The Seasonality of Tropical Cyclone Maximum Intensity in the North Atlantic and Western Pacific Regions

The destructive impacts of 2017 Hurricane season highlight the importance of understanding the intensities of powerful tropical cyclones (TCs). Potential intensity (PI)—the theoretical maximum TC intensity given environmental conditions—provides a useful framework to explore the drivers and climatology of TC intensity. A recent study by the authors showed that PI seasonality averaged over the North Atlantic (NA) main development region has a large seasonal amplitude that is completely controlled by SSTs. In contrast, the Western North Pacific (WNP) seasonal cycle is relatively flat, permitting powerful TCs any month of the year, in part because of near-tropopause TC outflow temperatures that damp WNP PI seasonality by 30-40%. In this work, we investigate whether observed TC lifetime maximum intensities exhibit a seasonal cycle similar to that predicted by potential intensity theory. To directly compare observed and potential intensities, we must account for seasonal differences in TC track and frequency. We employ TC best-track data to develop an along-track record of climatological PI for each observed storm in the satellite-era (1980-2015). When observed wind speeds are normalized by PI, the resultant distributions are approximately uniform. This property is used to determine the months where PI theory is valid for each basin. We find that the seasonal cycles of along-track PI and observed lifetime maximum intensity are similar to main development region averages: WNP TCs with observed maximum intensities near PI are observed throughout much of the year, whereas intense NA TCs are confined to the hurricane season. A log-additive model shows that along-track SSTs remain dominant in seasonal variations in NA maximum intensity, whereas TC outflow temperatures are the leading contributor to WNP seasonal cycles of along-track PI.