Dynamical Downscaling Projections of 21 st Century Atlantic Hurricane 1 Activity : CMIP 3 and CMIP 5 Model - based Scenarios

20 Twenty-first century projections of Atlantic climate change are downscaled to explore the 21 robustness of potential changes in hurricane activity. Multi-model ensembles using the 22 CMIP3/A1B (Late 21 st century) and CMIP5/RCP4.5 (Early and Late 21 st century) scenarios are 23 examined. Ten individual CMIP3 models are downscaled to assess the spread of results among 24 the CMIP3 (but not the CMIP5) models. Downscaling simulations are compared for 18-km grid 25 regional and 50-km grid global models. Storm cases from the regional model are further 26 downscaled into the GFDL hurricane model (9 km inner-grid spacing, with ocean coupling) to 27 simulate intense hurricanes at finer resolution. 28 A significant reduction in tropical storm frequency is projected for the CMIP3 (-27%), 29 CMIP5-Early (-20%) and CMIP5-Late (-23%) ensembles, and for five of ten individual CMIP3 30 models. Lifetime-maximum hurricane intensity increases significantly in the high-resolution 31 experiments-by 4 to 6% for CMIP3 and CMIP5 ensembles. A significant increase (+87%) in 32 the frequency of very intense (Category 4-5) hurricanes (winds >= 59 m s-1) is projected using 33 CMIP3, but smaller, only marginally significant increases are projected (+45% and +39%) for 34 the CMIP5-Early and CMIP5-Late scenarios. Hurricane rainfall rates increase robustly for the 35 CMIP3 and CMIP5 scenarios. For the late 21 st century, this increase amounts to +20 to +30% in 36 the model hurricane's inner core, with a smaller increase (~10%) for averaging radii of 200 km 37 or larger. The fractional increase in precipitation at large radii (200-400 km) approximates that 38 expected from environmental water vapor content scaling, while increases for the inner core 39 exceed this level.