On the signal of the 11-year sunspot cycle in the stratosphere over the Antarctic and its modulation by the Quasi-Biennial Oscillation (QBO)

Abstract

In earlier studies it was shown that during northern winters as well as northern summers the QBO must be introduced to identify a solar signal. A clear solar signal emerges only when the data are stratified according to the different phases of the QBO. This signal is often of opposite sign in the respective phases of the QBO. During the easterly phase of the QBO the height differences between solar maxima and minima imply an intensification of the polar vortex (positive Annular Mode) throughout the year during solar maxima: During the northern winter half-year positive height differences (anomalies) are observed from about 90 ◦ St o 60 ◦ N, with negative anomalies further north; and during the southern winter half-year positive anomalies exist from 90 ◦ Nt o 45 ◦ S, with negative anomalies further south. The structure of the anomalies is very symmetric with a change between hemispheres in May/June and October/November. The consistently positive height and temperature anomalies during solar maxima/easterly phase over the tropics and subtropics imply anomalous downwelling, i.e. a weakening of the Brewer-Dobson circulation throughout the year – during solar maxima. The antarctic polar vortex and the connected Final Warmings are influenced by the 11-year solar cycle in much the same way as the arctic polar vortex. During solar maxima/westerly phase the midwinter warmings over the Arctic and the Final Warmings over the Antarctic lead to the negative mode of the Annular Mode (weaker polar vortex). They influence the whole stratosphere with downwelling (adiabatic warming) over the polar regions and connected upwelling (adiabatic cooling) outside the respective polar regions, reaching far into the other (summer) hemisphere. This leads to an intensification of the Brewer-Dobson circulation – during solar maxima. Zusammenfassung