Abyssal Flow through Samoan Passage

a major choke point for the global overturning circulation

 
The Samoan Passage in the Pacific Ocean. a) Bottom temperature in the Pacific Ocean from the WOCE climatology. The Samoan Passage is located at the center. b) Bathymetry of the Samoan Passage with shallower channels to the west and a main channel to the east. c) Section of potential temperature (colors) and turbulent dissipation (black profiles) along the main channel (yellow in b). d) A towyo section across a major sill showing temperature (black contours) and turbulent dissipation estimates from density instabilities (color) at high spatial resolution.

The Samoan Passage in the Pacific Ocean. a) Bottom temperature in the Pacific Ocean from the WOCE climatology. The Samoan Passage is located at the center. b) Bathymetry of the Samoan Passage with shallower channels to the west and a main channel to the east. c) Section of potential temperature (colors) and turbulent dissipation (black profiles) along the main channel (yellow in b). d) A towyo section across a major sill showing temperature (black contours) and turbulent dissipation estimates from density instabilities (color) at high spatial resolution.

overview

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 ≡ 106 m3/s), 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.

 
Depth-integrated velocities from our long term mooring array (red arrows) deployed at the exact same location as moorings in the early 1990s (white arrows, Rudnick, 1997)

Depth-integrated velocities from our long term mooring array (red arrows) deployed at the exact same location as moorings in the early 1990s (white arrows, Rudnick, 1997)

Samoan Passage Abyssal Mixing experiment

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 10-3 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 channel 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.

A major sill in the Samoan Passage with observations from two towyo sections showing the three-dimensional structure of the flow in both the temperature field (colors) and turbulent mixing (black profiles).

A major sill in the Samoan Passage with observations from two towyo sections showing the three-dimensional structure of the flow in both the temperature field (colors) and turbulent mixing (black profiles).

hydraulic processes in the samoan passage

We got funded by the National Science Foundation to study abyssal hydraulic processes in the Samoan Passage. The project will heavily build upon data collected in the Samoan Passage Abyssal Mixing Experiment. In addition, we are planning on running a suite of numerical models spanning from idealized 2D to high-resolution 3D runs that will be tuned to observed turbulent mixing. This dataset will help us identify and study a number of processes that are loosely termed hydraulic - they arise when bottom topography restricts the free flow of the bottom current. Hydraulic processes may cause and alter turbulent mixing of the abyssal current with ambient water and impose restrictions on the maximum flow through the Samoan Passage.

Present theory is unable to account for a number of hydraulic aspects that let the flow through Samoan Passage stand out. Among these are flow through connecting channels, flow through channels with two sill configurations and flow across sills with complex bathymetry. Getting a handle on these processes will be important for better understanding the large scale circulation of the Pacific Ocean as the Samoan Passage is the major supplier for deep water renewal in the North Pacific. Moreover, these configurations are by no means unique to the flow through Samoan Passage and a better knowledge of hydraulic processes here will inform research of the flow at many other canyons and fracture zones of the world's oceans.