Continental cold-air-outbreaks under the varying stratosphere-troposphere coupling regimes during stratospheric Northern Annular Mode events

Abstract

AbstractA Stratospheric Northern Annular Mode (SNAM) phase-based composite analysis reveals that continental Cold Air Outbreaks (CAOs) can occur during both positive and negative SNAM events. CAOs tend to occur over Asia, characterized by a meridional-dipole surface temperature anomaly pattern (cold midlatitudes and warm high-latitudes) when the SNAM index is decreasing or the stratospheric polar vortex is weakening, but over North America and Europe with a meridionally-homogeneous pattern when the SNAM index is increasing or the stratospheric polar vortex is strengthening. While the decreasing SNAM is dominated by a stronger stratospheric poleward warm branch (WB-ST) of the isentropic meridional mass circulation and vice versa, the CAOs always follow a stronger tropospheric poleward warm branch (WB-TR) and an equatorward cold branch (CB) of the isentropic meridional mass circulation. The correspondence between the stronger/weaker WB-ST and stronger/weaker WB-TR&CB during majority of SNAM phases (referred to as stratosphere-troposphere coupling regimes) is responsible for the CAOs in Asia. During the remaining phases (stratosphere-troposphere decoupling regimes), in accompany with a weaker/stronger WB-ST, the WB-TR&CB are stronger/weaker and relates to the CAOs occurred in North America and Europe. The coupling regimes when the stratospheric polar vortex is weakening/strengthening are mainly attributed to the E-P flux convergence/divergence from the middle troposphere to the lower stratosphere, the larger wave amplitude throughout the column, and anomalous tropospheric wave flux mainly in the Asia in subpolar latitudes. The decoupling regimes, however, are mainly related to the anomalous westward-tilting of waves and the wave flux reflection toward the North America or Europe.