Tropical Cyclones and Upper Tropospheric Temperature Uncertainty in Global Climate Models

The potential intensity (PI) of tropical cyclones (TCs) is expected to increase in a warming climate. This provides more favorable thermodynamic environments for TCs and allows for the strongest TCs to become stronger in a warmed climate. The range in PI among the HighResMIP historical simulations is larger than the shift in PI from historical to future. HighResMIP present and future simulations use common surface boundary conditions. Present simulations use observed SSTs, and future simulations use a common projection of SSTs. Given the common surface boundary conditions in the HighResMIP Present and Future simulations, we propose that upper tropospheric temperature differences are responsible for the large range in PI in the HighResMIP historical and future simulations. PI is computed using the convective available potential energy (CAPE), which depends on upper tropospheric temperature to determine the level of neutral buoyancy. The level of neutral buoyancy is also the computed TC outflow level. In the Atlantic, East Pacific, and Western North Pacific TC Basins, we find a strong relationship between the spread in PI and upper tropospheric temperatures, with higher PI corresponding to cooler upper tropospheric temperatures. The relationship between the spread in PI and upper tropospheric temperatures is also noted in the CMIP6 historical simulations which use common surface boundary conditions. The spread of PI in HighResMIP simulations is within the range of the CMIP6 spread, but less concentrated. HighResMIP simulations can directly resolve TCs. Using two tracking algorithms, the frequency, lifetime maximum intensity (LMI), and accumulated cyclone energy (ACE) are retrieved or computed for TCs within the HighResMIP present and future simulations using common surface boundary conditions. Limitations of the resolved TCs in HighResMIP prevent us from concluding how the spread in PI may impact TC activity. Future work using higher resolution simulations is required to better establish the impact, if any, on the spread of PI on TCs within global climate models.