Marginal seas of the western North Pacific Ocean act as a hotspot in winter, warming the marine atmosphere. The atmospheric response to lower-level (below 700 hPa altitude) heating localized around this hotspot was studied using an idealized general circulation model with an assumed triangular hotspot. As simulated in previous studies, localized atmospheric heating over the mid-latitude hotspot enhances the westerly jet core, forming a stationary Rossby wave north of the core. This study found that the hotspot remotely influences the Rossby wave source (the sum of vortex stretching and vorticity advection caused by the divergent flow) and temperature deviations at upper levels (200–300 hPa). These results are qualitatively consistent with the winter climatology. The idealized experiment was applied to explosive and binary extratropical cyclones (pairs of surface cyclones located north and south of the main Japanese islands). The active area of transient eddies at lower levels splits into two in the hotspot, leading to regional explosive development and the formation of binary cyclones through an enhanced meridionally elongated trough. The hotspot is an essential factor driving the bifurcation of cyclone tracks into the southern and northern areas of Japan, leading to binary cyclones.
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