Stratospheric Influences on the Seasonality of Tropical Cyclone Potential Intensity

Because powerful tropical cyclones (TCs) pose significant risks to vulnerable societies, it is critical to understand the factors contributing to TC intensity. The theoretical maximum TC intensity (the TC “speed limit”) that is dictated by environmental conditions is known as the Potential Intensity (PI). PI is a function of the thermodynamic atmosphere-ocean disequilibrium and the TC thermodynamic efficiency—i.e. the difference between sea surface temperatures and TC outflow temperatures. While historical and future trends of TC PI have been studied extensively, relatively little attention has been given to TC PI seasonality or its controlling factors. Using reanalysis data from 1980-2013, this work presents the first comprehensive analysis of TC PI climatological seasonal cycles and their drivers in each main TC development basin. Results show that the altitude of TC outflow throughout the seasonal cycle in the Western North Pacific (WNP) basin is found above the tropopause, in the lower stratosphere. Consequently, lower stratospheric temperatures make an important contribution to WNP TC PI, thermodynamically permitting powerful TCs in all months of the year by damping SST-related PI seasonal variability. A log-additive model of the TC PI metric shows that lower stratospheric effects damp the WNP TC PI seasonal cycle amplitude by ~30%, and shift its phase by about a month. In contrast, TC outflow in the North Atlantic (NA) is found above the tropopause only during months of the peak hurricane season. Whereas lower stratospheric temperatures do influence NA TC PI, they do so only during peak hurricane season months, and do not prominently affect NA TC PI seasonality overall. Instead SST seasonality, through both atmospheric-ocean disequilibrium and thermodynamic efficiency, controls the NA TC PI seasonal cycle.