Role of tropical tropopause layer temperatures in tropical cyclone intensity trends and variability

Temperatures in the tropical tropopause layer (TTL) exhibit substantial variability, including large upward excursions associated with major tropical volcanic eruptions. TTL temperatures are important for controlling the flow of water vapor and tracers into the lower stratosphere, but recent studies have shown that they can also have a significant influence on the potential and actual intensities of tropical cyclones. In this study, observed trends in tropical cyclone potential intensity in each ocean basin are decomposed to investigate the relative contribution of trends TTL temperatures. Potential intensity, the theoretical maximum intensity of a tropical cyclone, depends on the thermodynamic disequilibrium between the ocean and atmosphere and a thermodynamic efficiency. Trends in thermodynamic efficiency primarily result from trends in the outflow temperature, which is approximately equivalent to the ambient tropopause temperature. It is found that across all basins, most of the trends in potential intensity are due to trends in the thermodynamic disequilibrium. The exception is the North Atlantic, where, as in previous studies, cooling near the tropical tropopause contributes a modest amount to the increase in potential intensity. The magnitude of the potential intensity trends and the relative contribution of TTL temperature to those trends are sensitive to the choice of reanalysis and radiosonde dataset, reflecting uncertainties in TTL temperature trends. In addition, significant correlations are found between detrended time series of outflow temperature and potential intensity. These results suggest that temperature in the tropical tropopause layer may play a role in controlling the interannual variability of tropical cyclone intensity despite its limited contribution to decadal trends.