Impacts of atmospheric temperature trends on tropical cyclone activity

Impacts of tropical temperature changes in the upper troposphere (UT) and the tropical tropopause layer (TTL) on tropical cyclone (TC) activity are explored. UT and lower TTL cooling both lead to an overall increase in potential intensity (PI), while temperature changes at 70 hPa and higher have negligible effect. Idealized experiments with a high-resolution global model show that lower temperatures in the UT are associated with increases in global and North Atlantic TC frequency, but modeled TC frequency changes are not significantly affected by TTL temperature changes nor do they scale directly with PI. Future projections of hurricane activity have been made with models that simulate the recent upward Atlantic TC trends while assuming or simulating very different tropical temperature trends. Recent Atlantic TC trends have been simulated by (i) high-resolution global models with nearly moist-adiabatic warming profiles and (ii) regional TC downscaling systems that impose the very strong UT and TTL trends of the NCEP-NCAR reanalysis, an outlier among observational estimates. The impact of these differences in temperature trends on TC activity is comparable to observed TC changes, affecting assessments of the connection between hurricanes and climate. Therefore, understanding the character of and mechanisms behind changes in UT and TTL temperature is important to understanding past and projecting future TC activity changes. The UT and TTL temperature trends in the NCEP-NCAR reanalysis are unlikely to be accurate and likely drive spuriously positive TC and PI trends and an inflated connection between absolute surface temperature warming and TC activity increases.