This study investigates the variation in available potential energy of stationary eddies in the mid‐troposphere over the midlatitude Northern Hemisphere (30°–60°N) from 1950 to 2021, based on the ERA5 reanalysis and its preliminary back extension. The variation is attributed mainly to changes in the wavenumber‐1 eddy. It is also noted that potential energy conversion from the zonal mean flow in early winter plays an important role in the 20‐year variation. In addition, statistical analysis and numerical simulation of an ICTP model demonstrate that the potential energy conversion is controlled by the North Pacific Gyre Oscillation, which induces anomalous meridional temperature advection in the mid‐troposphere over Northeast Asia.