Abyssal current and water mass in the Main Gap and an adjacent Small Gap of the Emperor Seamount Chain

Abstract

From 2016 to 2017, we conducted 1-year mooring current observation at the widest, deepest gap, widely known as the Main Gap, and a narrower, shallower gap just south of the Main Gap, called the Small Gap in this study, in the Emperor Seamount Chain, which divides the Northwest Pacific Basin (NWPB) from the Northeast Pacific Basin. We also conducted two hydrographic sections with a conductivity–temperature–depth sensor and a lowered acoustic Doppler current profiler around the two gaps in 2012 and 2016. At the Main Gap, the abyssal current flowed east–northeastward at a statistically significant mean velocity of 1.3–2.4 cm s−1, while the current was dominated by tidal and inertial variability with a magnitude of 3–4 cm s−1. At the Small Gap, the abyssal current had an eastward mean velocity of 9.4 cm s−1, accompanied by mesoscale variability with a magnitude of 7 cm s−1. These eastward currents carried abyssal water below a depth of 5000 m, which came from the northern part of the NWPB, with a volume transport of 1.6 × 106 m3 s−1 through the Main Gap and 0.5 × 106 m3 s−1 through the Small Gap. Cold, saline, oxygen-rich Lower Circumpolar Deep Water, which occupied the south of the zonal seamount range at 37°N, called the S–E Seamounts in this study, was found near the seafloor north of the S–E Seamounts to the west of the Small Gap; however, it did not extend to the Main Gap or to the east of the Small Gap.