Causes of the Delayed Seasonality of Northern Hemisphere Tropical Cyclones in CMIP6 Models

Observed tropical cyclone genesis frequency (TCGF) over the Northern Hemisphere (NH) exhibits a clear seasonal transition from the peak season (July‐September) to the late season (October‐December). Correct simulation of TCGF seasonality has profound impacts on TC activity and disaster predictions. This study finds slower seasonal transitions of NH TCGF in both uncoupled and coupled CMIP6 models, revealing that the atmospheric model is the source of this bias. Atmospheric models are biased in surface wind speed over the tropical oceans, which decreases (increases) the latent heat flux in July‐September (October‐December). The latent heat flux bias dominates the slower seasonal transition of the net atmospheric energy from July‐September to October‐December, resulting in a delayed southward movement of the Hadley circulation and TC‐related environmental fields in both coupled and uncoupled models. Thus, models simulate a slower seasonal transition of TCGF over the NH oceans. Our findings provide an energetic perspective on TCGF seasonality and aid in improving models' lower‐boundary physics.