In a sharp contrast to tropical cyclone (TC) genesis over the main development region of the western North Pacific (WNP), near-equatorial (0°–5°N) TCs exhibit a distinctive annual cycle, peaking in boreal winter and being inactive in boreal summer. The relative roles of dynamic and thermodynamic background states on near-equatorial TCs formation were investigated based on the observational diagnosis of the genesis potential index (GPI) and high-resolution model simulations. It is found that the background vorticity makes a major contribution to the distinctive annual cycle, while mean temperature and specific humidity fields are not critical. Numerical simulations further indicate that seasonal mean cyclonic vorticity in boreal winter has three effects on TC genesis near the equator. First, the environmental cyclonic vorticity interacts with TC vortex to promote a mid-level outflow, which strengthens boundary layer friction induced ascending motion and thus condensational heating. Second, it produces an equivalent Coriolis effect (via enhanced absolute vorticity), which strengthens positive feedback between primary and secondary circulation. Third, it helps to merge small-scale vortical hot towers (VHTs) into a mesoscale core through vorticity segregation process. However, background vorticity in boreal summer has an opposite effect on TC development near the equator.

中文翻译：

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背景动态和热力学状态对北太平洋西部近赤道热带气旋形成独特年周期的影响

与北太平洋西部主要开发区的热带气旋（TC）形成鲜明对比，近赤道（0°–5°N）TCs具有独特的年周期，在北方冬季达到顶峰并且不活跃在北方的夏天。基于成因潜力指数（GPI）的观测诊断和高分辨率模型模拟，研究了动态和热力学背景状态在近赤道TCs形成中的相对作用。已经发现，背景涡度对独特的年周期做出了重要贡献，而平均温度和特定湿度场并不关键。数值模拟进一步表明，北方冬季的季节平均气旋涡度对赤道附近的TC成因有三方面的影响。第一，环境气旋的涡度与TC涡旋相互作用，促进了中等水平的流出，这加剧了边界层摩擦引起的上升运动，从而增强了凝结加热。其次，它会产生等效的科里奥利效应（通过增强绝对涡度），从而加强一次和二次循环之间的正反馈。第三，它有助于通过涡度隔离过程将小型涡旋热塔（VHT）合并为中尺度核心。但是，北半球夏季的背景涡度对赤道附近的TC发育有相反的影响。这样可以加强一次和二次循环之间的积极反馈。第三，它有助于通过涡度隔离过程将小型涡旋热塔（VHT）合并为中尺度核心。但是，北半球夏季的背景涡度对赤道附近的TC发育有相反的影响。这样可以加强一次和二次循环之间的积极反馈。第三，它有助于通过涡度隔离过程将小规模的涡流热塔（VHT）合并为中尺度的核心。但是，北半球夏季的背景涡度对赤道附近的TC发育有相反的影响。