While eastern North Pacific (EPAC) tropical cyclones (TCs) most often move westward over open water, a subset deviates from this typical path, recurving toward higher latitudes or making landfall along the North American coastline. Because such events are relatively rare, they have received less attention, despite their significant contributions to rainfall and wildfire activity across the southwestern United States. This dissertation identifies and characterizes the spatial variability of EPAC TC tracks, analyzes the synoptic environments associated with TC recurvature near the North American coast, presents a detailed case study of two recurving EPAC TCs from late September to early October 2018, and constructs a climatology of upper-level interaction strength and its relationship to the relatively uncommon extratropical transition (ET) pathways observed in the basin. EPAC TCs from 1949–2022 were grouped into four track types using k-means clustering, enabling comparison of recurving and non-recurving cases within their respective clusters. Two of the four clusters were identified as most relevant for subsequent analyses of recurvature. Composite analyses for EPAC TCs from 1979–2022 were then used to diagnose the large-scale and local dynamics associated with recurvature, employing both storm-centered and recurvature-relative frameworks to capture the evolution of environmental fields throughout the normalized TC lifecycle. Results show that recurving EPAC TCs are typically associated with long-lived upstream Rossby wave trains and enhanced dual Rossby wave breaking anomalies, which together produce a highly amplified North Pacific trough–ridge pattern. These environments can also develop under regimes characterized by local subtropical ridge amplification and an increasingly wavy upstream flow. A detailed case study of an anomalously active sequence of recurving TCs across the western North Pacific (WPAC) and EPAC basins during late September–early October 2018 is presented. Using ERA5 reanalysis, diagnostics of layer-averaged potential vorticity advection by the irrotational wind (PVadv) and Q-vector divergence quantified TC–trough interactions, while synoptic analyses assessed Rossby wave breaking (RWB) and upper-tropospheric waveguide evolution. The ETs of WPAC TCs Trami and Kong-rey established a dynamically linked, highly amplified large-scale flow that persisted for over two weeks. Successive ETs triggered ridge amplification over the WPAC and EPAC, generated deep central Pacific troughs, and produced downstream cutoff lows that influenced the recurvature and inland decay of subsequent TCs, including Walaka, Rosa, and Sergio. These interactions indicate that coupled ET and RWB processes can sustain an anomalously amplified inter-basin flow regime, influencing precipitation and temperature extremes across North America, and may be modulated by subseasonal variability, including the Madden–Julian Oscillation (MJO). Upper-level forcing associated with EPAC TC recurvature was systematically quantified using PVadv, while ET was classified using cyclone phase space (CPS) parameters. Composite analyses revealed that strong upper-level interactions, typically occurring for higher-latitude recurving storms embedded within amplified trough–ridge patterns, promote rapid structural asymmetrization and transition to cold-core, with enhanced divergent outflow and poleward moisture transport producing extended precipitation anomalies over North America. Lower-latitude recurving storms can undergo ET when favorably phased with an upper-level trough, highlighting the critical role of trough–storm alignment and thermodynamic structure. Comparisons of variables associated with ET and precipitation anomalies further contextualize the environmental conditions conducive to transition, establishing a physically grounded framework linking upper-level dynamics, storm structure, and downstream hydrometeorological impacts.
Alexander Kyle Mitchell (Wed,) studied this question.