Intraseasonal Variation of East Asian Summer Monsoon under Multiyear La Niña Events: Contrast between 1999−2000 and 1984−1985

Based on multi-reanalysis data and observational data, this study focuses on the intraseasonal variation of the East Asian summer monsoon (EASM) during two multiyear La Niña events, occurred in 1984−1985 and 1999−2000, respectively. There is a significant discrepancy in intensity between two events, with moderate intensity for the former and strong intensity for the latter. The warm pool convection is enhanced during two events, inducing eastward retreat of the western Pacific subtropical high (WPSH) with weak intensity, but the intraseasonal variation is different between two events. During the strong event, convection begins to develop in June and reaches its maximum in July, then decays in August. This feature is consistent with the composite result in La Niña years, indicating that a La Niña signal dominates the EASM intraseasonal variation. By contrast, convection is enhanced in June and August with two peaks, but it is suppressed in July during 1984−1985 La Niña event. The result also shows that, when the sea surface temperature (SST) is higher in the preceding month, convection in the next month is enhanced along with a reduced solar radiation and local SST. Consequently, convection is suppressed due to the reduced SST in the preceding month. During the moderate La Niña event, the local air−sea interaction in the warm pool plays a key role. Therefore, the EASM exhibits a remarkable difference in the intraseasonal variation and impacting factors between two events. Due to eastward retreat of the WPSH, in general, there is less rainfall in the eastern part of China.