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Tropical cyclones (TCs) over the Bay of Bengal (BOB) can interact with the South Branch Trough (SBT) as they move northward and potentially amplify Rossby waves. This study evaluates the features of Rossby waves and their downstream impact on rainfall in South China. Results indicate that TC‐SBT interactions primarily occur in May and October‐November (Oct‐Nov), with probabilities of 59% and 53% respectively. Notably, the Rossby wave train associated with BOB TCs is more pronounced during Oct‐Nov due to the stronger subtropical westerly jet, in contrast to May. The downstream atmospheric response results in positive (negative) rainfall anomalies over South China in May (Oct‐Nov), particularly on the day following the maximum interaction day. Previous researches concerning TC‐extratropical flow interaction mainly focus on other basins where TCs move to higher latitudes, this study provides fresh insights into Rossby waves related to TC‐SBT interactions over the southern Tibetan Plateau. Plain Language Summary: During bimodal periods of tropical cyclone (TC) activity over the Bay of Bengal (BOB) in May and October‐November (Oct‐Nov), the Asian subtropical westerly jet is usually located at lower latitudes over the southern Tibetan Plateau, where South Branch Trough (SBT) is active. We quantified the extent of BOB TC‐SBT interaction based on the negative potential vorticity advection by TC‐associated irrotational wind. The mean location of the TC center and maximum interaction point are around (20°N, 88°E) and (33°N, 89°E), respectively. Rossby wave trains (RWT) associated with BOB TCs may extend eastward to 150°W, dispersing at a faster zonal group speed in May compared to Oct‐Nov. In May, the downstream response in South China featured by amplified upper‐level divergent outflow, intensified mid‐level warm and cold air convergence, and enhanced low‐level southwesterly water vapor transport, which was conducive to precipitation. In Oct‐Nov, the RWT shifted northward and the anticyclone anomaly related to the subtropical high dominated South China, leading to a northward shift in water vapor transport, unfavorable for precipitation in South China. The results are significant for the short‐ to medium‐range weather forecasts in downstream regions of BOB TCs. Key Points: Tropical cyclones (TC) over the Bay of Bengal (BOB) interacting with the South Branch Trough (SBT) can amplify mid‐latitude Rossby wavesThe Rossby wave associated with interactions between BOB TCs and SBTs is stronger in October‐November than in MayThe atmospheric response results in positive (negative) precipitation anomalies over South China in May (October‐November) [ABSTRACT FROM AUTHOR]