Response of middle atmosphere to short‐term solar ultraviolet variations: 2. Theory

Abstract

Ozone and temperature responses to solar variability, based on satellite data, have been reported in a companion paper (Keating et al., this issue). The present paper is intended to present a theoretical interpretation of this analysis with the purpose of better understanding the chemical behavior of the stratosphere and the coupling between temperature and ozone concentration, when a periodic forcing is applied to the solar ultraviolet (UV) flux. The response of the temperature and of the trace species concentrations, including ozone, to short‐term variations in the solar UV irradiance is calculated by a one‐dimensional chemical‐radiative time‐dependent model. The applied solar variability is assumed to be sinusoidal with a period of 27 days (in accordance with the rotation period of the sun) or 13.5 days (when two active regions are on opposite sides of the sun). The amplitude varies with wavelength, which is consistent with observations made by the Nimbus 7 solar backscattered ultraviolet (SBUV) experiment. The maximum ozone sensitivity in the stratosphere appears to be located near 3 mbar. The calculated amplitude and phase of the ozone response are significantly modified when the feedback between ozone and temperature is taken into account. The ozone/temperature coupling tends to modify the ozone phase lag such that, in the upper stratosphere and in the mesosphere, the ozone peak occurs a few days before the UV peak. Comparison of the model results with the observed ozone and temperature responses, based on satellite data, shows that the theory is consistent in many respects with observations. The calculated time lag of the temperature response, however, is approximately a factor of 2–4 smaller than the time lags derived from the measurements, suggesting evidence for some additional process not included in the model calculation. Large negative ozone sensitivities and positive temperature responses are predicted in the mesosphere as a result of the absorption by O2 of the solar irradiance at the Lyman α wavelength. The model also shows that the expected variation in the stratospheric nitric acid mixing ratio is a factor 2 larger than the corresponding opposite variation in the ozone concentration.

Sensitivity of tropical stratospheric ozone to rotational UV variations estimated from UARS and Aura MLS observations during the declining phases of solar cycles 22 and 23

“…On the other pressure levels (outside the 6.8-4.6 hPa range), there is at least a year out of three when the correlation is not statistically significant or even negative. Figure 8.a is similar to that found in previous studies (Hood, 1986;Brasseur et al, 1987;Brasseur, 1993;Hood and Zhou, 1998) with a negative lag above 3-4 hPa (ozone leading the solar flux) and a positive lag below (ozone lagging the solar flux). Above 1.5 hPa (upper stratosphere-lower mesosphere), the cross-correlation does not exceed 0.1 and is not statistically significant; this result is consistent with Hood and Zhou results (1998) also obtained with MLS ozone data.…”

“…Therefore, we apply a digital filter previously used (Hood, 1986;Chandra, 1986;Keating et al, 1987;Hood and Zhou, 1998;Zhou et al, 2000). The filtering procedure consists of smoothing data with a 7-day running mean which removes short-term fluctuations.…”

Decadal solar effects on temperature and ozone in the tropical stratosphere

“…This discrepancy in this region could be due to less penetration of UV radiation in the Herzberg continuum (200 nm-240 nm) caused by greater absorption at higher altitudes. Therefore, ozone becomes less sensitive to photochemistry (Brasseur and Simon, 1990) and the distribution of ozone is controlled primarily by dynamics (McCormack and Hood, 1996). Dynamical phenomenon may be responsible for the observed peak.…”

“…A number of both modeling and observational studies have reported the effects of 11-year solar variability on ozone and temperature over the low-latitude regions (Callis and Nealy, 1987;Stolarski et al, 1991;Brasseur, 1993;Fleming et al, 1995;Haigh, 1996). To assess the potential impact of solar UV changes on stratospheric ozone, both the 2-D photochemical transport models (Brasseur, 1993;Haigh, 1994;Fleming et al, 1995) and the General Circulation Models (GCMs) (Shindell et al, 1999) have been used.…”

The potential impact on atmospheric ozone and temperature of increasing trace gas concentrations

“…In the lower stratosphere, all contributions are significantly smaller so that the radiative lifetime becomes considerably longer. This lifetime has been estimated by Brasseur et al (1986) to be 3 days at 50 km, 9 days at 40 km and 17 days at 30 km. Gille and Lyjak (1986), from an analysis of the LIIvIS data (observations taken from the Nimbus 7 satellite), have derived radiative relaxation times of 8 days at 40 km, 22 days at 30 km, 40 days at 25 km and about 90 days at 20 km.…”