Annular Mode Variability of the Atmospheric Meridional Energy Transport and Circulation
Abstract Month-to-month variability in the meridional atmospheric energy transport is analyzed in the Modern-Era Retrospective Analysis for Research and Applications (MERRA) reanalysis for 1979–2012. The meridional transport of moist static energy (MSE) is composited onto the high and low phases of the northern and southern annular modes (NAM and SAM). While the high phase of the NAM and SAM is known to involve a poleward shift in the midlatitude storm track and jet, it is shown here that the distribution of poleward MSE transport shifts equatorward. This change is explained by examining the variability of the underlying meridional circulation. In particular, changes in the mass transport averaged on dry and moist static energy levels are considered. These circulations have an advantage over the conventional Eulerian circulation for explaining the total energy transport. They are computed using the statistical transformed Eulerian-mean (STEM) formulation, which provides a decomposition of the circulation into Eulerian-mean and eddy-driven components. The equatorward shift in the MSE transport is largely explained by a poleward shift of the Ferrel cell, while changes in the eddy-driven circulation have a comparatively small effect on the energy transport. The changes in the residual circulation and jet are shown to be consistent through momentum balance arguments. Mean-eddy feedback mechanisms that drive and sustain the annular modes are discussed at the end as a possible explanation for why the changes in the eddy-driven circulation are weak compared to the changes in the Eulerian circulation.