Vertical profile of the stratospheric solar cycle ozone variation at low latitudes: Observations and mechanisms (Invited Speaker)

Previous multiple regression statistical analyses of the solar cycle variation of stratospheric ozone have recently been improved (B. Soukharev and L. Hood, JGR, v. 111, 2006) by (1) analyzing three independent satellite ozone data sets with lengths extending up to 25 years and (2) comparing column ozone measurements with ozone profile data during the 1992 - 2003 period when no major volcanic eruptions occurred. Results show that the vertical structure of the tropical ozone solar cycle response has been consistently characterized by statistically significant positive responses in the upper and lower stratosphere and by statistically insignificant responses in the middle stratosphere (~ 28 to 38 km altitude). This vertical structure differs from that predicted by most models. The similar vertical structure in the tropics obtained for separate time intervals (with minimum response invariably near 10 hPa) is difficult to explain by random interference from the QBO and volcanic eruptions in the statistical analysis. The observed increase in tropical total column ozone approaching the cycle 23 maximum during the late 1990's occurred primarily in the lower stratosphere below the 30 hPa level. A mainly dynamical origin for the solar cycle total ozone variation at low latitudes is therefore likely.

The observed tropical minimum in the 11-year ozone response is centered at 10 hPa on the equator and has been reproduced using three independent satellite ozone data records (SBUV/(2), SAGE II, and UARS HALOE). At this location, ozone interannual variability is dominated by the QBO, which is caused by transport-induced changes in odd nitrogen. A possible explanation for the observed response minimum is therefore that a solar cycle modulation of the QBO exists such that the solar cycle variation of ozone is effectively reduced or eliminated near this location. As shown by Pascoe et al. (JGR, v. 110, 2005), the frequency of transitions from the west QBO phase to the east QBO phase at 44 hPa on the equator is greatest during the months of May, June, and July. Since the global Brewer-Dobson upwelling at the equator is weakest during these months, it has been argued that the west-to-east transition is inhibited when the upwelling rate is strong (Kinnersley and Pawson, JAS, v. 53, p. 1937, 1996). If so, then the strength of the Brewer-Dobson upwelling can effectively control the duration of the QBO west phase. Observationally, on average (!), the duration of the west phase is longer under solar minimum conditions, suggesting that the Brewer-Dobson ascent branch is weaker under solar maximum conditions (see also Kodera and Kuroda, JGR, v. 101, 2002). A weaker ascent branch under solar maximum conditions would also be consistent with higher ozone concentrations in the tropical lower stratosphere near solar maxima (Soukharev and Hood, 2006). During the west QBO phase (as conventionally measured at 40 hPa), the vertical wind shear near 10 hPa is easterly. This produces transport-induced decreases in odd nitrogen, which cause photochemical ozone increases. Therefore, if the west (east) QBO phase is longer (shorter) under solar minimum conditions, a decadal ozone variation could result that would be opposite in phase to the solar UV induced ozone variation.

Several recent model simulations have further addressed physical causes of the observed 11-year tropical ozone profile response. Simulations using a 2D photochemical transport model (J. McCormack, D. Siskind, and L. Hood, JGR, submitted, 2007), which combines the effects of solar UV variations, a solar-modulated zonal wind QBO, and an assumed 11-year variation in planetary wave 1 amplitude (which influences the strength of the tropical upwelling) produces an enhanced lower stratospheric ozone response of ~ 2.5% between the equator and 20S, and an enhanced upper stratospheric ozone response of ~ 1% between 45 and 55 km. On the other hand, recent 45-year transient simulations of a coupled 3D chemistry climate model (AMTRAC), which does not include a QBO and uses observed sea surface temperatures, have also yielded a tropical minimum in the solar cycle ozone response near 20 hPa and an enhanced response in the lower stratosphere, in approximate agreement with observations (J. Austin, L. Hood, and B. Soukharev, Atmos. Chem. Phys., in press, 2007). Further work is therefore clearly needed to understand fully the 11-year ozone profile response.