Moist formulations of the EP flux and their connection to surface westerlies in current and warmer climates

Abstract:

The Eliassen-Palm (EP) flux is an important diagnostic for wave propagation and wave-mean flow interaction in the atmosphere. Here we compare two moist formulations of the EP flux with the traditional dry EP flux and analyze their link to the position and strength of the surface westerlies using reanalysis data and both fully-coupled and idealized climate models. The first moist formulation of the EP flux modifies only the static stability to account for latent heat release by eddies, while the second moist formulation simply replaces all potential temperatures with equivalent potential temperatures. When moisture is taken into account, the latitude of maximum upward EP flux and maximum EP flux convergence shift equatorward and the strengths of both the flux and convergence increase, with larger changes for the second moist formulation.

In simulations with a coupled atmosphere-ocean climate model, both the peak surface winds and peak upward EP flux in the lower troposphere tend to be co-located throughout the seasonal cycle (especially in the moist formulations) and shift poleward by similar amounts in response to greenhouse warming. In simulations over a wider range of climates with an idealized atmospheric climate model we find that in cold climates the position of the surface westerlies coincides with the position of the maximum vertical EP flux and shifts poleward with warming, while in warm climates the surface westerlies coincide with an anomalous region of EP flux divergence near the subtropical jet. An isentropic potential enstrophy budget analysis reveals that in this model the anomalous EP flux divergence is balanced by vertical eddy PV fluxes associated with diabatic heating from large-scale condensation and radiation. The anomalous divergence is weaker when using moist EP fluxes, indicating that the moist formulations are partly capturing this effect.