The Effect of Alongcoast Advection on Pacific Northwest Shelf and Slope Water Properties in Relation to Upwelling Variability

Abstract

The Northern California Current System experiences highly variable seasonal upwelling in addition to larger basin‐scale variability, both of which can significantly affect its water chemistry. Salinity and temperature fields from a 7 year ROMS hindcast model of this region (43°N–50°N), along with extensive particle tracking, were used to study interannual variability in water properties over both the upper slope and the midshelf bottom. Variation in slope water properties was an order of magnitude smaller than on the shelf. Furthermore, the primary relationship between temperature and salinity anomalies in midshelf bottom water consisted of variation in density (cold/salty versus warm/fresh), nearly orthogonal to the anomalies along density levels (cold/fresh versus warm/salty) observed on the upper slope. These midshelf anomalies were well‐explained ( R 2 = 0.6) by the combination of interannual variability in local and remote alongshore wind stress, and depth of the California Undercurrent (CUC) core. Lagrangian analysis of upper slope and midshelf bottom water shows that both are affected simultaneously by large‐scale alongcoast advection of water through the northern and southern boundaries. The amplitude of anomalies in bottom oxygen and dissolved inorganic carbon (DIC) on the shelf associated with upwelling variability are larger than those associated with typical variation in alongcoast advection, and are comparable to observed anomalies in this region. However, a large northern intrusion event in 2004 illustrates that particular, large‐scale alongcoast advection anomalies can be just as effective as upwelling variability in changing shelf water properties on the interannual scale.

Drivers of Subsurface Temperature Variability in the Northern California Current

“…The spatial correlations are consistent with more than one mechanism, and with the tools used here, we cannot identify the main driver of that anomaly. This is the first time this precursory ocean condition has been shown to be so important, recognizing that prior work in the region has suggested that conditions further south influence the variability of ocean conditions in the N‐CCS region (Connolly et al., 2014; Stone et al., 2018). Potential mechanisms responsible for this signal include locally generated CTW and its interactions with the CUC, advection of spice in the CUC locally within the CCS, and interactions with the gyre circulation.…”

“…This suggests that the local generation of CTWs could occur during neutral years as a result of local alongshore wind variations. Remotely driven CTW also modulates the depth of CUC (Connolly et al., 2014; Stone et al., 2018), whose rectified effects persist through to the next summer after ENSO years. In our results, this mechanism was identified in the correlations of OND Niño 3.4 SST anomaly to the following MJJ depth of the CUC (lag of 4 months; Figure 5) south of N‐CCS.…”

Linking Chlorophyll Concentration and Wind Patterns Using Satellite Data in the Central and Northern California Current System

“…Overall, the model's temperature, salinity, and sea surface elevation had high Willmott Skill Scores (WS ≥ 0.92; Willmott, 1982), based on comparison with data from 2264 CTD casts in 2005 (Giddings et al, 2014). More information about the Cascadia model can be found in Davis et al (2014), Giddings et al (2014), and Siedlecki et al (2015), and more information about the particle tracking experiments can be found in Banas et al (2009b) and Stone et al (2018). Particles were released in on the shelf (i.e., in water shallower than 150 m, the depth of the shelf break) every 0.05 • FIGURE 6 | Model study domain for the particle tracking experiments.…”

“…They were released every 10 days over 2003-2009, resulting in 58,536 particles per year. Once released the surface-trapped particles were tracked with hourly time steps and daily output for up to 350 days, or until they moved outside the model boundaries or "beached" on land, defined as being close enough to land that a bilinear interpolation of the ROMS land mask (1 for ocean, 0 for land) at its location was less than 0.5 (Banas et al, 2015;Stone et al, 2018). Tidally-averaged model output was used for this particle release experiment.…”

“…In addition, the wide continental shelves of Washington and Oregon allow phytoplankton ample time to bloom as they are transported offshore (Strub et al, 1991). Remote wind forcing (defined here as alongshore winds at and south of 42 • N), through coastal-trapped waves, plays a large role in upwelling in the Northern CCS (Battisti and Hickey, 1984;Chapman, 1987;Hickey et al, 2006Hickey et al, , 2016Stone et al, 2018). Lastly, freshwater supply, particularly outflow from the Salish Sea and, on a smaller scale, the Columbia River, brings additional nutrients to the region (Davis et al, 2014).…”