Effect of Moisture on Wave-Mean Flow Interactions in Idealized Baroclinic Life Cycles

Past studies have shown that the moist Eliassen-Palm (EP) flux captures a greater momentum transfer than the dry EP flux in the midlatitude climate, which suggests that eddy moisture fluxes may play an important role in changing the baroclinicity of the zonal jet. This study investigates the role of wave-induced condensation on the eddy forcing and mean-flow development in moist baroclinic life cycles. The life cycles are simulated in an idealized general circulation model, which is nearly inviscid and moist adiabatic, with large-scale condensation as the only moist process. They are initiated from a baroclinically unstable background flow with a small wavenumber-6 perturbation and differ only by their initial value of relative humidity, which is set to be uniform throughout the atmosphere.

The changes in the jet are considered after 15 days, which corresponds to the end of an initial instability event. The separate contributions to the jet change from the eddy momentum, heat, and moisture fluxes can be diagnosed using a linear diagnostic based on the Kuo-Eliassen equation. It is shown that the wave-induced latent heating drives an indirect cell which reduces the baroclinicity on the equatorward flank of the jet, while the eddy sensible heat flux acts to reduce the baroclinicity near the center of the jet. The strength of the moist baroclinic forcing increases with the initially available moisture content.

From the transformed Eulerian mean (TEM) and isentropic perspectives, the eddy moisture flux enhances the meridional circulation and the eddy forcing of the mean flow. The circulation and EP flux are shown to be around four times as strong with a greater equatorward extent on moist isentropes than on dry isentropes. The equatorward extension of the moist EP flux overlaps 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 captured by the dry EP flux.