Subseasonal change in interannual variations of the Baiu precipitation

Many of previous studies have revealed that interannual variations of precipitation rate in the Baiu frontal zone (BFZ) are linked to large-scale oscillations in the world. This work examines subseasonal change of the interannual variations in the Baiu precipitation. The interannual variation of precipitation rate in the western BFZ is connected to the tropospheric biennial oscillation (TBO) of the Asian monsoon. This variation has a negative correlation between June and July. That is, the precipitation in the western BFZ shows negative anomalies in July when it has positive ones in June. On the other hand, the interannual variation of sea surface temperature (SST) in the western tropical Pacific, which is correlated to the TBO, keeps the similar anomalies from June to July. This suggests that the Baiu frontal activity is changed in seasonal progress even though the SST anomalies in the western tropical Pacific are almost same. The interannual variation of precipitation rate in the central BFZ is correlated to the El Niño/Southern Oscillation (ENSO), which is identified by approximately 4-year periodicity. This variation is prominent in June, but the anomalies become obscure in July. This is caused by weakening of anomalous high around the Philippine Sea, which controls the precipitation anomalies in the central BFZ. The interannual variation of precipitation rate in the eastern BFZ is associated with the summertime North Atlantic Oscillation (NAO) with about 6-year variability. This variation has a tendency to retain the anomalies from early June to middle July. Because strong westerlies from northern Europe to East Asia that acts as a waveguide of Rossby waves in the upper troposphere are maintained during this time period, stationary Rossby waves reach East Asia and induce anomalous precipitation in the eastern BFZ. As such, subseasonal change from June to July is differenct in interannual variations of the Baiu precipitation, i.e., in the western BFZ with 2-year periodicity, the central BFZ with 4 years, and the eastern BFZ with 6 years.