Asymmetric impact of the Scandinavian pattern on stratospheric circulation anomalies

Abstract

AbstractThis study investigates the stratosphere-troposphere coupling associated with the Scandinavian (SCA) pattern in boreal winter. The results indicate that the SCA impacts stratospheric circulation but that its positive and negative phases have different effects. The positive phase of the SCA (SCA+) pattern is restricted to the troposphere, but the negative phase (SCA−) extends to the upper stratosphere. The asymmetry between phases is also visible in the lead-lag evolution of the stratosphere and troposphere. Prominent stratospheric anomalies are found to be intensified following SCA+events, but prior to SCA−events. Further analysis reveals that the responses are associated with upward propagation of planetary waves, especially wavenumber 1 which is asymmetric between SCA phases. The wave amplitudes in the stratosphere, originating from the troposphere, are enhanced after the SCA+events and before the SCA−events. Furthermore, the anomalous planetary wave activity can be understood through its interference with climatological stationary waves. Constructive wave interference is accompanied by clear upward propagation in the SCA+events, while destructive interference suppresses stratospheric waves in the SCA−events. Our results also reveal that the SCA+events are more likely to be followed by sudden stratospheric warming (SSW) events, because of the deceleration of stratospheric westerlies following the SCA+events.