Keywords:modes of atmospheric circulation variability, heavy rain
A continuous heavy rain was observed in western Japan in early July 2018. The aim of this study is to investigate the atmospheric circulation from a view point of leading modes of atmospheric circulation variability and its relationship to external forcings. The first mode of the EOF analysis, applying to interannual variability of 300-hPa westerlies around Japan in early July, represents variability of intensity of the westerlies and precipitation associated with the Baiu front. The score of this mode in 2018 suggests that this mode contributed to the weakening of the westerlies and reduction of precipitation at the climatological position of the Baiu precipitation. The second mode represents meridional shifts of westerly jet axis and position of precipitation. The score of the second mode in 2018 suggests that this mode contributed to northward shift of westerlies and Baiu precipitation. A series of ensemble experiments of an atmospheric general circulation model (AGCM) was conducted under five boundary conditions. The emergence of the first (second) mode is often observed in an experiment in which observed sea surface temperatures are prescribed in the extratropics (tropics), while climatological-mean SST is prescribed in the tropics (extratropics). The score of the second mode is, however, not as large as observed in 2018, which may be one of reasons why the AGCM experiment can not reproduce the heavy rain event in July 2018.