Dependence of Global Tropical Cyclones on the Tropical Pacific Mean State in HighResMIP

Global tropical cyclone (TC) activity is known to depend on the variation of the zonal SST gradient in the tropical Pacific at an interannual time scale, known as the El Niño Southern Oscillation (ENSO)-TC modulation. The majority of the fifth and sixth Coupled Model Intercomparison Project (CMIP5 and CMIP6) simulations suggest that the mean state of the Pacific zonal SST gradient will weaken, exhibiting what is referred to as an El Niño-like pattern, as the climate warms. However, observations from the past few decades indicate a strengthening trend, a La Niña-like pattern. While the cause of the inconsistency between observations and model simulations is still under debate, questions arise: How do global tropical cyclones respond to different trends in the tropical Pacific mean-state? To what extent is the response similar to the interannual time scale ENSO-TC modulation?

To address these questions, we compare TCs, in both global and basin-specific perspectives, and their ambient environmental conditions using two sets of historical simulations in the High Resolution Model Intercomparison Project (HighResMIP): the SST-forced and the coupled simulations. During the historical period, HighResMIP models are forced with observed SST in the SST-forced simulations, resulting in a La Niña-like trend in the tropical Pacific. Similar to other CMIP6 models, the coupled simulations from HighResMIP exhibit an El Niño-like trend in the same region. The differences in TC activity trends will be described using density maps of genesis, track, and Accumulated Cyclone Energy (ACE). The trends in environmental conditions relevant to TC activity will be compared between the two data sets, and their contributions to the differences in TC activity trends will be discussed. The results will then be compared to the ENSO-TC relationship derived from HighResMIP models as well as from observations.