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

The variation of the zonal SST gradient in the tropical Pacific on an interannual time scale, known as the El Niño Southern Oscillation (ENSO), is recognized for its influence on global tropical cyclone (TC) activity. As the climate warms, the majority of the fifth and sixth Coupled Model Intercomparison Project (CMIP5 and CMIP6) simulations suggest the mean state of this zonal SST gradient to weaken, often referred to as an El Niño-like pattern. However, observations from the past few decades indicate a strengthening trend, a La Niña-like pattern. While the inconsistency between observations and model simulations is still under debate, a question arises: to what extent do global tropical cyclones respond to changes in the mean-state tropical Pacific similar to the well-known interannual time scale ENSO-TC modulation? In other words, how applicable is the El Niño-like and La Niña-like analogy when discussing the impact of the tropical Pacific on global TC activity under climate change?

To address this question, we compare TCs 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 change in the tropical Pacific. Similar to other CMIP6 models, the coupled simulations from HighResMIP exhibit an El Niño-like trend in the same region. We will use simulations with resolutions higher than 50 km, providing a better representation of the dynamic processes in TCs. 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 individual models as well as from observations. We will start from a global perspective and then zoom in on individual TC basins.