Characteristics of stratospheric warming events during Northern Winter
Résumé
the polar mid-stratosphere is characterized by the setting up of westerly winds around the pole during the wintertime; the so-called polar vortex. the polar vortex is one of the most variable features of the zonal-mean circulation of the earth atmosphere, due to a highly non linear interaction between planetary-scale rossby waves and the zonal flow. Indeed, the interaction between the upward tropospheric propagating waves and the polar vortex leads to a zonal flow weakening, implying a large day-to-day vortex variability. in the most dramatic cases, the polar vortex breaks down, the stratospheric polar flow can reverse its direction and the temperatures can rise locally by more than 50K in a span of a few days. such phenomena are known as major sudden stratospheric Warmings (SSWs) and constitute, since their discovery in 1952 (scherhag,1952) the most impressive dynamical events in the physical climate system. On the contrary, situations where the temperature increase is not associated to a polar vortex breakdown are known as minor SSWs.
There is actually a renewed interest about SSWs since Baldwin and Dunkerton (2001) have shown that the major SSWs can influence weather in the troposphere. This stratospheric-tropospheric linkage has been statistically highlighted but no physical explanation has been proposed. also, other studies, that incorporate both major and minor SSWs, show that events in the stratospheric zonal flow associated with SSWs present downward- propagating anomalies that can reach the troposphere (Limpasuvan et al 2004), implying that minor SSWs have to be also considered in the study of the stratospheric- tropospheric dynamical coupling.
in this study, we propose a global characterization of stratospheric warmings situations based on a temperature threshold in the 50-10hpa layer between 70n-90n, in order to better assess the properties of daily stratospheric temperature variability during the northern winter. the originality of this approach consists in evaluating the wintertime positive temperature anomalies in terms of intensity and duration without distinction between minor and major SSWs. We will show that there is a wide spectrum of warming types, where major SSWs are the most extreme, but other events – the minor SSWs – share some common properties with them. they can even have a surface signature if one look the stratospheric wave reflection on the polar vortex.