The variability of stratospheric ozone in a 29 year assimilated data set and sensitivity calculations

Abstract

Consistent observation-based data sets of stratospheric ozone are needed in order to resolve many of the pending questions regarding stratospheric ozone. Satellite observations are available since the late 1970s; however, as most observational methods rely on backscattered sunlight, these do not provide complete long-term coverage of the stratosphere, in particular during polar night. In this PhD thesis, a 29 year data set of stratospheric ozone is introduced that has been generated from sequential assimilation of satellite observations into the Bremen 3D Chemistry Transport Model (CTM). In the method of data assimilation, a three-dimensional physical computer model is used to close the gaps between single measurements. Observations constrain the CTM where available, and at the same time the information is propagated into areas where no observations are available. Here, profile ozone observations from the Solar Backscatter UV (SBUV and SBUV/2) instruments are used, which have been in orbit continuously since 1978. The resulting assimilated data set is validated against independent observations from other satellite platforms and in-situ observations with sondes. Agreement to independent observations is excellent throughout most of the stratosphere, and the assimilated data set can thus be used as a consistent extension of the satellite record beyond the limits of data coverage. The assimilated data set, in conjunction with sensitivity calculations with the unconstrained CTM, is used to analyse the variability of stratospheric ozone during the last three decades on two distinctly different temporal and spatial domains. The first research question deals with the short-term variability of polar ozone during winter. The Arctic ozone layer is subject to large inter-annual variations during spring; although statistical connections between dynamical quantities in winter and springtime total ozone abundance are known, little is known about how ozone anomalies develop and evolve in winter. With its coverage of polar latitudes during winter, the assimilated data set is ideally suited to address this issue. It is shown that ozone anomalies usually originate in the mid- to upper stratosphere and subsequently descend to the lower stratosphere, displaying a long lifetime of around six months. Ozone anomalies are strongly interrelated to anomalies in the stratospheric circulation, expressed here by the Northern Hemisphere Annular Mode (NAM). Extreme phases of the NAM, so-called strong and weak vortex events, lead to the formation of large and distinctively shaped ozone anomalies that traverse most of the stratosphere within days to weeks, and subsequently remain significant for five months in the lowermost stratosphere. A deeper analysis reveals that different mechanisms of interaction between chemistry and dynamics lead to the observed ozone anomaly pattern. Another source of mid-stratospheric Arctic ozone anomalies is the precipitation of energetic particles from the sun. Solar proton events lead to the formation of nitrogen oxides in the upper stratosphere and mesosphere that subsequently propagate down into the polar vortex and cause significant negative ozone anomalies lasting for up to six months in the assimilated data set. In a second research question, the long-term evolution of ozone is analysed on a global basis. Stratospheric ozone showed large declines during the 1980s and 1990s in both hemispheres. As a consequence of the regulation of ozone depleting substances (ODSs) by the Montreal Protocol in 1987, stratospheric chlorine loadings have peaked in the late 1990s and since begun to slowly decrease. A levelling off of negative ozone trends has been detected, raising the question whether this is already an onset of chemical recovery. The long-term evolution of column ozone is captured very well in the assimilated data set and the unconstrained CTM. Sensitivity calculations with the unconstrained CTM allow for a diligent attribution of observed ozone trends to their processes of origin. In particular, the relative contributions of anthropogenic (emissions of ODSs) and natural (changes in stratospheric circulation and temperature) factors are quantified. While a large part of ozone decreases in the 1980s and 1990s is attributable to ODS increases, only very small effects of chemical recovery are seen after the ODS turnaround. A significant trend change is observed between the phases of increasing and decreasing ODS loadings; however, this trend change is partly related to dynamics, and hence cannot yet be taken as significant evidence for an onset of ozone recovery in most regions.