapid changes in flow regimes can occur almost at random. The development and transition of weather regimes may then be purely internal (that is, depending only upon the properties of the fluid motions themselves) to the atmospheric dynamics. This internal quality suggests that the chaotic, abrupt short-range climatic behavior of the mid-latitudes is a natural behavior of the system that requires no assistance from the outside. The weather regime concept presents a different view of the extended-range atmospheric behavior than the stimulus-response model, such as the atmospheric response to the El Nimo.