An environmental synoptic analysis of tropical transitions in the central and Eastern North Atlantic

A tropical transition (TT) is the process whereby a baroclinic, high-to-moderate vertical wind shear, extratropical or subtropical cyclone is transformed into a warm-core, low vertical wind shear, tropical cyclone. Thirty TT events were identified over the central and eastern North Atlantic basin during the period 1979–2019. The TT process is here studied from a synoptic storm-centered composite climatology and an environmental classification perspective. The aim is to study their common features and highlight their differences. The storm-centered composite analysis reveals that a westerlies meridional trough with quasigeostrophic forcig acts as precursor. TT environments are characterized by a trough at 300 hPa geopotential and the increase of the 1000–500 hPa thickness, i.e., the system evolves into a warm-core, and a strong anticyclone is located north of the surface cyclone. The transition is accompanied by a large latent heat release which promotes the vertical redistribution of potential vorticity and a reduction of the 850–300 hPa vertical wind shear. The identified TTs in the central North Atlantic predominantly developed in environments with warm sea surface temperatures (> 25 ◦C) and low-to-moderate wind shear (10–15 m s − 1 ). In contrast, the eastern North Atlantic TTs transitioned in low sea surface temperature values ( 15 m s − 1 ). Finally, the statistically significant differences in the environmental classification encouraged further analysis of their environments via stormcentered composites, revealing that eastern North Atlantic cyclones have a more defined extratropical structure, while central North Atlantic cyclones show more tropical characteristics.This work was partially supported by research projects PID2019-105306RB-I00 (IBERCANES) and CGL2016-78702 (SAFEFLIGHT). This work is also supported by the ECMWF Special Projects SPESMART and SPESVALE. C. Calvo-Sancho acknowledges the grant awarded by the Spanish Ministry of Science and Innovation - FPI program (PRE2020-092343)