Influence of boreal summer intraseasonal oscillation on summer precipitation in North China

To understand the impact of the boreal summer intraseasonal oscillation (BSISO) propagation on summer precipitation in North China, this study analysed the quantitative relationship between the BSISO and summer precipitation in North China and its influence mechanism using summer precipitation data from 1981 to 2020, the NCEP/NCAR reanalysis data and the NOAA outgoing longwave radiation (OLR). The results show that there is a strong correlation between summer precipitation in North China and the BSISO. Over the past 40 years, the variance in summer precipitation anomalies in North China caused by BSISO1 accounted for 16.8%, whereas the impact of BSISO2 accounted for 13.1%. Their combined impact accounted for 30% of the variance in summer precipitation in North China. The BSISO influences summer precipitation process in North China primarily through upper tropospheric circulation and lower-level water vapour transport. At 500 and 200 hPa, the BSISO excited some disturbance centres of a deep barotropic structure propagating from west to east along the upper-tropospheric westerly jet. North China is controlled by the blocking circulation pattern of "high in the east and low in the west" while there is a negative (positive) anomaly near Lake Baikal and a positive (negative) anomaly near the Korean Peninsula. At 850 hPa, the BSISO stimulates an anticyclonic circulation anomaly on the north side of its convective area, and the southerly winds on the west side of the anticyclone enhance water vapour transport to North China. This high- and low-level circulation configuration is favourable for precipitation processes in North China. The results suggest that the influence of both low-frequency signals in the middle and high latitudes and the BSISO signal in the tropics should be considered to make extended-range forecasts of precipitation processes in North China.