Samoan Passage Abyssal Mixing Experiment

3rd & Final Cruise

Instruments + Measurements

Crush Cam

Cultural Exchanges

It’s just one of these incredibly special places in the ocean where you can measure the incoming properties of the fluid and measure the outgoing properties and there is really a big, measurable difference.

This Antarctic bottom water is of interest because it’s the main supply for deep water in the Pacific Ocean because there’s no deep water created anywhere in the North Pacific. All of the water in the North Pacific at the bottom has to have come through this pathway.

Motivation & Experiment

The Samoan Passage, 5500 m beneath the sea surface, is one of the "choke points" in the abyssal circulation. A veritable river of Antarctic Bottom water flows through it on its way into the North Pacific. As it enters the constriction, substantial turbulence, hydraulic processes, and internal waves must occur, which modify the water.

Because climate models do not do a good job of resolving flows like these, we will take our stable of instruments — moored profilers, conventional current meter moorings, and shipboard instruments — and measure the velocity, turbulence, and internal waves in the region. The overall goal is to understand these deep processes and the way they impact the flow, and to develop a strategy for eventually monitoring the flow through the Samoan Passage.

Experiment Sketch

The Samoan Passage Abyssal Mixing Experiment was three cruises over three years, each about 40 days long. During the first the seafloor was mapped as never before. On the second, the flow passages and pathways were mapped. We also deployed a long-term array of moorings that stayed in the water until the third cruise. On our final cruise to the region, the 'processes' cruise, we tried to measure in detail the incredible physics involved as the water flows through the passage.

We returned to the Samoan Passage for the third and final time to retrieve the long-term mooring array and to focus on the processes that are driving the turbulence and flow constrictions. We mapped the flow patterns around bumps in topography and over small sills as well as the turbulence response to those features. We are trying to capture as complete a picture as possible of the slightly denser fluids flowing under lighter fluids, causing mixing, and generating measurable responses in the flow. What we saw is a combination of internal waves that are generated by the flow and turbulence that is forced by the flow.

One of the challenges in this experiment is to take this basin scale property contrast and focus it down to a small region, about 200 km square. These flows are forced through small channels 20–50 km wide and we are also looking at responses to topographic features that are perhaps only 1–2 km in size.