Intra-seasonal variability of autumn rainfall in the Yangtze River Delta and its related atmospheric circulations

In the densely populated and economically developed Yangtze River Delta (YRD) region, abnormal rainfall in autumn (September-November) has important implications for economic development, food security, and livelihoods. Here we explore the intra-seasonal variability of autumn rainfall in the YRD and pre-evolution of related atmospheric circulations, which can help to improve intra-seasonal rainfall prediction skills in the autumn season. Intra-seasonal variability of autumn rainfall over the YRD exhibits two dominant timescales, a quasi-biweekly oscillation (QBWO) cycle associated with tropical cyclones, and an intra-seasonal oscillation (ISO) cycle controlled by large-scale atmospheric circulation. Considering the more stable and robust ISO than QWBO, the ISO is primarily addressed here. Two dominant ISO modes in autumn rainfall are identified by using an empirical orthogonal function analysis. The first ISO mode is characterized by a monopole with regional consistent rainfall anomalies (MP mode), whereas the second ISO mode has a meridional dipole structure with opposite rainfall anomalies over the south and north part of YRD (DP mode). These two ISO modes are modu-lated by the ISO propagation of atmospheric circulation in key areas, including the Bay of Bengal, the western North Pacific, and the Okhotsk Sea, from 25-day to 10-day lead when the rainfall mode is enhanced. The MP mode (regional enhanced rainfall) is related to regional active moist convection, strong western Pacific sub-tropical high (WPSH) and active cold air invasion via Lake Baikal. The DP mode (moist in the south and dry in the north) is accompanied by north-south convective dipoles, weak WPSH and inactive cold air outbreaks.