Investigating the Relationship between Sea Surface Temperature and the Mechanical Efficiency of Tropical Cyclones

Previous studies have investigated how sea surface temperature (SST) affects the potential intensity of tropical cyclones (TCs). However, this is an upper bound only on the maximum near-surface azimuthal winds and does not fully account for the effects of atmospheric moisture. Potential intensity might not vary in the same way as the total kinetic energy (W_KE) of a TC would with changing SST. The term W_KE is related, via the conceptualization of the TC as a heat engine, to TC mechanical efficiency. We investigate how TC mechanical efficiency varies with SST in a series of moist, axisymmetric, radiative–convective numerical experiments with constant SSTs ranging from 295 to 307.5 K. We find a 22.1% K²¹ decrease in the mechanical efficiency with SST. While the increase in the net heat energy gained by the TC heat engine acts to increase W_KE, the mechanical efficiency still decreases with SST due to the effects of moisture on W_KE and on the total heat input to the TC. Moist convection in an unsaturated atmosphere is associated with substantial irreversible entropy production, which detracts from the energy that the TC can use to power its winds. The increasing moisture content in a warmer atmosphere predicted by Clausius–Clapeyron scaling leads this irreversibility to increase in an unsaturated atmosphere, presenting a larger penalty on W_KE and decreasing the mechanical efficiency. Our results highlight the importance of giving full consideration to the effects of moisture on the TC heat engine in studies of how climate affects TCs.