In March 2019, the tropical western South Atlantic Ocean (∼16.5°S) registered the first pure tropical cyclogenesis since the beginning of the satellite era, which the Brazilian Navy named Iba. This study presents a description of the synoptic features and meteorological mechanisms contributing to Iba's initial cyclonic vorticity as well as the anomalous synoptic patterns linked to this unprecedented tropical cyclogenesis. Moreover, a general description of Iba's lifecycle is provided. The genesis of Iba was as a pure tropical cyclone (00:00 UTC on March 22), and it underwent subtropical (18:00 UTC on March 25) and extratropical (06:00 UTC on March 29) transitions. Anomalies for a 5‐day period prior to the genesis relative to the March climatology show environmental features appropriate for tropical cyclogenesis: (a) vertical wind shear in the 850–200 hPa lower than 10 m s⁻¹, (b) intense initial cyclonic relative vorticity at 925 hPa (−10 × 10⁻⁵ s⁻¹), (c) latent and sensible heat fluxes near the climatological values (∼150 W m⁻²), (d) warmer sea surface temperature (+2°C), and (e) relative humidity at 700 hPa 20% higher than the climatology. These features were associated with an anomalous wave pattern at 500 hPa, resembling a blocking structure over the southeastern South Pacific Ocean, and an anticyclonic circulation at upper levels over southeastern Brazil. This environment propitiated a weak vertical wind shear and convective cloud organization. Near the surface, the horizontal wind shear between the postfrontal southwesterly winds and the northeasterly winds of the South Atlantic subtropical anticyclone helped to generate Iba's initial cyclonic vorticity.

中文翻译：

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伊巴：西南部大西洋上的第一个纯热带气旋

2019年3月，热带南部大西洋南部（〜16.5°S）记录了自卫星时代开始以来的首次纯热带气旋，巴西海军将其命名为Iba。这项研究描述了导致Iba初始气旋涡的天气特征和气象机制，以及与这种前所未有的热带气旋作用有关的异常天气模式。此外，提供了Iba生命周期的一般描述。伊巴的起源是一个纯热带气旋（3月22日00:00 UTC），它经历了亚热带（3月25日18:00 UTC）和温带（3月29日06:00 UTC）的过渡。相对于三月的气候，发生前五天的异常表现出适合热带气旋发生的环境特征：^-1，（b）925 hPa（-10×10 ^-5 s ^-1）强烈的初始气旋相对涡度，（c）接近气候值（〜150 W m ^-2）的潜热通量和感热通量），（d）较温暖的海面温度（+ 2°C）和（e）700 hPa的相对湿度比气候高20％。这些特征与500 hPa处的异常波型有关，类似于南太平洋东南部的阻塞结构，以及巴西东南部的高层反气旋环流。这种环境导致垂直风切变和对流云组织较弱。在地表附近，南大西洋亚热带反旋风的后额西南风和东北风之间的水平风切变有助于产生Iba的初始旋风涡。