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Ray Yamada and Olivier Pauluis

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.

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Ray Yamada and Olivier Pauluis

Abstract

Previous formulations for the zonally averaged momentum budget and Eliassen–Palm (EP) flux diagnostics do not adequately account for moist dynamics, since air parcels are not differentiated by their moisture content when averages are taken. The difficulty in formulating the momentum budget in moist coordinates lies in the fact that they are generally not invertible with height. Here, a conditional-averaging approach is used to derive a weak formulation of the momentum budget and EP flux in terms of a general vertical coordinate that is not assumed to be invertible. The generalized equation reduces to the typical mass-weighted zonal-mean momentum equation for invertible vertical coordinates.

The weak formulation is applied here to study the momentum budget on moist isentropes. Recent studies have shown that the meridional mass transport in the midlatitudes is twice as strong on moist isentropes as on dry isentropes. It is shown here that this implies a similar increase in the EP flux between the dry and moist frameworks. Physically, the increase in momentum exchange is tied to an enhancement of the form drag associated with the horizontal structure of midlatitude eddies, where the poleward flow of moist air is located in regions of strong eastward pressure gradient.

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Ray Yamada and Olivier Pauluis

Abstract

Previous studies show that the moist Eliassen–Palm (EP) flux captures a greater eddy momentum exchange through form drag than the dry EP flux in the midlatitude climate. This suggests that the eddy moisture flux acts to decrease the baroclinicity of the zonal jet. This study investigates such a mechanism in moist baroclinic life cycles, which are simulated in an idealized general circulation model with large-scale condensation as the only moist process. The runs are analyzed using a linear diagnostic based on the Kuo–Eliassen equation to decompose the jet change into parts driven by individual forcing terms. It is shown that the wave-induced latent heating drives an indirect Eulerian-mean cell on the equatorward flank of the jet, which acts to reduce the baroclinicity in that region. The eddy sensible heat fluxes act to reduce the baroclinicity near the center of the jet. The moist baroclinic forcing strengthens as the amount of initially available moisture increases.

The effect of the eddy moisture flux on the transformed Eulerian-mean (TEM) and isentropic dynamics is also considered. It is shown that the circulation and EP flux on moist isentropes is around 4 times as strong and extends farther equatorward than on dry isentropes. The equatorward extension of the moist EP flux coincides with the region where the baroclinic forcing is driven by latent heating. The moist EP flux successfully captures the moisture-driven component of the baroclinic forcing that is not seen in the dry EP flux.

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