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 degrees 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 (< 25 degrees C) and high wind shear (> 15 m s(-1)). Finally, the statistically significant differences in the environmental classification encouraged further analysis of their environments via storm-centered composites, revealing that eastern North Atlantic cyclones have a more defined extratropical structure, while central North Atlantic cyclones show more tropical characteristics.

Automated summary

The study revealed that tropical transition is a process where extratropical or subtropical cyclones transform into tropical cyclones, with environmental factors such as troughs and geopotential heights playing key roles. Central North Atlantic TT events usually occur in environments with warm sea surface temperatures and low-to-moderate wind shear, while eastern North Atlantic TT events take place in environments with lower sea surface temperatures and higher wind shear. The environmental classifications also indicate that eastern North Atlantic cyclones have a more defined extratropical structure, while central North Atlantic cyclones show more tropical characteristics.