Downward migration of the zonal-mean circulation in the tropical atmosphere


The annular modes of the extratropical atmosphere have received much attention for quantifying and predicting variability of the jet streams and storm tracks, despite the limited zonal coherence of midlatitude variability. In the tropics, annular Huctuations of the circulation have not been investigated, despite the comparative dominance of zonal-mean variations in this region, associated with weak temperature gradients at low latitudes.

Here, the zonal-mean signal in equatorial geopotential height is investigated and found to be strong, with distinct signatures on global surface temperature and sea level pressure. Furthermore, evidence is found of a downward migration of anomalies from the upper troposphere to the surface on a 10 to 20 day time scale, associated with variations of the strength and extent of the Hadley Circulation. Both reanalyses and idealized modeling demonstrate that annular Huctuations of the tropical circulation are robust and distinguishable from known modes of tropical variability, including the Madden-Julian Oscillation, El Nino- Southern Oscillation, and the Quasi-Biennial Oscillation.