Diapycnal Upwelling Over the Kyushu‐Palau Ridge in the North Pacific Ocean

A significant portion (∼2.1 Sv, 1 Sv = 106 m3 s−1) of deep water penetrates into the Philippine Sea through the Yap‐Mariana Junction, the sole passage of the Philippine Sea below 4,000 m, and is then upwelled into shallower layers, closing regional overturning circulation. Yet, the structure and variability of this diapycnal upwelling remain poorly understood. Here, we report on a fine‐resolution hydrographic observation conducted at the most significant topographic feature in the Philippine Sea, the Kyushu‐Palau Ridge (KPR). Enhanced mixing up to O(10−2) m2 s−1 near the KPR is manifested, indicating the presence of substantial upwelling herein. Besides, the ridge‐related topography contributes more deep‐water mass transformation than abyssal basins in the Philippine Sea. This study highlights the significant role of rough bathymetry features in generating diapycnal upwelling in the North Pacific. Plain Language Summary In the North Pacific, a significant portion of deep water from the Antarctic enters the Philippine Sea below 4,000 m, where it is upwelled into shallower layers, closing regional overturning circulation. However, where the deep water gains buoyancy in the deep Philippine Sea required to upwell into shallower layers remains largely unknown, rendering our understanding of the western Pacific overturning circulation incomplete. To fill this gap, we conducted observations at two transects over the Kyushu‐Palau Ridge (KPR), the most significant topographic feature in the Philippine Sea. Based on this observational program, we reveal the mixing structure down to the bottom across the KPR and estimate the deep water transformation in the deep Philippine Sea. This study illuminates the role of ridge‐related topography in driving diapycnal upwelling in the deep ocean and improves our understanding of the deep circulation in the North Pacific Ocean. Key Points A three‐dimensional distribution of diapycnal mixing over the Kyushu‐Palau Ridge is presented The spatially variable of mixing across the Kyushu‐Palau Ridge results in varying vertical velocities The Kyushu‐Palau Ridge contributes more deep‐water mass transformation than abyssal basins in the Philippine Sea