Large-Scale Controls on Tropical Cyclogenesis: Statistical and Idealized Modeling

Abstract

This thesis examines the complex dynamics between environmental conditions, seed genesis, and tropical cyclone (TC) genesis in the Eastern North Pacific (ENP) and North Atlantic (NA). Basin- and timescale-specific Genesis Potential Indices (GPIs) are constructed using Poisson regression, highlighting the importance of local upwelling and wind shear for seed genesis and TC development, respectively. To capture the influence of non-local factors, a novel trajectory-based GPI (traj-GPI) is introduced, merging adjacent non-local environments based on observed seed trajectory densities. The traj-GPI outperforms the original GPIs by simultaneously capturing seed activity, driven mainly by upward motion, and the transition to TCs, controlled primarily by vertical wind shear. The thesis further explores the differential responses of TC genesis to seed activity through a Hadley Circulation (HC) perspective. Reanalysis data reveals that the interannual variation in the Northern and Southern cells of the HC is more pronounced in the ENP and NA, respectively. These variations amplify seed frequency via enhanced upward motion within the ITCZ. In the ENP, however, an intensified Northern cell leads to increased shear over the TC genesis areas, hindering seed maturation and resulting in a trade-off balance. Finally, idealized aquaplanet simulations are conducted to investigate the impact of tropical SST warming patterns on the seed-TC relationship. We perform two sets of experiments with prescribed SST profiles replicating the equatorial and off-equatorial tropical warming associated with enhanced seed activity in the ENP and NA, respectively. The results not only show consistency with observed seed and TC activity but also align with HC-related upwelling and wind shear as evidenced by reanalysis data. This thesis thus offers an integrated framework that combines statistical modeling and dynamics, providing a explanation of how large-scale environmental conditions influence seed and TC genesis across different basins.