On the relationship between extreme and mean values in total ozone and atmospheric dynamics and volcanic eruptions at mid-latitudes

We present an analysis of the relationship between mean and extreme values in total ozone and atmospheric dynamics and volcanic eruptions at mid-latitudes. In this work we analyze observational data (NIWA2.7 data, spatial resolution: 1°longitude x 1.5° latitude, temporal resolution: daily values) on large spatial/temporal scale for the northern and southern mid-latitudes. To analyze extremes we apply a so-called r-largest order statistics model, a special case of the Poisson point process model, and compared the results with output from a standard ARMA model for mean values. Spatial analysis of coefficient estimates and related p-values for significance provide interesting results regarding the influence of dynamics (i.e., ENSO, NAO, AAO) on total ozone at mid-latitudes. The results show that ENSO influences ozone extremes in both Northern and Southern mid-latitudes (especially during spring), especially towards low latitudes. This pattern indicates the enhanced transport of ozone from the tropics to the extra tropics during strong El Niño events, a feature described so far only based on data from ground based stations but not shown on large spatial scale. Further, the results provide evidence for a relationship between the thickness of the southern ozone "collar" and the ENSO phase (especially La Niña) during winter at Southern mid-latitudes. Next to ENSO significant influence was found for the NAO and its southern counterpart the AAO. Especially the later one is of importance as at central Southern mid-latitudes highly significant negative coefficient estimates are found which can be related to enhanced wave activity in the tropics leading to enhanced ozone transport from the tropics to the extra-tropics. Further the results indicate that the Mt. Pinatubo eruption was amplified by the dynamical state of the atmosphere in the Northern hemisphere but was strongly weakened/masked in the Southern hemisphere. Here the strength of the ozone hole and ozone "collar" together with the state of the AAO leads to a suppression of the Mt. Pinatubo signal at Southern mid-latitudes. The Southern polar vortex was shifted towards the Antarctic Peninsula and the Southern parts of South America in 1991, the first year of the Pinatubo eruption, which might explain the negative coefficient estimates found in this region. In the following years (1992-93) the Southern ozone "collar" was displaced and richer than normal, which can be related to the negative mode of the AAO and its relation to the wave activity in tropical regions leading to a strengthening of the Lagrangian mean circulation and thereby to enhanced transport of ozone from the tropics to extra-tropics and to a strong blocking/separation between the vortex and "collar" area.