Heatwaves as extreme events are becoming increasingly severe under global climate change, posing serious threat to socioeconomy, ecosystems, and human health. In July 2025, a deep and persistent high-pressure anomaly led to an unprecedentedly long-lasting atmospheric heatwave over East Asia and surrounding Northwest Pacific, and also caused a record-breaking marine heatwave over this region. Specifically, the previously crucial driver of mid-latitude atmospheric circulation anomalies from remote tropical oceans exhibits limited signal at this time. Hence, the maintenance mechanisms underlying these compound heatwaves remain poorly understood. Here, we show that this marine heatwave intensified the pivotal high-pressure anomaly after early July through active feedback. Specifically, direct diabatic heating mainly contributed to the upper troposphere, while indirect transient eddy modulation is important throughout the troposphere. These processes jointly served as an important booster for sustaining these compound heatwaves. Thus, from a local perspective, our findings highlight the overlooked but important role of sea surface warming over the Northwest Pacific in modulating the local atmospheric circulation and promoting the maintenance of both atmospheric and marine heatwaves, which facilitates sub-seasonal climate prediction and helps to understand the coupling between atmospheric and marine compound extremes.