Abstract
Extreme warm stratospheric events during polar winters from ERA-Interim reanalysis and CMIP5-ESM-LR runs were separated by duration and strength of the polar-night jet oscillation (PJO) using a high statistical confidence level of three standard deviations (strong-PJO events). With a composite analysis, we demonstrate that strong-PJO events show a significantly stronger downward propagating signal in both, northern annular mode (NAM) and zonal mean zonal wind anomaly in the stratosphere in comparison with non-PJO events. The lower stratospheric EP-flux-divergence difference in ERA-Interim was stronger in comparison to long-term CMIP5-ESM-LR runs (by a factor of four). This suggests that stratosphere–troposphere coupling is stronger in ERA-Interim than in CMIP5-ESM-LR. During the 60 days following the central date (CD), the Arctic oscillation signal was more intense during strong-PJO events than during non-PJO events in ERA-Interim data in comparison to CMIP5-ESM-LR runs. During the 15-day phase after CD, strong PJO events had a significant increase in stratospheric ozone, upper tropospheric zonally asymmetric impact, and a regional surface impact in ERA-Interim. Finally, we conclude that the applied high statistical threshold gives a clearer separation of extreme warm stratospheric events into strong-PJO events and non-PJO events including their different downward propagating NAM signal and tropospheric impacts.
Highlights
During winter, the boreal stratosphere couples with the underlying troposphere in two pathways: bottom-up and top-down
Polvani and Waugh [1] showed that zonal-mean eddy heat flux, which is a proxy for the upward wave activity flux, averaged over prior 40 days, was highly (−0.8) anti-correlated with the northern annular mode (NAM) index at 10 hPa, suggesting a physical link
The main question remains: under which specific oscillation behavior during such warm events is the stratosphere more strongly coupled with the troposphere? In this study we focus on extreme warm events with a major sudden stratospheric warming (MSSW) followed by a strong downward propagating NAM signal, and accompanied by a pronounced polar-night jet oscillation (PJO) event, which have the potential for forecasting improvements [11]
Summary
The boreal stratosphere couples with the underlying troposphere in two pathways: bottom-up and top-down Often it undergoes different extreme circulation states: cold events with a strong polar vortex and zonal mean westerlies (polar-night jet); and warm events with a weakening or a breakdown of the polar vortex and zonal mean easterlies. Warm events are caused by increased upward wave activity fluxes associated with ultra-long planetary waves as demonstrated by Polvani and Waugh [1]. This observation is in agreement with the wave-mean flow interaction theory (e.g., [2]). That Baldwin and Dunkerton [4] demonstrated that during weak vortex regimes (warm events), mean surface pressure anomalies are in the negative phases of Arctic oscillation (AO)
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