Changes in Stratospheric Transport Induced by Sudden Stratospheric Warmings in Reanalysis and Model

The extreme disruptions of the wintertime stratospheric circulation during sudden stratospheric warmings (SSW) have potentially important effects on tracer concentrations through alterations in transport properties. The goal of this study is to provide a quantitative analysis of those changes examining the anomalies of the advective Brewer-Dobson circulation and the effective diffusivity as a measure of isentropic mixing. We composite data around the central day of the warming from thirty-six years of reanalysis data (ERA-Interim), and from five ensemble members (60 years each) of the Whole Atmospheric Community Climate Model version 4 (WACCM4) performed for the Chemistry Climate Model Intercomparison (CCMI) exercise.

Our results show an overall qualitative and quantitative agreement between ERA-Interim and WACCM4. The acceleration of the BDC is characterized by enhanced polar downwelling and tropical upwelling throughout the whole depth of the stratosphere starting at lag -10 days before the central day of the SSW, reaching anomalies larger than 1.5 standard deviations throughout the stratosphere at around lag -3 days. These anomalous stratospheric circulation changes sign right after the warming central day, which can be traced back to the cease of wave forcing soon after that date. Interestingly, the anomalies in the effective diffusivity reach their maximum around 1-2 weeks after the SSW at 800 K (~30 km), and even later in the lower stratosphere. The implications of the above changes on the ozone tendencies during SSWs in the model will also be discussed.