[A-CG38] Climate Variability and Predictability on Subseasonal to Centennial Timescales

[ACG38-P01] Interannual variations of large-scale circulations associated with winter precipitation in Tokyo

*Tatsuya Tanaka¹, Hiroshi G. Takahashi¹, Yoshihiro Iijima¹ (1.Tokyo Metropolitan University)

Keywords:storm track, monsoon

This study investigates the atmospheric circulations contributing to the interannual variations in winter rainfall and snowfall in Tokyo, focusing on interannual variations of the activities of the south coast cyclones of Japan and northwesterlies of the East Asian winter monsoon using JRA-55 reanalysis data and long-term observations from the Japan Meteorological Agency. Generally, the south coast cyclones are a major cause of winter precipitation in the Kanto region, which may be associated with the Northwest Pacific storm track activity. Also, as large-scale conditions, the East Asian winter monsoon and El Niño-Southern Oscillation (ENSO) can modulate the interannual variations of the rainfall and snowfall in Tokyo. However, the influences of these factors on rainfall and snowfall in Tokyo have not been fully understood. The results show that the activity of the storm track south of Japan is more active when the winter monsoon is weakened. The active storm track south of Japan can be consistent with the low-level cyclonic anomalies over and around southeastern China, which may contribute to south coast cyclone genesis. When the East Asian winter monsoon is weak on the interannual timescale, the upper tropospheric jet stream also tends to weaken, which may not prevent the coupling between upper and lower vortices, suggesting active storm tracks.
Because a previous study reported the active storm track during El Niño events, this study also confirmed the influences of the ENSO on storm track activity based on NOAA’s ONI. The results confirmed that storm tracks become more active near the south of Honshu Island during El Niño. However, the influences of El Niño may be weaker than those of interannual variations of the East Asian winter monsoon. In addition, we investigated the interannual variations in the low-tropospheric temperature over Japan. The activity of the storm track resulted in higher temperatures around Honshu during El Niño and weak monsoon conditions. This result implies that increased storm track activity south of Japan does not always lead to more snowfall. In addition, it is noteworthy that the interannual correlation between precipitation and snowfall was very weak. Thus, not only south coast cyclones but also ENSO and other large-scale circulation fields may play a complex role in influencing snowfall events in Tokyo.