Seasonal and interannual variability in the poleward undercurrent off the US West Coast: inferences from observations and a high-resolution regional ocean model

Alexander L Kurapov, NOAA National Ocean Service, Office of Coast Survey, Silver Spring, MD, United States, John A Barth, Oregon State University, College of Earth, Ocean, and Atmospheric Sciences, Corvallis, OR, United States, Jennifer L. Fisher, Oregon State University, Cooperative Institute of Marine Resources Studies, Newport, OR, United States and Daniel L Rudnick, Scripps Institution of Oceanography, La Jolla, CA, United States
Outputs from a 2-km resolution regional ocean model, a dynamical core of the NOAA West Coast Ocean Forecast System (WCOFS), and available hydrographic data records are analyzed to understand seasonal and interannual variability in water properties at the level of the coastal undercurrent. The WCOFS domain extends along the west coast of the North American continent from 24N to 54N. The model is run continuously without data assimilation for a period of 2013-2017, which includes the 2014 “warm blob” and 2015-16 El Niño events. Analyses are focused at a level of the 26.5 kg/m3 isopycnal surface, characteristic of the poleward undercurrent along the continental slope. This surface is found over the continental slope at depths of 150-300 m below the ocean surface. Compared to the multi-year CTD profile time series over the continental slope off Oregon (44.6N), the model reproduces seasonal and interannual variability in the depth of that surface. Using the model and glider data in the Northern and Central California (CA), scatterplots of temperature (T) vs. background potential vorticity (PV) are analyzed near the 26.5 kg/m3 potential density level using July-September data. In these T-PV diagrams, slope waters (inshore of the 2000 m isobath) generally exhibit relatively warmer T and lower PV compared to waters sampled offshore (farther than 100 km from the slope). Off Trinidad Head (41N), the slope water properties are more distinctive from those farther offshore in 2016 compared to the other years sampled (2015, 2017, 2018). 2016 stands out in this way in the model as well. The longer term glider data record off Monterey Bay (36.5N, 2008-18) exhibits a similar pattern in 2016 and to some degree in 2010 (following a moderate 09-10 El Niño event). The model shows that the continuity in the undercurrent is interrupted as a result of instability and eddy generation on the slope. In 2016, the continuous, coherent current was maintained in late summer and fall, which contributed to the more clear separation of the offshore and slope waters. While research to relate the intensity of the undercurrent to the remote oceanic forcing is ongoing, we see in the model that variability in the placement of the 26.5 kg/m3 isopycnal surface over the slope at the southern boundary (24N) does not correlate with the undercurrent intensity in Central CA (34-36N).