Although global mean temperature has been rising since the mid-twentieth century and can be attributed to increases in emissions of greenhouse gasses, the annual number of global tropical cyclones (TCs) has remained steady at around 86 since 1980. The limited length of observed TC data and the effect of multi-decadal variability have made it challenging to detect trends in TC activity on a global scale.
There is a distinct spatial pattern to the trends in TC frequency of occurrence at a global scale since 1980. Significant decreases in the frequency of occurrence in the South Indian Ocean, as well as the western North Pacific, can be seen. Conversely, increases in the North Atlantic and Central Pacific are evident. Using a suite of high-resolution dynamical model experiments, the authors of a new study, led by former AOS Associate Research Scholar Hiroyuki Murakami (UCAR/GFDL) and published in the Proceedings of the National Academy of Sciences, demonstrate for the first time that the observed spatial pattern in trends cannot be explained only by underlying multi-decadal natural variability, but external forcing such as greenhouse gasses, aerosols, and volcanic eruptions likely played an important role. AOS Faculty Member Tom Delworth is among the paper's co-authors.