Impact of the Madden－Julian Oscillation on Pre-flood Season Precipitation in South China

Impact of the Madden-Julian Oscillation (MJO) on pre-flood season precipitation in South China is investigated based on a real-time MJO index and observed daily rainfall from gauge stations. It is shown that the rainfall patterns change from enhanced to suppressed in South China at intraseasonal timescales as the MJO moves from the Indian Ocean to the western Pacific. The most significant positive and negative anomalies are found in phase 4 and phase 7, respectively. The maximum positive and negative anomalies of regional mean rainfall are 17% and 11% relative to the climatological regional mean, respectively. Such different rainfall regimes are associated with the corresponding changes in large-scale background fields such as the western Pacific subtropical high (WPSH), moisture, and vertical motion. In phase 4, when the MJO is mainly over the Indian Ocean, the WPSH shifts farther westward, the moisture and upward motion in South China are increased, such environment is in favor of rainfall enhancement. In contrast, in phase 7, when the MJO enters the western Pacific, the WPSH retreats eastward, the moisture and upward motion in South China are decreased, such environment is in favor of rainfall suppression.