Insights from Ex-Typhoon Halong (2025) -- An Arctic Cyclone of Tropical Origin

An Arctic cyclone, Ex-Typhoon Halong, produced strong winds and devastating flooding in southwestern Alaska during 11-12 October 2025. This study examines the evolution of Halong after its transition into an extratropical cyclone through the analysis of ERA5 reanalysis and WRF model simulations. It is found that warm sea surface temperature (SST) anomalies over the western North Pacific preconditioned ex-Halong for intensification by increasing water-vapor content and reducing static stability. Quasi-geostrophic lifting associated with a subsequent interaction with another extratropical cyclone led to the rapid deepening of ex-Halong. This case demonstrates that tropical cyclones can transition into extratropical systems that are intensified by anomalously warm ocean waters, exacerbating impacts in high latitudes. Further analyses indicate that an increasing fraction of Alaskan cyclones has originated in tropical latitudes (south of 30°N) in recent decades. In particular, the frequency of Arctic cyclones of tropical origin increased by a factor of four in August and by a factor of three in September during 1980-2025 compared with 1940-1979.

💡 Research Summary

This paper investigates the evolution and rapid re‑intensification of Ex‑Typhoon Halong, which transformed from a tropical cyclone into an Arctic extratropical cyclone and caused severe wind, storm‑surge, and flooding impacts along southwestern Alaska on 11–12 October 2025. Using ERA5 reanalysis, a cyclone‑tracking algorithm, quasi‑geostrophic (QG) diagnostics, and a suite of Weather Research and Forecasting (WRF) model experiments, the authors identify the key physical mechanisms that enabled Halong’s unusual northward migration and subsequent intensification.

The study begins by placing the event in the context of a rapidly changing Arctic environment: declining sea‑ice extent, lengthening open‑water seasons, and increasing coastal erosion rates have amplified the vulnerability of high‑latitude communities to storm impacts. While most North Pacific storms follow a west‑to‑east track and rarely penetrate the Arctic, Halong originated in the western North Pacific where sea‑surface temperatures (SST) were >1 K above the climatological mean. This anomalously warm ocean supplied abundant water vapor, reduced static stability, and helped preserve a deep warm core as the storm moved poleward.

Halong’s extratropical transition (ET) occurred on 10 October, as shown by cyclone phase‑space diagnostics: the system shifted from a symmetric warm‑core structure to a highly asymmetric cold‑core configuration. Despite the loss of tropical characteristics, the residual moisture and instability allowed the storm to maintain a strong low‑level jet. The authors then demonstrate that a subsequent interaction with an existing extratropical cyclone triggered strong QG lifting. By solving the Q‑vector form of the omega equation on a storm‑centric 5 000 × 5 000 km grid, they isolate the adiabatic QG forcing term as the dominant contributor to vertical motion, with diabatic heating playing a secondary role. This forced ascent deepened the surface low by more than 10 hPa, producing the observed rapid re‑intensification.

To quantify the role of SST, three WRF sensitivity experiments were conducted over 10–13 October 2025: (1) CTRL using the actual ERA5 SST field, (2) LTM employing the long‑term mean SST for the same date, and (3) DSST in which the linear warming trend was removed, leaving only interannual variability. Each experiment included 18 ensemble members with varied physical parameterizations (convection‑permitting, tropical, and Arctic‑specific schemes). The CTRL runs reproduced the observed pressure drop and wind maxima, whereas both LTM and DSST experiments showed markedly weaker re‑intensification, confirming that the warm SST anomaly in the western Pacific was essential for supplying latent heat and maintaining instability.

Beyond the case study, the authors perform a climatological analysis of North Pacific cyclones from 1940 to 2025. By classifying cyclones that originated south of 30° N and later reached ≥65° N as Arctic Cyclones of Tropical origin (ACTs), they find that the frequency of ACTs has increased dramatically in recent decades. Specifically, the number of ACTs in August rose by a factor of four and in September by a factor of three during 1980–2025 compared with 1940–1979. This trend aligns with the observed warming of the western North Pacific and suggests that tropical‑origin storms are becoming a more common source of high‑latitude extreme weather.

The paper also notes systematic biases in operational forecasts of Halong’s track and intensity, highlighting the difficulty of predicting storms that undergo complex tropical‑to‑extratropical transitions and interact with other mid‑latitude systems. The authors argue that improved representation of SST anomalies, QG forcing, and multi‑storm interactions in numerical models, together with higher‑resolution observational networks, are critical for enhancing forecast skill.

In summary, the study provides a comprehensive mechanistic explanation for the re‑intensification of Ex‑Typhoon Halong, demonstrates the pivotal role of anomalously warm western Pacific SSTs, and documents a clear upward trend in Arctic cyclones of tropical origin over the past eight decades. These findings have important implications for Arctic risk assessments, coastal infrastructure planning, and climate‑change adaptation strategies in high‑latitude regions.