Downward Migration of the Zonal‐Mean Circulation in the Tropical Atmosphere. Issue 18 (19th September 2020) - DallaSanta, Kevin
Abstract: The annular modes of the extratropical atmosphere have received much attention for quantifying variability of the jet streams and storm tracks, despite the fact that the midlatitude circulation itself does not vary uniformly with longitude. While tropical fluctuations in geopotential height have lower amplitude than in the extratropics, they exhibit stronger zonal coherence, or dynamical annularity. A simple index is developed 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. Plain Language Summary: Earth's atmosphere has certain recurring patterns which are "annular, " spanning an entire latitude circle. Annular patterns have proven to be surprisingly useful for weather and climate prediction. However, in the tropics, annular patterns have not been studied. We show that annular variability in the tropics does exist, and it contains interesting features that may be useful for prediction. In particular, anomalies in the circulation migrate downward from the tropopause (about 16 km) to the surface over 10 days. These features are also apparent in a relatively simple climate model, which helps direct further research. Key Points: Zonally coherent, or annular, fluctuations capture a higher fraction of circulation variability in the tropics than they do in midlatitudes Annular anomalies in tropical geopotential height and zonal wind migrate from the tropopause to the surface over approximately 10 days Downward migration is associated with a pulsing of the overturning Hadley circulation on subseasonal time scales