Is the Tropical Cyclone Weakening or Transitioning? An Assessment of North Atlantic Extratropical Transition

The extratropical transition (ET) of tropical cyclones (TCs) tends to affect regions well removed from the tropics with hazards often produced by TCs such as strong winds and heavy rain. When a TC enters an environment characterized by drier air, increasing vertical wind shear, and decreasing sea surface temperatures, it often weakens and/or undergoes ET. Distinguishing between a TC that is falling apart (e.g., rapidly weakening) and one that is transitioning to an extratropical cyclone while over open ocean can be challenging as both processes can result in increasingly asymmetric convection and decreasing maximum wind speeds. In addition, storms that complete ET can produce extreme rainfall, such as Sandy (2012) and Floyd (1999), and increasingly extreme rainfall events are expected in a warming climate. However, findings conflict regarding whether ET frequency will increase due to climate change, thus more work is needed to evaluate the latest trends in ET.

To extend existing assessments of ET and support the differentiation between a weakening TC and a transitioning TC, this project first constructs a 45-year ET climatology in the North Atlantic during 1979-2023 using EX-labeled cases from the National Hurricane Center’s best track dataset (HURDAT2). It then evaluates the evolution of precipitation associated with transitioning cases using the CMORPH (Climate Prediction Center MORPHing technique) dataset with approximately 8-km resolution every 30 minutes from 1998 to present. Particular emphasis will be placed on extreme rainfall with respect to both rate and total accumulation. Future work will then compare rainfall patterns associated with ET against the patterns found for over-ocean weakening events that are not undergoing ET.