The role of North Pacific bomb cyclones in the onset of January 2021 sudden stratospheric warming (SSW) is examined by conducting a set of numerical model experiments. The control simulation, initialized 10 days before the SSW onset, successfully reproduces the SSW. As this event is preceded by the bomb cyclones in the North Pacific, their impact is tested by initializing the model without them. This sensitivity experiment shows much weaker polar‐vortex deceleration than the control simulation, resulting in no distinct SSW onset. This difference is attributable to the dampened constructive linear interference between the climatological wave and the cyclone‐related wavenumber‐one anomaly in the sensitivity experiment. It weakens the vertical propagation of wavenumber‐one wave into the stratosphere, thereby reducing wave breaking in the polar stratosphere. This result suggests that bomb cyclones should be considered for better understanding SSW and improving its predictability.

The rapid intensification of extratropical cyclones (ETCs) around the Korean peninsula and their surface impacts are investigated by detecting and tracking ETCs in East Asia using ERA‐Interim data over the period of 1979–2017. The ETCs approaching the Korean peninsula, identified on the 850‐hPa relative vorticity field, are mainly initiated in the leeward side of the Altai‐Sayan Mountains and southern Tibetan Plateau, featuring northern and southern cyclone tracks, respectively. The northern‐track ETCs are typically accompanied by a developing upper‐level trough. In particular, those that intensify rapidly are characterized by a strong upper‐level potential vorticity (PV) anomaly related to the enhanced trough, with a rather weak influence of diabatic heating. This is in contrast with the southern‐track ETCs, which are primarily organized by a lower‐level PV anomaly produced by diabatic heating. The southern‐track ETCs rapidly intensify in the presence of a migrating upper‐level trough through the interaction of upper‐ and lower‐level PV anomalies. Although their intensification processes differ, surface winds reaching 10 m s−1 are observed with the rapid intensification of both northern‐ and southern‐track ETCs. The southern‐track ETCs, traveling across the East China Sea and the Yellow Sea, also result in substantial precipitation in Korea and Japan.

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Observations show Arctic sea ice area in late summer and early autumn is rapidly declining (Serreze & Stroeve, 2015), leading to a reduction of reflected shortwave at top of the atmosphere (Hartmann & Ceppi, 2014). Climate models project an ice-free summertime Arctic ocean within this century under a high-emission scenario (Notz & Community, 2020;Overland & Wang, 2013). Reanalysis data also show that storminess, as measured by eddy kinetic energy (EKE), has weakened significantly during the Northern Hemisphere (NH) summertime (Coumou et al., 2015;Gertler & O'Gorman, 2019). Consistently, the number of intense midlatitude cyclones has decreased significantly since 1979 (Chang et al., 2016). Summertime storminess in the NH is projected to further weaken throughout the 21st century (

Abstract. The development processes of the extratropical cyclones passing the Korean Peninsula during the period of 1979–2017 are quantitatively evaluated in the potential vorticity (PV) perspective. A feature tracking algorithm is applied to the ERA-Interim reanalysis data to objectively identify the distinct northern- and southern-track (NT and ST) cyclones affecting the region in the cold season. The dynamic and thermodynamic contributions to the development of these two categories of cyclones are then comparatively assessed in terms of the relative vorticity tendency resulting from the PV tendency inversion. It is quantified through inversion that the NT cyclones develop 87.9 % dynamically and 6.2 % thermodynamically. In contrast, the respective contributions to the ST cyclones are 71.8 % and 43.5 % for the ST cyclones, with negative effects from non-explicit processes. In both NT and ST cyclones, the zonal PV advection in the upper troposphere is the most influential for the dynamic development, while nonlinear advection being more important in the former. The larger thermodynamic contribution of the latter is attributed to more latent heating being involved in the development, which produces more lower-level PV and reduces damping from vertical PV advection. These results indicate that East Asian cyclones passing the Korean Peninsula have different development processes depending on their tracks.