Moored observations of the deep western boundary current east of the Gagua Ridge in the Northwestern Pacific

The deep western boundary current (DWBC) of the Pacific serves as a lower limb of the meridional overturning circulation that has been identified as a key component of the global climate system. Here, we report the results of observations from a set of full-depth subsurface moorings deployed in the Northwestern Pacific from August 2017 to May 2020. The observations reveal a southward, bottom-intensified DWBC east of the Gagua Ridge, featuring a core velocity of 4.5 cm/s and a mean southward volume transport of 1.79 ± 0.75 Sv (1 Sv = 10 6 m 3 /s). Seasonally, this DWBC shows a tendency to move southward with relatively cold water in summer and northward with warm water in winter. On intraseasonal time scales, its variability is closely related to surface-detectable mesoscale eddies, suggesting that some of these eddies are deep ocean reaching and can directly modulate the bottom current as deep as ~ 5500 m east of the Gagua Ridge. These results enhance our understanding of the deep circulation and pathway of deep water in the Pacific.