Transition Path Theory Insights into Hurricane Rapid Intensification

We examine hurricane and ocean reanalysis data to understand how rapid intensification (RI) of tropical cyclones is influenced by the upper ocean density structure, focusing on barrier layer (BL) thickness and thermocline depth in the eastern Caribbean Sea and the nearby western tropical North Atlantic. This was done using transition path theory (TPT), complemented with basic statistical analysis. The framework for TPT is established using Markov chains, which are constructed by discretizing pertinent data time series, namely, intensity of weather systems, changes in intensity, translational speed, and BL thickness and thermocline depth along system paths. These elements are considered trajectories within abstract state spaces, in which evolution is portrayed as a memoryless stochastic process. RI imminence is rigorously framed using a newly derived TPT statistic, which gives the time distribution to first reach a target---the RI state---from a source---for instance, the state determined by a certain BL range and system intensity---conditional on connecting paths exhibiting minimal detours. We find a higher frequency of RI in the eastern Caribbean and adjacent Atlantic influenced by river runoff, predominantly in tropical storms and category 2 hurricanes. RI is often associated with a well-developed BL; however, intensifying systems require an increased translational speed. The TPT analysis reveals a stronger link between RI and thermocline depth than with BL presence, with the likelihood of RI increasing for hurricanes with a thin BL, particularly category 1. Across strength categories, a deep thermocline consistently increases the probability of RI, a factor overlooked by basic statistical methods. Translational speed emerges as an important factor, with faster and stronger hurricanes being more prone to RI, while slower systems are less likely.