Climate Change Effects on the Extratropical Transition of Tropical Cyclones in High-Resolution Global Simulations Using the Model for Prediction Across Scales (MPAS)

The extratropical transition (ET) of tropical cyclones (TCs) occurs when a tropical cyclone translates into the midlatitudes, interacts with midlatitude features, such as an upper-level trough or extratropical cyclone (ETC), and undergoes a transformation from a symmetric, warm-core tropical system to an asymmetric, cold-core extratropical system. TCs undergoing ET can develop into rapidly intensifying systems bringing high-impact weather to areas far removed from the original TC. While previous research has looked extensively at the ET process and climate change effects on tropical cyclones, relatively few studies have looked into how ET events may be affected by climate change.

This study utilizes the Model for Prediction Across Scales (MPAS) to conduct ten multi-seasonal simulations using a variable resolution mesh with 15-km grid spacing throughout the Northern Hemisphere expanding out to 60-km across the Southern Hemisphere. These simulations years include varying strengths of El Niño-Southern Oscillation (ENSO) as well as levels of activity in the North Atlantic, Eastern Pacific, and Western Pacific basins. These ten multi-seasonal simulations are then repeated with altered initial conditions and sea surface temperature (SST) fields as well as elevated carbon dioxide concentrations following the IPCC AR5 RCP 8.5 emissions scenario to represent a future thermodynamic environment.

Simulated tropical cyclones are tracked using the TempestExtremes objective tracking algorithm (Ullrich and Zarzycki 2016). TCs are initially detected as minima in SLP, and then retained as candidate cyclone centers if certain criteria are met (e.g., closed SLP contour and presence of a warm core). TC tracks are then extended to identify and track ET events using the ExTraTrack ET tracker (Zarzycki et al. 2017), together producing modeled current-day TC and ET climatologies. Results show the generation of TC activity in all Northern Hemispheric basins, as well as simulated full-strength TCs, which exhibit realistic characteristics such as rainbands and an eye. Applying the tracking algorithm to the set of future simulations produces a future-day TC and ET climatology. Preliminary results show an increase in the North Atlantic basin primarily north of 30ºN, indicating a potential increase in TCs likely to undergo ET. Ongoing research will test this hypothesis, and include examination into changes in the percentage of ET events, timing of ET completion, latitude of transition, and post-ET intensity and impacts (e.g., high winds and rainfall) due to climate change, all of which will be presented here.