One third of the world’s tropical cyclones occur in the western North Pacific, so any changes in the region where they are generated could affect billions of people in Asian and Pacific Island nations. Because previous studies have suggested that global warming could shift the location of tropical cyclone genesis in the Pacific, understanding how cyclonic activity has changed, or potentially could change, in this region in response to global warming is of great societal concern.
The mean genesis longitude of tropical cyclones is roughly controlled by the east–west shift of the tropical upper tropospheric trough (TUTT), a semipermanent region of relatively low pressure over the North Pacific. Previous studies have suggested that the TUTT can both promote and inhibit the genesis of tropical cyclones along its eastern edge. The objective of a study by Wu et al. was to demonstrate that a westward shift in the mean typhoon genesis longitude over the western North Pacific is controlled by changes in the tropical upper tropospheric trough.
By analyzing data sets of tropical cyclone tracks and intensities along with multiple sets of temperature and wind field data, the researchers identified a mean westward shift in the TUTT of 12.6° longitude (0.37° longitude/year) during the period from 1979 to 2012. They also found a westward shift in the annual mean tropical cyclone genesis location by 5.8° longitude (0.17° longitude/year). The researchers argue that as the low pressure in the TUTT has shifted westward, the strong vertical wind shear—which inhibits tropical cyclone genesis—along the trough’s eastern edge has suppressed tropical cyclone genesis in the eastern portion of the area, resulting in the corresponding shift in the mean typhoon genesis location.
The team suggests that both westward shifts are associated with enhanced tropical tropospheric warming. This conclusion is consistent with their maps of wind speed and temperature, which show enhanced warming trends in the upper troposphere over the equator and cooling trends in the lower stratosphere, precisely the troposphere temperature response to global warming predicted by many climate models. (Geophysical Research Letters, doi:10.1002/2015GL063450, 2015)
—Terri Cook, Freelance Writer
Citation: Cook, T. (2015), Birthplace of Pacific tropical cyclones on the move, Eos, 96, doi:10.1029/2015EO038375. Published on 2 November 2015.