Effects of the Quasi-Biennial Oscillation and Stratospheric Semiannual Oscillation on Tracer Transport in the Upper Stratosphere

Abstract

Abstract Using satellite observations together with a chemistry–climate model (CCM), the effect of the stratospheric semiannual oscillation (SAO) and quasi-biennial oscillation (QBO) on the equatorial double peak in observed CH4 and NO2 is reexamined. It is concluded that the lower-equatorial Halogen Occultation Experiment (HALOE) CH4 mixing ratio of the April double peak in 1993 and 1995 was associated with the prominent first cycle of the SAO westerlies, which causes local vertical downwelling in the upper equatorial stratosphere. The observational evidences imply that the strong westerlies of the first cycle of the stratospheric SAO in 1993 and 1995 were driven by enhanced lower-stratospheric gravity wave activity in the early parts of those years. The CCM simulations further verify that the gravity wave source strength has a large impact on the development and strength of the SAO westerlies. This result suggests that the equatorial long-lived tracer mixing ratio near the stratopause (which is associated with the strength of the SAO westerlies) was not only modulated by the QBO phase, but was also significantly influenced by interannual variation in the gravity waves. It is also found that the deeper equatorial trough of the double peak is unlikely to be always accompanied by the more prominent Northern Hemispheric lobe, and the Northern Hemispheric lobe of the double peak can be mainly attributed to subtropical upwelling. The altitude of greatest chemical destruction anomalies associated with the SAO and QBO is below the trough of the double peak, implying that the effect of the chemical process on the double peak is insignificant.