Middle atmospheric ozone and temperature responses to solar irradiance variations over 27‐day periods

Abstract

A one‐dimensional (1‐D), photochemical‐radiative, time‐dependent model with vertical diffusion is developed to investigate the response of ozone in the middle atmosphere to solar UV irradiance variations over 27‐day solar rotation periods. The model, based on equinoctial conditions at the equator, extends from 10 to 100 km with a vertical resolution of 0.5 km and employs an approximation based on photochemical families. The amplitudes of the wavelength‐dependent solar UV variation over 27‐day periods as observed by the Upper Atmosphere Research Satellite (UARS) are adopted in the model. The results of the 1‐D model show that at altitudes of 60–80 km, ozone decreases with increasing solar irradiance at Lyman α. The amplitude of the ozone variation is about 1% of its equilibrium value at about 70–75 km. Below 50 km an ozone increase is found due to increased photolysis of molecular oxygen by increased solar irradiance over the Herzberg continuum. The peak of this ozone increase (about 0.3%) is located at about 35 km. The peaks of the ozone oscillation in the upper stratosphere precede those of the solar oscillation as a result of the strong influence of the temperature feedback on the ozone response through the temperature dependent chemical reaction rates. A modified NCAR 2‐D model is also used to study the short‐term ozone response. Comparison of the 2‐D and 1‐D results shows that horizontal transport increases the phase lag of the temperature response. This large phase lag is essential to simulate the observed ozone response given by Nimbus 7 solar backscattered ultraviolet observations. The positive temperature response around 70 km observed by Nimbus 7 stratospheric and mesospheric sounder data is confirmed by the UARS microwave limb sounder temperature observation, but neither 1‐D nor 2‐D models are able to reproduce this observed temperature response at this level. The effects of the exothermic chemical heating and vertical diffusion coefficient on the mesospheric temperature and ozone responses are discussed. The relative response of ozone and temperature to normalized solar variations over different spectral regions and the roles of different photochemical families in the ozone response are presented. Finally, the seasonal and latitudinal (up to middle latitudes) variations in the ozone response are evaluated with the use of the 2‐D model.