Contrast change of cyclogenesis over tropical and extratropical in the Eocene

Modern tropical cyclones (TCs), which originate in the tropics, feature deep warm-core symmetric structures. However, the characteristics of TCs in the early Eocene remain unclear. Here, we showed evidence from proxy data, climate modeling, and cyclone phase space that cyclones with deep warm-core symmetric structures appeared at high latitudes during the early Eocene. Under the favorable conditions of warm sea surface temperature and weak baroclinicity, most of these cyclones originated from the transition of extratropical cyclones. Meanwhile, in the tropics, only 36.91% of symmetric cyclones had a deep warm core, while 49.86% had a warm core at lower levels. These shallow cyclones tend to induce more intense rainfall than modern TCs. Our results provide unique insights into TC changes under high carbon dioxide levels and highlight the growing threat of extreme rainfall and winds associated with TCs at high latitudes.