Equatorward Eddy Heat Flux in the Boreal Winter Subtropics and Its Impacts on the Residual Mean Circulation

Poleward heat flux driven by large-scale eddies is a widely recognized component of the general circulation. Although less known, a weak but persistent equatorward eddy heat flux (EEHF) also exists in the subtropical upper troposphere–lower stratosphere (UTLS). In this study, we solve a Transformed Eulerian Mean (TEM) form of the Kuo–Eliassen equation and find that in the subtropical UTLS, the EEHF alone weaken the boreal winter TEM circulation. This EEHF fluctuates on daily time scales. When the EEHF strength exceeds 1 standard deviation, the UTLS TEM circulation substantially weakens. This EEHF cannot be explained by background temperature gradients. Instead, the evolution of the Eliassen–Palm flux and precipitation rate indicates that it is likely driven by horizontal propagation of planetary-scale waves which, at least in part, is triggered by tropical convection over the western tropical Pacific.