[A-CG33] Extratropical oceans and atmosphere

[ACG33-06] Rapid increase of explosive cyclone activity over the midwinter North Pacific in the late 1980s

*Akira Kuwano-Yoshida¹, Satoru Okajima², Hisashi Nakamura² (1.Disaster Prevention Research Institute, Kyoto University, 2.Research Center for Advanced Science and Technology, The University of Tokyo)

Keywords:explosive cyclone, storm track, climate variability

Long-term changes in the activity of explosively developing “Bomb” cyclones over the wintertime North Pacific are investigated by using a particular version of a global atmospheric reanalysis dataset into which only conventional observations have been assimilated, JRA-55C. Bomb cyclones in January are found to increase rapidly around 1987 in the midlatitude central North Pacific. Some of the increased “Bomb” cyclones formed over the East China Sea and then moved along the southern coast of Japan before developing explosively in the central North Pacific. The enhanced cyclone activity is found to be concomitant with rapid warming and moistening over the subtropical western Pacific, the South and East China Seas under the weakened monsoonal northerlies, leading to the enhancement of lower-tropospheric Eady growth rate and equivalent potential temperature gradient, setting a condition favorable for cyclone formation in the upstream of the North Pacific storm track. Along the storm track, poleward moisture transport in the warm sector of a cyclone and associated precipitation along the warm and cold fronts tended to increase and thereby enhance its explosive development due to latent heat release. After the transition around 1987, a Bomb cyclone has become more likely to develop without a strong upper-level cyclonic vortex propagating from Eurasia than in the earlier period. The increased Bomb cyclone activity in January is found to contribute to the diminished midwinter minimum of the North Pacific storm track activity after the mid-1980s.