Iba: The First Pure Tropical Cyclogenesis Over the Western South Atlantic Ocean

In March 2019, the tropical western South Atlantic Ocean (similar to 16.5 degrees 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 x 10(-5) s(-1)), (c) latent and sensible heat fluxes near the climatological values (similar to 150 W m(-2)), (d) warmer sea surface temperature (+2 degrees 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.