On the inter-model spread of tropical cyclone genesis frequency over the North Atlantic in CMIP6 models

The simulation of tropical cyclone (TC) genesis in the North Atlantic (NA) has long posed a challenge for coupled models, showing significant inter-model spread of TC numbers in the NA main development region and its ratio to the Northern Hemisphere total. This ratio is referred to as relative tropical cyclone genesis frequency (RTCGF). By explicitly detecting TCs from the historical simulations of 30 Coupled Model Intercomparison Project Phase 6 (CMIP6) models, we found that the RTCGF ranges from 0.2 % to 18.9 %, compared to 14.8 % in the observations. This diversity in RTCGF across models is mainly affected by large-scale conditions and African easterly waves, whereas the impact from model resolution is limited. The differences in large-scale conditions originate from the relative sea surface temperature (SST) in the NA compared to the tropics. A higher relative SST triggers the convergence of westerly from the Pacific Ocean and easterly from Africa toward the NA, which enhances low-troposphere vorticity, mid-troposphere ascending motion, and moist convection, creating more favorable large-scale conditions for TC genesis. However, vertical wind shear is less affected by relative SST and has a minor role in the inter-model spread of TC genesis. Further analysis demonstrates that the horizontal resolution is not the key for accurately simulating relative SST and the activity of African easterly waves in the current coupled models. Our findings suggest that model physics may be more relevant to TC activity in coupled models with a resolution of approximately 1–2°.