15 Jul (CLIM) Reed, PhD Defense – Changes in Extratropical Cyclones

Austin Reed, GMU Climate Dynamics PhD Program 

Discerning the Role of Latent Heat Release in Changes of Observed Extratropical Cyclones Over the Atlantic and Pacific Storm Tracks: Eulerian and Lagrangian Approaches 

Wed, 15 Jul, 2:00pm,  

Exploratory Hall 3301 and via zoom (contact ikinter@gmu.edu for link) 

Advisor: Jim Kinter 

Extratropical cyclones (ETCs) are a primary driver of cool-season extreme precipitation across the Northern Hemisphere midlatitudes. They dominate the midlatitude atmospheric energy transport within the Atlantic and Pacific storm tracks, which reduces the equator-to-pole temperature gradient. How global warming will affect the frequency and intensity of ETCs is of great interest but hard to predict because of competing influences from changes to baroclinicity and atmospheric moisture.  

This dissertation extends the Multiple Object Tracking (Lagrangian) framework to ETCs for the first time, applying it to analyze changes in cool-season ETC intensification over 1965–2023 using ERA5 reanalysis, comparing the early (1965–1993) to the late (1993–2023) segments of that period. Identifying individual ETCs clarifies the role of processes such as Latent Heat Release (LHR) in driving trends, enabling a direct comparison of changes between the Atlantic and Pacific storm tracks. Over the Pacific storm track, ETCs intensified and have shifted poleward and eastward, in conjunction with a broad 7% increase in low-level moisture availability over the western Pacific. The broad moistening appears to precondition the environment for stronger ETC development by reducing moist static stability and amplifying LHR for a given amount of ascent. During the intensification stage, Pacific ETCs exhibit an 11% increase in ascent and a 15% increase in LHR near the Warm Conveyor Belt. This enhanced diabatic forcing promotes lower-tropospheric potential vorticity generation and storm self-amplification. Pacific ETC intensification is further accompanied by increases in surface latent and sensible heat fluxes driven by enhanced warm-sector inflow, suggesting a positive feedback between ETC development and moisture availability. Atlantic ETCs, despite experiencing broad low-level moistening, show no trend in intensification, largely attributed to an 11% decline in baroclinicity throughout the atmospheric column and a decline in upward motion. 

Basin-scale changes in the transport of Dry Static Energy (DSE) and Latent Heat (LH) were also analyzed with an Eulerian (time-filtering) approach. In the Atlantic, DSE and LH transports have significantly shifted equatorward (18%) and westward (14%), whereas in the Pacific, both transports have shifted poleward (11% for DSE and 23% for LH), further corroborating the ETC track shifts in the Lagrangian framework. Together, these findings suggest fundamental differences in how Atlantic and Pacific ETCs respond to global warming, with implications for extreme precipitation and coastal flooding projections across the U.S. coastlines, Europe, and East Asia.