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.
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