Interdecadal variations of Northern Hemispheric Polar Cell during winter over the past 60 years

The Polar Cell is an important but often ignored component of the mean meridional circulations. This study identified and investigated the long-term variations in the Northern Hemispheric Polar Cell during Winter (NPCW) by using the meridional and regional Mass Stream Function derived from ERA5 reanalysis data. The results show that the intensity and central location of the NPCW have an evident rhythm of interdecadal change from 1959 to 2022. Compared to the preceding and following decades, the NPCW was weaker and shifted farther poleward during 1987~1997. It is found that the interdecadal variability might be modulated by the interdecadal variation in the quasi-stationary planetary wave activity. Compared to the period 1976~1986, the amplitude of the quasi-stationary planetary wave during 1987~1997 increased at high latitudes but decreased around 40–50°N. This pattern altered the lower-tropospheric pressure systems, inducing southerly wind anomalies over the mid-high latitude North Atlantic, which acted to weaken the NPCW intensity. Concurrently, the upward propagation of quasi-stationary planetary waves into the stratosphere over high latitudes along the polar wave-guide was reduced，which decreased the convergence of Eliassen-Palm (E-P) fluxes in the lower stratosphere over high latitudes of the Northern Hemisphere. These resulted in an enhanced polar front jet through planetary wave-mean flow interaction．The AO was then enhanced and the polar vortex strengthened. Meanwhile, anomalous cooling in the Arctic troposphere coupled with anomalous warming in the mid-latitudes increased the mean meridional temperature gradient. This enhanced gradient promoted eddy activities over the mid-high latitudes of the North Atlantic and North Pacific, driving a poleward shift of the NPCW"s ascending branch and thus altering the circulation"s central location. In contrast, during 1998~2008, opposite anomalies in quasi-stationary planetary wave activity led to northerly wind anomalies in the lower troposphere and a weakened meridional temperature gradient, causing the NPCW to strengthen and shift southward compared to the previous period.