The ocean has an overturning circulation in which dense waters that form at high latitudes as a result of cooling and ice formation sink and become part of a complex network of deep currents that carry water all over the globe and eventually back to the surface. The overturning circulation is an important player in determination of Earth's climate.  The North Pacific Ocean is unusual in that it does not produce water that sinks all the way to the bottom: the bottom water instead comes the Southern Ocean in the form of a deep northward current. All of this water is funneled through a choke point just south of the Equator: the Samoan Passage and two neighboring passages (Figures 1 and 2). The main purpose of this project was to analyze recent measurements of the flow in Samoan Passage in order to quatify and better understand the volume transport and its variability, the causes of mixing with other water masses and the overall importance of this mixing, and the factors that determine how the flow is distributed betwenn the two main passages: the Samoan Passage and the region east of the Manihiki Plateau. The data had previously been collected as part of a three-year campaign that involved three cruises and an array of measurements of water properties, velocities, and turbulence. 

The main advancements include an idealized model (FIg. 3) explaing why roughly 5-6 Sv. (1 Sv. = 1 million cubic meters per second) of transpoft passes directly through the Samoan Passage, whereas another 2.8 Sv. is diverted around the Manihiki Plateau. This partitioning is important because the flow through the Samoan Passage spills over multiple sills (Figure 4), creating turbulent mixing with warmer overlying water, whereas the Manihiki branch is slower and does not esperience overflows. Another outcome of the project is a map showing where mixing hotspots occur within the Samoan Passage (Figure 5).  As one can see, the topography is complex and there are several routs that the throughflow can take, each with its own set of sills and bumps. Identificaiton of the hotspots leads to evaluation of the physical mechnisms responsible for the mixing. This list includes hydraulic jumps, shear flow instabilities, breaking of lee waves, and generation at the bottom.  A surprising finding is that the general level of mixing remains temporally constant even as the overall volume transport fluctuates.  We also present an estimate suggesting that the mixing accounts for more than half of the total mixing that bottom waters experience during their transit through the South and North Pacific. 

We have also tried to better understand how it is possible that one branch of a deep flow can be hydraulically controlled while the other not so.  An idealized model suggests that while possible, this situation requires a rethinking of the that deep sill flow play in the regulation of abyssal flows. Instead of influencing the flow far upstream, the way that a dam backs up the flow in a reservoir, the sills in the Samoan Passage influence how much water is diverted around the Manihiki Plateau. These considerations would inform the use of hydraulic principles to design an instrument array for monitoring the deep transport, a desirable activity for those interested in detecting climate change in the deepenst limb of the Pacific overturning circulation.  

Our project also engaged in an outreach activity intended to bring ocean science to disadvantaged youth in a fun way (Figure 6).  We set up a two-week class for 5th and 6th grade students who live in southern California and come from families that live beneith the poverty level. The course was offered though A Step Beyond, an Escondido, CA nonprofit, and we used lectures, embodied learning  and a field tripin to excite interest among the students in physical oceanography. The group was largely hispanic and was co-taught with a dance instructor.  The class was co-taught by Pratt and Roxanne Rojas de Blanco, a dance teacher who works in the San Diego public school system.  Gunnar Voet from the Samoan Passage team guided the students on a field trip to the Scripps Institute of Oceanography. 

Last Modified: 06/06/2020
Modified by: Lawrence J Pratt