What Controls the Annual Cycle of the North Atlantic Tropical Cyclone Frequency?

Abstract:

The North Atlantic (NA) tropical cyclone (TC) frequency usually peaks in August and September and is much less active in other seasons, which is often understood as being controlled by TC genesis indices. However, these indices generally predict a more smooth annual cycle than observations over the NA basin. In particular, the relative frequency of NA TC in July is over estimated. Here, we demonstrate that the NA TC frequency annual cycle can be better modeled by combing TC genesis probability and vortex seeds that have the potential to develop into TCs. The sharpness of the observed annual cycle is better captured and the relative frequency in July is more consistent to observations. Our results highlight the power and usefulness of the probability and seeds framework that has been used to successfully explain the diverging TC responses from a range of climate models under the same radiative forcing.