Momentum balance and Eliassen-Palm flux in moist isentropic coordinates

Eddy momentum fluxes make up the dominant means by which angular momentum is transported into the midlatitudes. Eliassen-Palm (EP) flux diagnostics have proven to be a useful tool for understanding how eddies exchange momentum with the zonal-mean flow. One limitation of the classic formulation of the EP flux lies in that it is based on purely dry dynamics. We address this problem here by developing a weak formulation of the momentum balance in moist coordinates, which involves a generalized form of the EP flux.

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