May 14th, 2014
Tropical cyclones (TCs), including hurricanes and typhoons, are a major hazard around the globe, including North America. TCs have exhibited variability and change on a variety of timescales, including multi-decadal changes.
Scientists used a novel method to measure changes in TC activity over the past 30 years: the location at which a tropical cyclone achieves its lifetime-maximum intensity (or LMI). Over the past 30 years the typical location of LMI has moved poleward in both hemispheres – at a rate of over 30 miles per decade. According to this study, the amount of poleward migration varies by region. The greatest migration is found in the northern and southern Pacific and South Indian Oceans, but over the past 30 years the poleward migration of peak intensity is not seen in Atlantic hurricanes.
The position in peak cyclone intensity has moved poleward as the environment has become more favorable for tropical cyclones in the poleward regions of the tropics (and less favorable closer to the equator), through decreases in wind shear (difference in wind with height that hinder tropical storms) and increases in potential intensity (the largest cyclone intensity that the local temperatures can support). However, the underlying causes for these multi-decadal changes are still unknown, and these results highlight the need to understand the mechanisms controlling multi-decadal changes in TC location. A poleward expansion of the tropics over this same period, that has been connected, in part, to anthropogenic radiative forcing changes such as stratospheric ozone and greenhouse gas forcing, suggests an influence from anthropogenic forcing in the observed TC expansion.
This study focused on the location of LMI, as this quantity is less influenced by changes in observing practices over time. These changes in observing practices, and the resulting potential for discrepancies in TC records, have been a key limitation in understanding the character of past multi-decadal changes of TC activity. Overcoming these limitations may provide critical clues to understanding the likely future course of TC activity over the decades to come.