The Influence of ENSO Flavors on Western North Pacific Tropical Cyclone Activity
We discovered that the location of sea-surface temperature (SST) warming during El Niño strongly controls seasonal Western North Pacific tropical cyclone (TC) activity, with central Pacific El Niño driving greater TC activity enhancements than east Pacific El Niño. This is because the central Pacific is warmer and therefore requires less warming to reach the SST threshold for deep convection, a key physical mechanism connecting El Niño and TCs.
This research shows that in order to quantify and reduce uncertainty in future TC projections, Earth System Models must be evaluated and improved for their ability to represent not only the frequency, but also the spatial characteristics of El Niño.
The El Niño-Southern Oscillation (ENSO) is a major source of seasonal western North Pacific (WNP) tropical cyclone (TC) predictability. However, the spatial characteristics of ENSO have changed in recent decades, from warming more typically in the eastern equatorial Pacific during canonical/Cold Tongue El Niño, to warming more typically in the central equatorial Pacific during non-canonical/Warm Pool El Niño. We investigated the response in basin-wide WNP TC activity and spatial clustering of TC tracks to the location and magnitude of El Niño using observations, TC-permitting tropical channel model simulations, and a TC track clustering methodology. We found that simulated Western North Pacific TC activity, including accumulated cyclone energy (ACE) and number of typhoons and intense typhoons, is more effectively enhanced by sea-surface temperature warming of the central, compared to the eastern, equatorial Pacific. El Niño also considerably influenced simulated TC tracks regionally, with a decrease in TCs that were generated near the Asian continent, and an increase in clusters that were dominated by TC genesis in the southeastern WNP. This response corresponds with the spatial pattern of reduced vertical wind shear and is most effectively driven by central Pacific SST warming. Finally, internal atmospheric variability generated a substantial range in the simulated season total ACE (+/- 25% of the median). However, extremely active WNP seasons were linked with El Niño, rather than internal atmospheric variability, in both observations and climate model simulations.