A Two-way Stratosphere–Troposphere Coupling of Submonthly Zonal-Mean Circulations in the Arctic

This paper examines the dominant submonthly variability of zonally symmetrical atmospheric circulation in the Northern Hemisphere(NH)winter within the context of the Northern Annular Mode(NAM),with particular emphasis on interactive stratosphere–troposphere processes.The submonthly variability is identified and measured using a daily NAM index,which concentrates primarily on zonally symmetrical circulation.A schematic lifecycle of submonthly variability is developed that reveals a two-way coupling process between the stratosphere and troposphere in the NH polar region.Specifically,anomalous tropospheric zonal winds in the Atlantic and Pacific sectors of the Arctic propagate upwards to the low stratosphere,disturbing the polar vortex,and resulting in an anomalous stratospheric geopotential height(HGT)that subsequently propagates down into the troposphere and changes the sign of the surface circulations.From the standpoint of planetary-scale wave activities,a feedback loop is also evident when the anomalous planetary-scale waves(with wavenumbers 2 and 3)propagate upwards,which disturbs the anomalous zonally symmetrical flow in the low stratosphere,and induces the anomalous HGT to move poleward in the low stratosphere,and then propagates down into the troposphere.This increases the energy of waves at wavenumbers 2 and 3 in the low troposphere in middle latitudes by enhancing the land–sea contrast of the anomalous HGT field.Thus,this study supports the viewpoint that the downward propagation of stratospheric NAM signals may not originate in the stratosphere.