Samoan Passage Abyssal Mixing Experiment

The Samoan Passage is the major gateway for deep water renewal in the North Pacific where virtually no bottom waters are formed locally. About 6 Sv (1 Sv $\equiv 1 \times 10^6$ m$^3$ s$^{-1}$), more than half of the abyssal northward limb of the Pacific Meridional Overturning Circulation (MOC), flows through the Samoan Passage while smaller amounts flow across Robbie Ridge to the west and around the Manihiki Plateau to the east. As the bottom current is forced through the narrow channels and across the sills of the Samoan Passage it is strongly modified by turbulent mixing.

In our recent NSF-funded Samoan Passage Abyssal Mixing Experiment we gathered moored and shipboard measurements to re-assess the long term volume transport through the Samoan Passage and to study flow pathways and turbulent mixing processes in the Samoan Passage. Moored long-term observations of the abyssal flow through the Samoan Passage, when compared to historical measurements, showed a slightly weakened volume transport by about 0.6 Sv or 10% and a significant warming of $1\times10^{-3}$ K/yr over the past two decades. Shipboard CTD/LADCP sections from a cruise in 2012 revealed the flow pathways through the Samoan Passage with the unanticipated result of about half the volume transport going through shallower channels to the west while the other, denser, half goes through the main eastern channel. Topographic sills along both pathways lead to hydraulic response and strong associated acceleration and mixing of the flow. Detailed measurements at the primary sill of the eastern channelw showed flow acceleration to more than 0.5 m/s. The first direct turbulence measurements with an autonomous Vertical Microstructure Profiler in the Samoan Passage revealed 1000 to 10,000 times stronger turbulence than oceanographic background levels. Heat-budget derived estimates of turbulent mixing levels were 2 to 6 times higher than area-averaged mixing levels in the Samoan Passage, pointing to the importance of mixing concentrated at so-called hot-spots. These mixing hot-spots are located downstream of the sills, where the flow, likely hydraulically controlled, accelerates, and hydraulic jumps with the highest mixing levels in the region were observed.