Bottom marine heatwaves (BMHWs), i.e., anomalous ocean warming at the seafloor, can happen independently of concurrent surface marine heatwaves (SMHWs), posing a serious threat to marine ecosystems and presenting a challenge to detect and study them adequately. This study examines BMHWs on the continental shelf of the East China Sea (ECS) using the GLORYS ocean reanalysis data from 1993 to 2020. Our results show that summertime BMHWs in the ECS are generally more intense than SMHWs, with some BMHW events occurring without surface expression. Through heat budget analyses of the 2016 SMHW event and the 2019 BMHW event, we investigate the drivers of summertime BMHWs. It is indicated that the independent occurrences of bottom temperature anomalies in summer are predominantly attributed to oceanic horizonal advection. Specifically, the anomalous heat advection associated with the Taiwan Warm Current is essential for the development of summertime BMHWs, while the advection by the exchange flows between the Kuroshio and the ECS shelf currents also plays a role. Conversely, winter BMHWs are more prevalent in coastal regions under the influence of coastal currents, and typically feature consistent warming from surface to bottom. These findings provide important insights into the driving mechanisms of BMHWs in the ECS, which are essential for predicting and managing these extreme events in the future.

bottom marine heatwaves, East China Sea, oceanic currents, heat advection