Characteristics of stratospheric warming events during Northern winter

The strong interest in Sudden Stratospheric Warmings (SSWs) is motivated by their role in the two-way stratospheric-tropospheric dynamical coupling. While most studies only investigate major SSWs (vortex breakdown), the minor ones (strong vortex deceleration) are overlooked. This work aims at overcoming this gap by providing a comprehensive description of stratospheric warming events without a priori distinctions between major and minor SSWs, leading to a more complete estimate of the stratospheric variability. Warming events are extracted from reanalysis datasets by means of a mid-stratospheric polar-cap temperature daily index. Events are characterised by a bimodal distribution in amplitude, with a broad peak at small amplitudes (inferior to 5K) and a sharp peak at around 9K. Due to the intrinsic polar vortex dynamics, the warming amplitude presents a distinct seasonal distribution. Small amplitude warmings mainly occur during early- and late-wintertime by contrast with the larger amplitude ones occurring during mid-wintertime. From mid-November to mid- March, the large amplitude warmings (i.e. Strong Warming Events, SWEs) include both major and minor SSWs, as well as Canadian and Final warmings. Although major SSWs belong to the tail of the SWEs distribution, there is no clear distinction between the major and minor SSWs according to the considered properties of the events. Such result brings out the idea of “warming continuum”. Furthermore, diagnostics of heat flux reveal that there is no statistical difference between SWEs with regard to their feedbacks on the planetary waves, and hence on their potential influence into the troposphere.

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