Preconditioning for Stratospheric Sudden Warmings: Sensitivity Studies with a Numerical Model

Abstract

A three-dimensional primitive equation model of the stratosphere is used to investigate the phenomenon of preconditioning for sudden warmings. 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 initial fields are based on zonal-mean observations for individual days during these two winters. The observed wave evolution over a 20-day period is imposed at the lower boundary (near 250 mb). All of the days chosen for initialization and for the beginning of the lower-boundary wave evolution correspond to a time about nine days prior to either one of the major sudden warmings or a minor sudden warming of the same winter. For comparison, model runs are initialized with hybrid initial fields constructed by splicing together upper-level (above 10 mb) data from one day and lower-level data from another time during the same winter. 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.