Dominant contribution of Arctic sea ice cover to the bimodally interannual variation of the polar night jet in late November

Abstract The polar night jet (PNJ), characterized by strong zonal-mean zonal winds at 60–80° N and 50 hPa, shows interannual variability that is distinctively observed in late November. The autumnal acceleration tendency of the PNJ exhibits the alternative of monotonous acceleration and temporary deacceleration marked by a “short break” and consequently represents bimodal fluctuations. Statistical analyses using reanalysis and objective analysis datasets in this study reveal that the reduction of Arctic sea ice concentration (SIC) predominantly contributes to the PNJ short break, rather than other potential factors such as the quasi-biennial oscillation (QBO) and the El Niño-Southern Oscillation (ENSO) variability. The PNJ short break is usually accompanied by the upward propagation of Rossby waves from the troposphere to the stratosphere over Siberia. When the Arctic SIC is lower than climatology, particularly in the Barents–Kara Sea, the increased heat flux at the sea surface causes deceleration events inherent to late November. In contrast, the intensity of the submonthly-scale PNJ deceleration and acceleration is affected little by the phase of the QBO, while the phase of the QBO is thought to contribute to the seasonal means of the PNJ. While ENSO variability can also give favorable conditions for the deceleration of the zonal-mean westerly winds, the upward wave propagation is observed mainly over the North Atlantic and Europe rather than over Siberia.