[A-CG36] Extratropical oceans and atmosphere

[ACG36-14] Weakening of bomb cyclones over the North Pacific in the early 21st century

*Jiaxiang Gao¹, Rong-Hua Zhang¹, Hai Zhi¹ (1.Nanjing University of Information Science and Technology)

Keywords:bomb cyclones, maximum near-surface wind, Eady growth rate, meridional air temperature gradient, the North Pacific

Bomb cyclones are rapidly deepening extratropical cyclones predominantly found in mid-latitude regions. These extreme events are particularly frequent over the North Pacific (NP), posing significant societal and environmental risks. Currently, our understanding of the variability of bomb cyclones over the NP remains limited. This study analyzes the variations in multiple NP bomb cyclone characteristics from 1980 onward using four major reanalysis datasets. The results show an weakening trend of bomb cyclones since the beginning of the 21st century, which is characterized by significant reductions in maximum near-surface wind speeds, increases in minimum sea-level pressure, and slower deepening rates. Further analysis reveals that the observed weakening trend of bomb cyclones is closely linked to a reduction in maximum 850 hPa Eady Growth Rate, driven primarily by reduced vertical wind shear within the 30°N–45°N latitudinal band. Furthermore, our findings indicate that the Aleutian Low acts to modulate the meridional air temperature gradient over the mid-latitude NP, which is corroborated by climate model outputs. This modulation provides a pathway for the Aleutian Low to affect low-level baroclinicity and thus bomb cyclone characteristics. These results have important implications for future projections of bomb cyclone activity over the NP, aiding in risk assessment and mitigating the impacts of these extreme events.