Climatology of the Extratropical Transition of Tropical Cyclones in the North Atlantic Using a Variable-Resolution GCM with Regional Refinement

The climatology of the extratropical transition of tropical cyclones between 1980 and 2002 is analyzed using the Community Atmosphere Model (CAM) version 5 with the variable-resolution Spectral Element (SE) dynamical core. The horizontal resolution is 28 km over the North Atlantic basin and transitions to 111 km over the global domain. The high-resolution region sufficiently resolves the structure of tropical cyclones and their transition into extratropical systems. The low-resolution global domain makes this approach computationally efficient compared to traditional uniform high-resolution grids. The use of a single global model with a variable-resolution grid allows for long-term simulations without introducing the errors associated with boundary conditions in regional models. The 23-year simulation follows protocols from the Atmospheric Model Intercomparison Project, with prescribed sea surface temperatures and sea ice.

Cyclone phase space parameters that represent the cyclones thermal structure and symmetric distribution are used to objectively determine the beginning and end of the extratropical transition. This cyclone phase space analysis shows the evolution of storms as they transition from symmetric, warm-core tropical cyclones to asymmetric, cold-core extratropical storms. The climatology of extratropical transitions is evaluated and compared to reanalysis data from the National Centers for Environmental Prediction Climate Forecast System Reanalysis (NCEP CFSR). About 63% of all tropical cyclones underwent extratropical transition in CAM-SE and 54% in NCEP CFSR, compared 46% in observations using subjective analysis. The seasonal cycle, phase space distribution, and duration of extratropical transition in CAM-SE are comparable to reanalysis data. The results show that high-resolution CAM-SE adequately simulates the long-term climatology of the extratropical transition of tropical cyclones.