Examination of Multiple Pathways to Atlantic Tropical Cyclone Formation within the Marsupial and Rotating Convection Paradigms

A popular oversimplification of the Marsupial Paradigm (MP) for tropical cyclone (TC) formation first presented by Dunkerton, Montgomery, and Wang (2009) is that this technique is only applicable when following an individual African Easterly Wave (AEW). The marsupial framework, when combined with the rotating convection (RC) paradigm, is more general and can be applied to a variety of tropical flow configurations. Recently, Naval Postgraduate School students have examined the formation of multiple TCs during the highly active 2020 Atlantic season. Their scrutiny of satellite imagery and GFS analyses using the MP and RP frameworks has revealed that TC formation can be achieved via a multitude of pathways involving a variety of kinematic, dynamic, and thermodynamic evolutions of very different incipient circulations. Britton (2021) focused on three AEWs that became TCs in the eastern Atlantic and found a trackable 700-hPa circulation starting near the African coast in all three cases. However, the relatively straightforward pathway of gradual moistening of a single, small circulation for TS Josephine was noticeably different from the extensive moistening of a dry low-level circulation for H Isaias and the complexity of multiple circulations merging for H Laura. Huff (2023) and Corretjer (2023) each tracked the pouch structure of a single AEW across the Atlantic until eventual TC formation in the Caribbean Sea as H Delta and H Eta, respectively. The pouch structure of eventual H Delta was relatively simple for much of its long track, and a stalled track that coincided with increased vertical alignment supported subsequent TC formation. Although the track of the pouch of eventual H Eta appeared like that of H Delta, the pouch of the initial AEW underwent a complicated merger with a non-AEW pouch that spun up over the central Atlantic. McKaig (2021) focused on the formation of three late-season TCs in the western Caribbean and discovered that while H Iota formed from a tropical wave, H Gamma formed from the interaction of a tropical wave and a low-level strip of cyclonic relative vorticity that rolled up, and H Zeta formed from the interaction of a tropical wave and a Central American Gyre (CAG). Finally, two years prior to these cases, H Michael (2018) formed in association with a small cyclonic circulation that spun off the Colombian coast and experienced only limited interaction with two weak AEWs before interacting more with a CAG that tapped into moisture and cyclonic vorticity from the eastern Pacific. The application of techniques, including pouch-centric maps of multiple variables at multiple levels as well as circulation tendency calculations to determine the relative contribution of various circulations, allows examination of these various TC pathways within the context of the overarching MP and RP paradigms.