The effects of imposed stratospheric cooling on the maximum intensity of tropical cyclones in axisymmetric radiative-convective equilibrium

The effects of stratospheric cooling and sea surface warming on tropical cyclone (TC) potential intensity (PI) are explored using an axisymmetric cloud-resolving model run to radiative-convective equilibrium (RCE). Almost all observationally constrained datasets show that the tropical lower stratosphere has cooled over the past few decades. Such cooling may affect PI by modifying the storm's outflow temperature, which together with the sea surface temperature (SST) determines the thermal efficiency in PI theory. Results show that cooling near and above the model tropopause (90 hPa), with fixed SST, increases the PI at a rate of 1 m s-1 per degree cooling. Most of this trend comes from a large increase in the thermal efficiency component of PI as the stratosphere cools. Sea surface warming (with fixed stratospheric temperature) increases the PI by roughly twice as much per degree, at a rate of about 2 m s-1 K-1. Most of the PI trend under increasing SST comes from large changes in the air-sea thermodynamic disequilibrium. The predicted outflow temperature shows no trend in response to surface warming, however the outflow height increases substantially. Under stratospheric cooling, the outflow temperature decreases and at the same rate as the imposed cooling. These results have considerable implications for global PI trends in response to climate change. Tropical SSTs have warmed by about 0.15 K decade-1 since the 1970s, but the stratosphere has cooled anywhere from 0.3 K decade-1 to over 1 K decade-1, depending on dataset. Therefore, global PI trends in recent decades appear to have been driven more by stratospheric cooling than by surface warming.