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

We compare two moist formulations of the Eliassen-Palm (EP) flux with the traditional dry EP flux and analyze their link to the position and strength of the surface westerlies using 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 increases, 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 the opposite behavior in response to warming: an equatorward shift of the surface westerlies. In cold climates, the position of the surface westerlies in the idealized simulations coincides with the position of the maximum vertical EP flux, while in warm climates the surface westerlies coincide in latitude with an isolated region of upper-tropospheric EP flux divergence that has previously been shown to relate to poleward fluxes of potential vorticity variance.