Chemical fluctuations associated with vertically propagating equatorial Kelvin waves

Satellite retrievals of ozone and nitrogen dioxide from the Nimbus-7 Limb Infrared Monitor of the Stratopshere (LIMS) reveal distinct spectral features which are collocated in frequency with Kelvin wave temperature fluctuations. These features represent a significant component of the unsteady variance in retrievals of O₃ and nighttime NO₂ in the tropics and are very similar to Kelvin wave temperature disturbances. Chemical fluctuations occur symmetrically about the equator, in phase across the tropics, and propagate downward, all consistent with the behavior of equatorial Kelvin waves. The phase structure of ozone perturbations mirrors that of temperature fluctuations in the upper stratosphere and mesosphere, only shifted 180°. The regular phase tilt with altitude disappears in the middle to lower stratosphere, where it is replaced by more or less barotropic behavior. That change in phase structure marks a transition from photochemical control in the upper stratosphere and mesosphere to dynamical control in the lower stratosphere. Fluctuations in NO₂ propagate downward throughout the observed region, with a constant shift of 180° from temperature fluctuations. Only a slight indication of equatorial Kelvin waves is found in nitric acid, in accord with the weaker temperature sensitivity, relatively long lifetime, and small vertical gradient of HNO₃ in the tropics. Fluctuations in ozone are consistent with dynamical and chemical mechanisms operating on that species. The response of ozone in a detailed photochemical calculation driven by observed temperature variability locks into agreement with the observed ozone variability above about 4 mbar, where O₃ is under photochemical control. At lower altitudes, vertical transport is able to explain both the magnitude and phase of the observed fluctuations in ozone. The same considerations have only mixed success in explaining the observed variability of nitrogen dioxide. The amplitude of nighttime NO₂ fluctuations is underestimated in the photochemical calculation by about a factor of 2. Although large enough to explain the discrepancy, contributions from vertical transport have the wrong phase. Observed fluctuations in daytime NO₂, which have a much smaller signal-to-noise ratio, are at odds with both chemical and dynamical explanations. Contamination in the NO₂ channel of LIMS by water vapor may be responsible for these discrepancies.