Equatorial middle atmospheric chemical composition changes during sudden stratospheric warming events

Abstract. Chemical composition data obtained from the Microwave Limb Sounder (MLS) and Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) instruments onboard EOS-Aura and ENVISAT satellites are used to investigate the variation of ozone (O₃), water vapor (H₂O) and methane (CH₄) volume mixing ratios (VMRs) in the height range 20–60 km over the equatorial region during the sudden stratospheric warmings (SSWs) of 2004, 2009 and 2012, the occurrence of which are identified using the ERA (European Center for Medium Range Weather Forecasting Reanalysis) Interim temperature and zonal wind data sets. It is found that the O₃ VMR shows increment whereas H₂O VMR shows decrement during the SSW event and the possible reasons for these changes in the VMRs are investigated. In the upper stratosphere, the source of water vapor is oxidation of CH₄ which takes place either by hydroxyl (OH) or by atomic oxygen (O). However, the OH VMR available for the year 2009 SSW event does not show any significant variation. The decrease of zonal mean MLS temperature over the equator during the SSW suggests that the rate of the reaction of ozone production (O + O₂ ⟶ O3 + M), which increases with decreasing temperature, shifts the O/O₃ ratio towards O₃, resulting in the decrease of CH₄ oxidation and consequent decrease in water vapor.