[Eoas-seminar] PhD Defense - Evan Jones, Monday October 27 9 AM

[eoas-seminar at lists.fsu.edu]

Dear all,

Please join us on Zoom for Evan Jones’ PhD defense next Monday October 27 at 9 AM. This will be held on Zoom *only*.  See below for more info.

Cheers,

Allison

Name: Evan Jones
Date: Monday, October 27, 2025 at 9 AM Eastern
Advisors: Allison Wing and Rhys Parfitt
Location: Zoom
https://fsu.zoom.us/j/95439837109<https://nam04.safelinks.protection.outlook.com/?url=https%3A%2F%2Ffsu.zoom.us%2Fj%2F95439837109&data=05%7C02%7Ceoas-seminar%40lists.fsu.edu%7C349706ab768643ba022208de100ab0d6%7Ca36450ebdb0642a78d1b026719f701e3%7C0%7C0%7C638965835263283893%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=XgwjlpzUZYpKj0fOLIpu1%2F1OXGV10tqEBJow%2FMQIHkM%3D&reserved=0>
Meeting ID: 954 3983 7109

Title: Influence of Western Boundary Currents and their Sea Surface Temperature Gradients on the Extratropical Transition of Tropical Cyclones

Abstract: The extratropical transition (ET) of tropical cyclones (TCs) results in extratropical cyclones that can pose significant risks to populations in the midlatitudes, yet the role of western boundary currents (WBCs) and their sea surface temperature (SST) gradients in modulating the ET process remains poorly quantified. This dissertation examines how the SST gradients of WBCs, especially the Gulf Stream (GS), influence the lower atmosphere and the structural evolution of TCs undergoing ET. By analyzing global reanalysis diagnostics, statistical composites, and highŠ\resolution atmospheric simulations, it develops and tests a mechanistic framework linking SST gradients to sensible heat flux (SHF) gradients, diabatic frontogenesis, storm frontal structure during ET, and ET characteristics.

The first section of this work develops a global climatology of frontal development during ET using the ERA5 reanalysis. Using an objective front metric implemented within the cyclone phase space (CPS) framework, it quantifies how warm and cold fronts develop and intensity as TCs transition, and how their orientation reflects the combined imprint of environmental shear, storm motion, and climatological alignment with WBCs. These results demonstrate that TCs with stronger or preexisting frontal signatures complete ET more quickly, and that WBCs such as the GS possibly imprint a systematic, basin-scale influence on storm frontal geometry.

The second component analyzes the GS role in influencing the lower atmosphere and frontal development during ET in the North Atlantic. Composite analyses within ERA5 reveal that TCs that fully complete ET are associated with warmer GS SSTs, stronger large-scale SST gradients, and enhanced grid-scale, local SST gradients. The converse is also true for TCs that do not successfully complete ET. Using an calculated index of GS strength, a high value of this index corresponds to stronger SHF gradients and enhanced diabatic frontogenesis, indicating the region could be more favorable for ET to occur. The opposite is also true for a low value of the GS index. These findings demonstrate evidence for the importance of local, fine-scale SST gradients in determining the ultimate fate of ET in the region.

The last results chapter uses the Weather Research and Forecasting (WRF) model to run high-resolution simulations of Hurricane Teddy (2020) to test the causal chain proposed by the observational analyses of the previous chapters. Through prescribed changes to GS SSTs, the experiments show that sharper local, fine-scale SST gradients yield stronger SHF gradients, more intense diabatic frontogenesis, and show a somewhat mixed signal of enhanced adiabatic frontogenesis within the transitioning storm when the SST gradients are sharpened. Even subtle changes in SST gradient sharpness at sub¨C50 km scales produce measurable differences in storm evolution, CPS trajectory, and ET duration, though the results are somewhat mixed. Nevertheless, these results confirm that fineŠ\scale oceanic features like SSTs can imprint directly onto storm structure in ways that coarser models or purely statistical analyses may miss.

Taken together, these analyses provide a multiŠ\scale, processŠ\based understanding of how WBCs influence the ET of TCs. The dissertation demonstrates that the oceanĄ¯s mechanistic impact on ET extends beyond large-scale baroclinicity or basin-wide SSTs to support the role of a third mechanism via grid-scale, local SST gradients. There is evidence provided for an effect on the structure and timing of the development of lower-atmospheric fronts within the transitioning storms and an effect on CPS trajectories and the duration of ET. The work bridges two traditionally separate research areas: dynamical lowerŠ\atmospheric meteorology and midlatitude air-sea interaction. It underscores the importance of high-resolution observations and the incorporation of the links within weather forecast models as a mechanistic frameworks for improving ET prediction and determining worsening impacts for TCs undergoing ET in mid-latitude regions. They also suggest that real-time monitoring of fine-scale SST gradient strength could provide an early indicator of which TCs are most likely to complete ET successfully.


——————————————————
Allison Wing, Ph.D.
Werner A. and Shirley B. Baum Professor
Associate Professor, Department of Earth, Ocean and Atmospheric Science
Florida State University
awing at fsu.edu





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