Northern vs. southern hemisphere differences in the stratospheric influence on variability in tropospheric nitrous oxide

Abstract. We present a chemistry-climate model with a tagged stratospheric nitrous oxide (N₂O) tracer that predicts distinct seasonal cycles in tropospheric N₂O caused by descent of N₂O-depleted stratospheric air in polar regions. We identify similar phenomena in recently available aircraft profiles from global campaigns and routine monitoring. Long-term atmospheric measurements from the National Oceanic Atmospheric Administration (NOAA) global surface monitoring network provide additional support for a significant impact on surface N₂O originating from the stratosphere. In the northern hemisphere, the NOAA surface N₂O atmospheric growth rate anomaly is negatively correlated with the previous winter’s polar lower stratospheric temperature. This negative correlation is consistent with increased (decreased) transport in years with a strong (weak) Brewer Dobson circulation of warm, N₂O-depleted air from the middle and upper stratosphere into the lower stratosphere, with subsequent cross-tropopause transport of the N₂O-depleted air into the troposphere. In the southern hemisphere, polar lower stratospheric temperature is correlated to monthly summertime anomalies in tropospheric N₂O as it descends into its seasonal minimum, a result that is supported by aircraft data as well as the chemistry-climate model. However, the N₂O atmospheric growth rate anomaly in the southern hemisphere is better correlated to the stratospheric quasi-biennial oscillation (QBO) index, as well as the El Niño Southern Oscillation index, than to polar lower stratospheric temperature. These hemispheric differences in the factors influencing the N₂O atmospheric growth rate are consistent with known atmospheric dynamics and the complex interaction of the QBO with the Brewer Dobson circulation.