40–70 day meridional propagation of global circulation anomalies

Abstract

This paper reports a diagnostic study of circulation anomalies in a semi‐Lagrangian θ‐PVLAT coordinate by following contours of the daily potential vorticity (PV) field on isentropic (θ) surfaces using the NCEP/NCAR reanalysis II data set from 1979 to 2003. The leading EOF mode, which explains about 69% of the total variance of daily Northern Hemisphere PV anomalies in the θ‐PVLAT coordinate and is highly correlated with the stratosphere Northern Annular Mode (NAM) index, is used to construct a composite life cycle of the NAM variability. Composite circulation anomalies of both signs propagate poleward in the stratosphere and equatorward in the troposphere with an average time of about 44 days for warm anomalies and 72 days for cold anomalies to travel from the equator to the pole. Accompanying the meridional propagation, there exist a simultaneous downward propagation of stratospheric circulation anomalies in both the tropics and extratropics. A global mass circulation paradigm is proposed to explain the simultaneous meridional and vertical propagation of global circulation anomalies that appears responsible for the annular mode variability.