Structure of cross-shelf circulation in a buoyancy-influenced, wind-driven Eastern Boundary Current system

University Of Washington, Seattle WA

Investigators

Abstract

The overarching goal of this project is to understand the patterns and driving mechanisms of subtidal cross-isobath (inshore/offshore) circulation in a wind-forced Eastern Boundary Current (EBC) system such as the California Current System (CSS) where buoyancy sources affect stratification and the mean circulation is seasonally-dependent. Specifically, project objectives are: 1) Determine the response of cross-isobath circulation over the mid-shelf to local and remote forcing during the upwelling season as a function of stratification, 2) Determine the cross-margin extent of the identified cross-isobath circulation patterns, 3) Determine how the cross-isobath response changes seasonally. The objectives will be met through application of a newly developed method of isolating the cross-isobath circulation to 1) analysis of an existing and untapped set of observations and 2) analysis of upcoming NSF-funded Ocean Observatory Initiative (OOI) Endurance Array observations that will span the continental margin year-round at two separate along-shelf locations in the northern CCS. Cross-margin exchange remains one of the least understood processes in EBC systems. Although progress has been made on understanding near-surface dynamics and transports, the origin, structure, and timing of the deeper compensating return flows have proven elusive. Much of our lack of understanding stems from the fact that cross-isobath flows are difficult to accurately isolate: 1) cross-isobath flows are generally much weaker than coincident along-isobath flows, and 2) water column velocity measurements often do not span the crucial surface and bottom boundary layers. Analyses often hinge on removal of the instantaneous depth-averaged cross-isobath velocity in order to relate the residual structure and its temporal variability to external forcing. This approach avoids the otherwise unbalanced depth-averaged along-shelf momentum that results from a slight veering-over of along-isobath currents, as occurs when a meander or eddy passes by a measurement location. However, this methodology can leave significant biases in the vertical structure of both the mean and fluctuating cross-isobath velocity profiles if the along-isobath flow is vertically sheared, as is often the case in EBC systems such as off the western US, Peru and Chile, and parts of Africa. A newly-developed technique that eliminates these biases and has shown promise at a mid-shelf location in the northern CCS will be employed to isolate the cross-isobath circulation in existing and upcoming observational data sets. Except during spring the seasonal mean surface pressure gradient opposes the along-isobath flow in the northern CCS. This offers a dynamical counter-example to other shelf regions such as those off the northeast US where flows are down the pressure gradient. Cross-margin exchange regulates water property distributions (heat, salt, and oxygen), nutrient availability, larval recruitment of invertebrates and fish, and pollutant and sediment dispersal, including export to the open ocean. Because of these wide-ranging and important applications and because understanding of cross-isobath circulation has historically been so elusive, the outcomes of this project will themselves be of broad impact. The project will provide new methodology and insights that may be used and tested in other coastal settings with regional current jets and/or significant eddy fields. The project will provide educational activities for a Postdoctoral Fellow/early career scientist, graduate student teaching, local resource managers, and an undergraduate student. In particular, results will be incorporated into a graduate level class at the UW that attracts biological and fisheries oceanography students with interdisciplinary interests in along- and cross-shelf motion. Regional partners, e.g., from WA Dept. of Health and WA Dept. of Fish & Wildlife, will be trained on the use of OOI data streams and products as tools for coastal resource management. Last, an undergraduate student will gain experience with the use and application of cutting-edge OOI datasets, with the goal of presenting their results at the annual UW Undergraduate Research Symposium.