Modulation of the impacts of Madden-Julian Oscillation on winter rainfall in China by El Nino-Southern Oscillation

This study revealed the modulation of the impacts of Madden-Julian Oscillation (MJO) on winter rainfall in China by El Nino-Southern Oscillation (ENSO) using reanalysis data and statistical analysis. When MJO is active over the Indian Ocean, its impacts are strengthened by El Nino, leading to enhanced positive rainfall anomalies in eastern China. However, the effects of MJO are counteracted by La Nina; thus, significant positive rainfall anomalies exist only in Zhejiang Province and adjacent areas. MJO activity over the western Pacific induces opposite rainfall anomalies in South China between El Nino and La Nina winters; rainfall in South China increases (decreases) in El Nino (La Nina) winters. Such differences in rainfall anomalies are closely related to anomalous circulation and moisture transport. Anomalous southwesterlies in eastern China are enhanced in El Nino winters when MJO is active over the Indian Ocean, which leads to increased moisture transport towards China and stronger ascending motion in eastern China. But the anomalous southwesterlies are weakened in La Nina winters, thereby resulting in less moisture transport and convergence. When MJO is active over the western Pacific, anomalous northeasterlies in mainland China are enhanced (weakened) in La Nina (El Nino) winters, which strengthens (weakens) the moisture divergence and descending motion in China. Moisture convergence anomalies that explain the rainfall anomalies in ENSO winters are mainly attributed to the effects of low-frequency background state (LFBS) moisture and meridional winds. It was found that ENSO modulates the impacts of MJO by two main ways. First, ENSO can regulate the intensity and distribution of MJO activity. Therefore, MJO in the same phase will lead to different circulation and moisture transport anomalies in China. Second, ENSO-induced LFBS circulation and moisture anomalies can also influence the moisture convergence in China, which will strengthen or weaken the effects of MJO.