Relation between Equatorial Mesospheric Wind Anomalies during Spring and Middle Atmosphere Variability Modes

Abstract

Strong easterly wind anomalies in the equatorial upper mesosphere were observed during northern hemispheric spring equinox. While the anomalies are mainly related to the semi-annual oscillation (SAO) in the stratopause region and to the quasi-biennial oscillation (QBO) in the stratosphere, an additional influence of sudden stratospheric warmings (SSWs) was recently suggested. In order to analyze this relation, we use an 18-year simulation of the Kühlungsborn Mechanistic Circulation Model (KMCM) that includes these middle atmosphere variability modes. We compute composites for the different phases of the QBO and intensities of SSWs. The results support the notion that easterly equatorial mesospheric wind anomalies occur during spring equinox if the QBO is in its westerly phase. In addition we show that this relation does not hold when the preceding winter is characterized by strong SSWs. We interpret this phenomenon as a result of a persistent strengthening of the residual circulation in the equatorial stratopause region.