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. This process can induce modifications to the TC itself, such as expanded asymmetric distributions of wind and precipitation, which allows the system to impact a wider area as it continues to transition. Additionally, TCs undergoing ET can develop into rapidly intensifying systems bringing TC-like conditions (e.g., intense rainfall, strong winds, large waves) to areas far removed from the original TC, such as Canada, Europe, the northeast US, or the Pacific Northwest, that are not accustomed to such events. 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.

In this study, we utilized 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 span a series of years representing a variety of environments (e.g., a strong El Niño, a strong La Niña, an anomalously active year in each Northern Hemispheric TC basin, an anomalously inactive year in each Northern Hemispheric TC basin, etc.). Tropical cyclones in the model output are tracked using the Geophysical Fluid Dynamics Laboratory (GFDL) TSTORMS tracking algorithm to produce a modeled current-day TC and ET climatology. These ten multi-seasonal simulations are then repeated using a pseudo-global climate change (PGCC) technique following the IPCC AR5 RCP 8.5 emissions scenario. Applying the tracking algorithm to this set of PGCC simulations produces a future-day TC and ET climatology; comparison of the current-day and future-day climatologies allows for an assessment on how climate change effects the frequency, geographical location, and intensity of ET events.