Downward Migration of the Zonal Mean Circulation in the Tropical Atmosphere


A virtual visit to London…


From the first time we could observe global weather patterns in the mid 20th century, meteorologists have sought out “annular”, or ring-like, modes of variability that characterize zonally uniform vacillations of the atmosphere.  The annular modes of the extratropical atmosphere and their antecedents have received much attention for quantifying the variability of the jet streams and storm tracks (and their response to external forcing) despite the fact that the midlatitude circulation itself does not vary uniformly with longitude.  The prominence of these modes rather reflects the statistical annularity of the circulation.  While tropical fluctuations in geopotential height have lower amplitude than in the extratropics, they exhibit stronger zonal coherence, or dynamical annularity.

In this talk, I’ll present a simple index to characterize zonal-mean anomalies of the tropical circulation. It reveals that anomalies in geopotential height and zonal wind migrate downward from the upper troposphere to the surface on a time scale of about 10 days. These features are distinguishable from known modes of tropical variability, the Madden–Julian Oscillation in particular. Evidence from reanalysis and idealized model experiments confirms that this downward migration is quite generic and driven by mechanically forced variations in the strength of the Hadley circulation on subseasonal time scales.