Extratropical Transition of Tropical Cyclones in the North Atlantic: Multi-Decadal Climatology and Phase Space Analysis using a Variable-Resolution GCM

The climatology of the extratropical transition of tropical cyclones between 1980 and 2002 is analyzed using the variable-resolution Spectral Element (SE) dynamical core of the Community Atmosphere Model (CAM). The horizontal resolution is 28 km over the North Atlantic basin and transitions to 111 km over the rest of the globe. The high-resolution region fully resolves the structure of tropical cyclones and the subsequent transition into extratropical systems. The low-resolution global domain makes this approach computationally efficient compared to traditional uniform high-resolution grids. The transition from warm-core, symmetric tropical cyclones to cold-core, asymmetric extratropical cyclones is quantified using cyclone phase space parameters that represent the symmetry and thermal structure of the cyclone. This phase space analysis is used to evaluate the extratropical transition in CAM-SE and results are compared to reanalysis data from the National Centers for Environmental Prediction Climate Forecast System Reanalysis (NCEP CFSR). Extratropical transition occurred in about 63% of tropical cyclones 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 the long-term climatology of the extratropical transition of tropical cyclones is successfully simulated in CAM-SE.