Abstract:
This paper recalculates the BSISO (boreal summer intraseasonal oscillation) index by making an EOF (empirical orthogonal function) decomposition using the OLR (outgoing longwave radiation) data and 850-hPa zonal wind velocity (U850) data over (10°S–50°N, 40°E–160°E) from May to October in 1981–2010. The evolution characteristics of the BSISO and its influence on the summer precipitation in North China were analyzed. The results are as follows: (1) There are two obvious low-frequency signals in the Indian Ocean–Northwest Pacific region in summer. The first is the BSISO1, which is inclined from northwest to southeast and propagates from the tropical Indian Ocean to the northeast with an oscillation period of approximately 45 days. The second is the BSISO2, which is inclined from southwest to northeast and propagates from the Northwest Pacific to the northwest with an oscillation period of approximately 20 days. (2) The BSISO affects the summer precipitation process in North China mainly by influencing the atmospheric circulation and water vapor transport. At 500 hPa, the BSISO signal will cause the North–South movement and intensity change of the subtropical high in the east of North China to affect the summer precipitation in North China. At 850 hPa, the BSISO signal will affect the water vapor transport to North China through the associated cyclonic or anticyclonic anomalous circulation to influence the summer precipitation in North China. (3) Although the MJO signal exists throughout the year, its amplitude is largest in the winter half-year, especially in winter, and is lowest in summer. However, the amplitude of the BSISO signal is largest in the summer half-year, especially in summer. Therefore, the MJO and BSISO signals can respectively be taken into account in winter and summer when using low-frequency signals of the tropical atmosphere to predict the precipitation process in an extended range.