First, we used the CSIRO Mk2 coupled ocean–atmosphere climate model, as previously documented by Gordon and O’Farrell (16). The model version and experimental design are the same as previously published by Hunt and Watterson (15). The atmospheric model has nine vertical levels and a horizontal resolution of 350 km × 625 km, rather coarse by the standards of contemporary climate models. The model was set up for a “present” climate with a fixed atmospheric CO2 concentration of 330 parts per million. This equilibrium simulation thus only reflects the internal variability generated in the coupled climate model system. The simulation was run for 10,000 years, but only the last 5000 years were analyzed here. Note that, in this project, we did not detect TCs directly as generated by the model. The model generated some low-pressure systems that have some of the structural characteristics of TCs. Nevertheless, because of the coarse resolution of the model, its simulation of TCs is unlikely to be very skillful. Instead, we derived the basin-wide climate variables simulated by the model and applied them to the GPIs defined below.

EC-Earth is developed by a consortium of European research institutions (17). The atmospheric component of EC-Earth is based on Integrated Forecasting System (IFS), which is developed at the European Centre for Medium-Range Weather Forecasts (ECMWF), coupled with the land model H-TESSEL. The IFS and H-TESSEL components were also used to produce ERA-Interim reanalysis. The ocean component is based on Nucleus for European Modelling of the Ocean (NEMO) (24), including the sea ice model LIM3 (25). EC-Earth model has relatively high resolution, a horizontal resolution of 125 km, and the atmosphere has 62 vertical levels; the ocean model NEMO has a horizontal resolution of 110 km with 46 vertical levels. Here, we used the last millennium simulation with EC-Earth v3.1. The initial condition started from an equilibrium state at 850 CE after 300 years of spin-up, and the imposed forcings were changed from 850 to 1850. This millennium simulation provides a 1000-year transient climate, contains both externally forced variability and internal variability, and is thus closer to a real climate condition than an equilibrium climate simulated by the CSIRO model.

The third model we used was the MRI Atmospheric General Circulation Model version 3.2 (MRI-AGCM3.2) (20). The model atmosphere has a high horizontal resolution of 60 km with 64 vertical levels. A randomly varying ensemble of 100 members was performed for the historical period 1951–2011, forced by the prescribed SST, sea ice concentration, global mean concentrations of greenhouse gases, and three-dimensional distributions of ozone and aerosols. The data from these simulations are freely available to download from the Database for Policy Decision Making for Future Climate Change (d4PDF) (26). We analyzed two subsets of these data here. First, we analyzed one ensemble over the full 61 years (ensemble member m050) to provide an assessment of the model’s TC climatology over the Atlantic. Second, we analyzed all 100 ensembles for the year 2005 to determine whether, given exactly the same climate forcing, random variation in formation rates could cause years of high TC formation.

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