QBO-induced anomalous transport in the Northern Hemisphere stratosphere: the exceptional 2018/2019 late boreal winter

Abstract. Large positive anomalies of nitrous oxide (N₂O) were observed in the northern hemisphere lower stratosphere in the late 2018/2019 boreal winter. Thanks to its long lifetime in the lower stratosphere, N₂O is a robust tracer for stratospheric transport. This study investigates the magnitude, vertical structure, and dynamical origin of the late 2018/2019 boreal winter N₂O anomaly using multiple chemical transport model simulations, chemical reanalyses, a specified-dynamics chemistry-climate model, and merged satellite observations. All datasets consistently show pronounced N₂O positive anomalies in February 2019 in the northern mid-latitudes at 50 hPa. The N₂O Transformed Eulerian Mean budget indicates that the N₂O anomalies are primarily driven by enhanced meridional residual advection, which is in turn determined by enhanced planetary wave forcing. The extratropical effects of the quasi-biennial oscillation (QBO) determine these transport anomalies: the poleward QBO secondary circulation was unusually strong in the late 2018/2019 boreal winter, and a marked northward displacement of the zero wind line induced the enhanced planetary wave forcing. The combined strengthening of both the northward QBO secondary circulation and the planetary wave forcing led to unusually strong poleward advection in the northern lower stratosphere, which ultimately built up the N₂O positive anomalies. These results indicate the value of long-lived stratospheric tracers such as N₂O for diagnosing dynamically driven extreme events and for disentangling the contributions of different transport processes. We highlight the importance of the QBO teleconnections, particularly as a warming climate may change the frequency and intensity of extreme events thereby impacting these teleconnections.