The upward movement of the world's "oldest" water found in the Northeast Pacific Basin is one of the least understood aspects of the global ocean circulation and depends on complex local effects. Globally, it has become increasingly clear over the last few decades that the return flow of bottom water to the surface ocean in the "conveyor belt" is far from the quasi-uniform upward motion and circulation that was originally envisioned. A high resolution analysis of the deep and abyssal circulation for the entire Northeast Pacific basin will be conducted using all available historical data and model statistical and numerical methods. The project will also answer a long-standing question about the origin of the North Pacific double silica maximum. The investigator for this project has a leading role in undergraduate education in Oceanography at the University of Washington and will use the results from this study to illustrate to students the complexities of the real-world processes underpinning the simple schematics that are commonly found in discussions of the role of the deep ocean circulation in long-term climate variation. The investigator will publish and archive the three-dimensional fields of ocean properties on the University of Washington Oceanography website. These fields may be helpful to other researchers for uses ranging from experiment design to better understanding of tracer distributions in the deep North Pacific. Finally, the work will provide support and training for an undergraduate and a graduate student.
The first objective of the project is to test a hypothesis about how circulation fields relate to the North Pacific double silica maximum. The North Pacific hosts a striking mid-depth maximum of dissolved silica (Si), a regenerated nutrient important for ocean biogeochemistry. Previous work noted a second much denser maximum of Si over the western flank of the Juan de Fuca Ridge, with an intervening Si minimum that precludes a simple upwelling connection between bottom water and mid-depth in this region. The hypothesis then was that the mid-depth Si plume is fed from the bottom of Cascadia Basin, located east of the ridge. Recent work revealed that Cascadia Basin sources are not sufficient to maintain this feature. The working hypothesis for this project is that the additional source lies west of the ridge, requiring a connection - one that is surprisingly complex - via a convoluted upward-spiraling route south along the western flank to a deep gap just north of the Mendocino Fracture Zone, and then north into Cascadia Basin along the eastern flank of the ridge system. Along this flow path, bottom heating and diapycnal mixing decrease the density of the fluid until it spills over the bounding ridges to feed the mid-depth plume. The downstream part of the flow path through Cascadia Basin has been documented in recent work by the investigator who proposes a detailed analysis of the upstream western flank flow, applying a similar geostrophic inverse methodology. The second objective is to extend the inverse calculation to the entire region east of the dateline and north of 25 N at unprecedented resolution. An analysis using data available before 1990 revealed surprising complexity to the deep circulation in the Pacific, in the form of distinct regions of meridional transport in the west and east, connected by multiple zonally-elongated deep gyres, but more recent studies disagree. A detailed analysis of the northeast Pacific overturning at higher resolution, and using a fundamentally different inverse technique based on the potential vorticity budget, will be used to yield new insight on this issue, as well as other unanswered questions.
