Seed Theory and ENSO: Evaluating the Distribution of Tropical Cyclogenesis

Abstract

Tropical cyclones (TCs) are defined as organized, rotating storms that spiral about low- pressure systems. TCs are among the most damaging known weather events. Understanding climatic factors that affect the behavior of TCs is integral to preserving human livelihood. It is well established that different phases of the El Nino Southern Oscillation (ENSO) shift the spatial distribution and global frequency of TCs. The general theory accounting for this shift cites changes in vertical wind shear (VWS): during different phases of ENSO, VWS distribution changes. Regions of increased VWS have a decreased probability of TC formation. Recent work by Tsung-Lin Hsieh and the Vecchi Group suggests a novel mechanistic un- derstanding of TC formation, referred to as "Seed Theory". This framework posits that pre-TC "seeds" – weakly rotating storm clusters – precede TCs. The probability of transition from "seed" to TC is shown to be inversely proportional to Tang and Emanuel’s ventilation index, dependent (in part) on VWS. The spatial distribution of "seeds" is shown to be proportional to vertical ascent and background vorticity. Our project seeks to employ this framework to re-examine causes for TC change during different phases of ENSO. We use HiRAM-HadISST coupled climate model outputs to argue that changes in seed distribution, in addition to changes in VWS, are responsible for changes in TC activity during different ENSO phases. We further argue that changes in seed distribution can be primarily explained by vertical ascent velocity, an environmental condition directly affected by ENSO. Using gross moist stability analysis, this can be further decomposed into such fundamental first-principles quantities as radiative flux and column humidity.