Aerosol vs. Greenhouse Gas Influences on Tropical Cyclone Potential Intensity and the Hydrologic Cycle

Previous work has shown that aerosol cooling reduces tropical cyclone (TC) potential intensity (PI) more strongly than greenhouse gas warming increases it, by something like a factor of two, measured by the change in PI per degree change in sea surface temperature (SST), in model experiments where only a single forcing is applied. This has the consequence that PI shows only small increases in simulations of the historical period (and, arguably, in observations) despite considerable global warming over that period: the net historical warming results from a combination of forcings in which the greenhouse gas warming is stronger, but the aerosol cooling, due to its outsize effect on PI, is nonetheless able to keep PI increases relatively small, both in the Atlantic (where historical aerosol forcing has been particularly strong) and in the global tropical mean. A full understanding of TC intensity changes with climate requires an understanding of the physical reasons for this outsize aerosol effect on PI. We investigate this using single-forcing and historical experiments from the Fifth Coupled Model Intercomparison Project (CMIP5).

We first show that latent heat flux can be used as a proxy for PI (as theory leads us to expect if changes in outflow temperature and surface wind speed are relatively small). This allows us to interpret PI changes using the surface energy budget, considering the changes in the different budget terms per degree SST. Offline calculations with radiative kernels allow us to estimate what fraction of the surface radiative fluxes can be considered a feedback due to temperature and water vapor changes, as opposed to direct forcing by greenhouse gases, aerosols, or the combination. All calculations are carried out for the global tropical oceans, hemisphere by hemisphere (e.g., 0-30N), during the relevant TC seasons.

The results illustrate that the outsize effect of aerosol forcing is a consequence of the fact that aerosols act in the shortwave while greenhouse gases act in the longwave. As shown in previous studies of the global hydrologic cycle, shortwave forcing has a greater impact on latent heat flux – and thus also PI – than does longwave, primarily because of the differences in the response of the surface energy budget to the direct, temperature-independent component of the forcing. Shortwave forcing drives the climate system in large part at the surface, while greenhouse gases do so at the top of the atmosphere, with the surface being insulated by the greenhouse effect, so that the net surface longwave flux associated with a temperature change can be small due to cancellation between its upward and downward components, especially at high surface temperature. Our kernel results also indicate that the temperature-dependent longwave feedback component is greater by approximately a factor of two for the shortwave than the longwave forcing. Though smaller than the difference in the direct effect, this also contributes to the stronger aerosol compared to greenhouse gas response.