熱帯収束帯
Whether it be a depression in the Intertropical Convergence Zone (ITCZ), a tropical wave, a broad surface front, or an outflow boundary, a low level feature with sufficient vorticity and convergence is required to begin tropical cyclogenesis. Even with perfect upper-level conditions and the required atmospheric instability, the lack of a surface focus will prevent the development of organized convection and a surface low. Tropical cyclones can form when smaller circulations within the Intertropical Convergence Zone come together and merge.