This project investigated the structure of cross-shelf circulation in the northern California Current System (CCS), both vertically and across the continental shelf and upper slope.

To achieve project objectives we first devised a simple technique utilizing a streamwise-normal coordinate system to isolate a two-dimensionally balanced estimate of the cross-shelf circulation from a variable flow field. The technique projects the stream-normal flow onto the cross-shelf coordinate in order to remove meander- or eddy-induced biases in the cross-shelf flow that are unaccounted for with an alternative, commonly-applied approach. In general, this method can be applied to upwelling- or downwelling-favorable systems alike, and should therefore have broad appeal in the community. This technique was published in McCabe et al. (2015).

Applying this approach to data collected from a mid-shelf mooring located off Washington State in 2005, we then described the seasonal development of a shallow onshore "return" flow and a deeper offshore-directed flow during the summer upwelling season (McCabe et al. 2015). The upwelling-mean cross-shelf circulation profile during the latter half of the upwelling season was three-layered: offshore flow existed at the surface, an onshore return flow existed within the interior water column, and a third offshore-directed layer existed at depth. We evaluated mechanisms to explain this mean structure and suggested that the timing of the development and strengthening of both the interior onshore return flow and the offshore near-bottom layer were consistent with the seasonally-changing direction and magnitude of the large-scale along-shelf sea-level gradient. We argued that the change to a poleward sea-level gradient initiated a seasonal relaxation of upwelled isopycnals that likely led to the near-bottom flow. Late-season enhancement of the interior onshore return flow was related to the along-shelf surface wind stress but appeared to form as a consequence of offshore transport in the near-bottom layer and the need to satisfy coastal mass balance. Seasonal changes in stratification did not appear to explain the observed changes in the cross-shelf circulation.

We then used data collected by the Ocean Observatories Initiative (OOI) Endurance Array in summer 2016 to investigate whether or not the cross-shelf circulation patterns observed in 2005 were present and, if so, the extent to which they existed across the continental shelf. Results confirmed that the same three-layer upwelling circulation was present in 2016 and furthermore that the pattern was evident at all three OOI sites (inner shelf, middle shelf, and upper slope). These findings suggest that such patterns may be commonplace during the summer upwelling season in the northern CCS.

Because continental shelf water properties are intimately tied to cross-shelf circulation, we also used historical data to investigate the seasonal and interannual water property variability on the middle shelf throughout the northern CCS. Results were published in Hickey et al. (2016) and may be summarized as follows. As a result of seasonality in the regional alongshore wind structure, remote forcing has little effect on shelf water properties in winter, but is particularly effective in setting shelf water properties in summer when the alongshore wind stress gradient is significant. When the summertime alongshore wind stress gradient is relatively weak, alongshore differences in shelf water properties become significant. Both theoretical models and observations suggest that it is the along-shelf wind stress gradient that sets up large-scale along-shelf sea level gradients (i.e. along-shelf pressure gradients) that then alter the depth structure of the cross-shelf flows, and shelf water properties. These results are consistent with our argument that the along-shelf sea level gradient is important in driving changes in the cross-shelf circulation over the northern CCS shelf.

Given the importance of cross-shelf flows, the findings of this research themselves are of broad impact. At present, results have been published in two directly related scientific manuscripts (McCabe et al. 2015; Hickey et al. 2016); another manuscript is in preparation. Results in one additional manuscript (McCabe et al. 2016) were partially funded by this project. This latter manuscript led to news articles at the national level, as well as radio and local television interviews, meaning that coastal oceanographic issues and upwelling flows, in particular, were prominently discussed in the public domain. In addition, eight presentations at national (5) and international (1) conferences and invited seminars (2) were also made during this project. A tutorial on OOI data access and on-line plotting was provided to local State and Tribal shellfish managers. Results of this research were incorporated into a graduate level coastal oceanography class at the University of Washington. A project PI served on a female student's Masters committee at the University of Washington. Results from this project helped shape that student's Masters research. Finally, this project supported an early career scientist.

Last Modified: 11/30/2019
Modified by: Ryan M Mccabe