Under the influence of Asian summer monsoon, extremely heavy summer rainfall could occur in Southeast China, causing catastrophic social-economic impacts to the region. However, due to limited historical records, it is difficult to anticipate the risks of unprecedented extreme rainfall episodes. In this study the UNSEEN method (UNprecedented Simulated Extremes using ENsembles) is applied for studying these events, by sampling large ensemble climate model simulations of Version 6 of the Met Office Global Seasonal Forecasting System (GloSea6). We first perform fidelity tests on gridboxes in Southeast China and show that the climate model can represent observations realistically. Probabilities of unprecedented Southeast China rainfall are then computed for each of the five gridboxes within Southeast China, revealing probabilities of about 6% (up to 9%) in summer. The circulation dynamics associated with such events is also examined. For gridboxes close to the western north Pacific, a strengthened western Pacific subtropical high (WPSH) leads to anomalous low-level southerlies onshore to Southeast China, thereby contributing to heavy rainfall there. For gridboxes along 35° N, unprecedented events are often associated with an upper-level stationary wave pattern along the climatological jet, inducing vertical motion conducive to convection. For gridboxes along 25° N, extreme rainfall events are often associated with a southward shifted subtropical jet. Finally, the importance of the shifted jet and the WPSH strength in generating the unprecedented rainfall in the southern gridboxes along 25° N is also investigated. A southward-shifted jet could lead to extreme rainfall in the southern gridboxes even when the WPSH displacement is not strong. Overall, anomalous upper-level flow and WPSH activity can be equally important in triggering unprecedented summer rainfall in Southeast China. In short, we demonstrate that the UNSEEN method can be applied for estimating the risk of extreme monsoon rainfall in Southeast China and understanding the associated mechanisms.

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