Meridional and Downward Propagation of Atmospheric Circulation Anomalies. Part I: Northern Hemisphere Cold Season Variability

As in the Northern Hemisphere, there exists a simultaneous poleward propagation of temperature anomalies in the stratosphere and equatorward propagation in the troposphere in the Southern Hemisphere’s cold season. It takes about 110 days for anomalies of one polarity to propagate from the equator to the pole (or half the period of the complete cycle), nearly twice as long as in the Northern Hemisphere. The earlier poleward propagation of temperature anomalies in upper levels compared with those in lower levels results in an apparent downward propagation in the stratosphere. Accompanying the polewardand downward-propagating warm (cold) anomalies is a successive leveling (steepening) of isentropic surfaces, reflecting a simultaneous reduction (strengthening) of the meridional temperature gradient and increase (decrease) of the vertical static stability. Following changes in the thermal fields are polewardand downward-propagating zonal wind anomalies of the opposite sign. The arrival of the poleward-propagating stratospheric thermal anomalies over the polar region coincides with the beginning of the compensating equatorward advancement of tropospheric thermal anomalies of the opposite sign. The synchronized meridional propagation in the thermal fields results from the temporal variation between stronger and weaker meridional mass circulations, which is responsible for a meridional out-of-phase variability pattern in both the stratosphere and troposphere and a vertical out-of-phase pattern between stratospheric and tropospheric temperature anomalies at high latitudes. The vertical alignment of a warm high (cold low) anomaly in the stratosphere overlying a cold high (warm low) anomaly at the surface explains the apparent “equivalent barotropic” structure in the height anomalies.