Relative Importance of Mechanisms Governing Interannual Stratosphere to Troposphere Ozone Transport

Factors governing the strength and frequency of stratospheric ozone intrusions over the Pacific-North American region are considered for their role in modulating tropospheric ozone on interannual timescales. The strength of the association between two major modes of climate variability -- ENSO and the Northern annular mode (NAM) -- and the amount of ozone contained in stratospheric intrusions is tested in the context of two mechanisms that modulate stratosphere-to-troposphere transport (STT) of ozone: (StratVarO₃) the winter season buildup of ozone abundances in the lowermost stratosphere (LMS), and (JetVar) North Pacific jet and wave breaking variability during late spring. In essence, StratVarO₃ corresponds to variability in the amount of ozone per intrusion, while JetVar corresponds to the frequency of intrusions.

The resulting analysis suggests that StratVarO₃ is more important than JetVar for driving interannual variations in STT of ozone over the Pacific-North American region. In particular, the abundance of ozone in the LMS at the end of winter is shown to be a robust indicator of the amount of ozone that will be contained in stratospheric intrusions during the ensuing spring and summer. Additionally, it is shown that the winter season time-summed strength of the stratospheric NAM is a useful predictor of ozone intrusion strength. The results also suggest a nuanced relationship between the phase of ENSO and STT of ozone. It is found that while ENSO-related jet variability is associated with STT variability, it is wave breaking frequency rather than typical ENSO teleconnection patterns that are responsible for the ENSO-STT relationship.