Quantifying the Imprints of Stratospheric Contributions to Interhemispheric Differences in Tropospheric CFC-11, CFC-12, and N2O Abundances

For trace gases destroyed in the stratosphere, mass flux across the tropopause can substantially influence observed surface hemispheric differences (NH-SH). Here, we quantify associations between observed stratospheric and tropospheric NH-SH growth rate anomalies of CFC-11, CFC-12, and N2O. We employ a chemistry climate model along with satellite and global surface station observations. Our model explains 60% of observed N2O NH-SH growth rate variability from 2005 to 2019, compared to 30% for CFC-11% and 40% for CFC-12, supporting evidence that unexpected anthropogenic emissions caused sustained positive NH-SH anomalies in these CFCs from 2012 to 2017. Between 2012 and 2015, the observed CFC-11 NH-SH difference grew by 1.7 ppt; our model explains 0.5 +/- 0.1 ppt of this growth, but not the duration. Our model suggests that in the absence of further emission anomalies, new NH-SH positive tracer anomalies should have occurred in 2020, and predicts small negative anomalies in 2021. Plain Language Summary Changes in the North-South difference of surface trace gas abundances, denoted NH-SH, are often used as evidence of new emissions. However, atmospheric dynamics can also influence measurements of NH-SH. Importantly, anomalies in trace gas abundances in the stratosphere are transported down to the troposphere, influencing tropospheric NH-SH values. We quantify the stratospheric influence on NH-SH in models and observations for CFC-11, CFC-12, and N2O. We provide a simple model to account for future stratospheric anomalies at the surface. Our model suggests that 60% of the variability in NH-SH can be explained by stratospheric anomalies. Following 2013, there was a sustained positive NH-SH anomaly in observations. Our results indicate that stratospheric influences can only partially explain this anomaly, supporting earlier work that an expected emission led to the observed positive NH-SH anomaly. This work shows that stratospheric observations improves interpretation of future emissions.

Automated summary

The exchange of trace gases between the stratosphere and troposphere can influence surface hemispheric differences, with a chemistry climate model explaining a significant portion of observed variability in NH-SH growth rates. Positive NH-SH anomalies in CFCs observed from 2012 to 2017 were likely caused by unexpected anthropogenic emissions, and a model suggests the occurrence of new anomalies in 2020. This study highlights the importance of stratospheric influences on interpreting surface emissions.