A Global convection circulation paradigm for the Annular Mode

Based on the observational evidence deduced from the 24-year NCEP/NCAR reanalysis II dataset (1979-2003) in an isentropic PV-latitude Lagrangian coordinate, a global convection circulation paradigm is proposed to explain dynamical mechanisms for the annular mode. Due to the Earth's rotation, the westerly flow associated with the vertically sloped meridional temperature contrast that prevails from the troposphere to stratosphere and from the subtropics to the pole is a physical barrier for a direct exchange of warm and cold air masses. The cross-frontal ageostrophic circulation associated with an intensification of the meridional temperature contrast acts as a dynamic “pump” that moves warm (cold) air poleward (equatorward) over (under) the westerly jet. The mixing of warm and cold air masses by the cross-frontal ageostrophic circulation leads to frontolysis (frontogenesis) at upper levels in the warm (cold) air sector, and frontogenesis at lower levels. This results in a poleward and downward propagation of the cross frontal circulation accompanied with a leveling of the vertically slopped baroclinic zone. Behind (ahead of) the poleward and downward advancement of the cross-frontal ageostrophic circulation is a deceleration (acceleration) of the westerly wind due to the subsequent geostrophic adjustment associated with the development of frontolysis (frontogenesis). A succession of poleward and downward advancements of cross-frontal circulations and the companying leveling of baroclinic zone and the subsequent geostrophic adjustments from the tropics to the winter pole are responsible for the simultaneous poleward and downward propagations of anomalies of both signs with a meridionally dipole structure. The average traveling time from the tropics to the pole is about 40-50 days. We will discuss the role of the stratosphere bridge that links tropical thermal forcing and high-latitude circulation in winter season as well as its implications for climate predictions.