Preconditioning for stratospheric sudden warmings: sensitivity studies with a numerical model

A flow is said to be preconditioned when the stratosphere is in a state such that a realistic planetary wave pulse from below will lead to a major sudden stratospheric warming. Two observed sudden warmings (February 1979 and February 1989) are simulated by the model. The results of the model runs indicate that a major warming occurs for certain initial fields regardless of whether the lower-boundary wave evolution corresponds to that observed or was taken from another period with a minor warming. In most of the model integrations, the occurrence of a warming in the model depends only on the lower-level winds and is independent of the upper-level winds. -from Author