Seasonal and interannual variability in along-slope oceanic properties off the US West Coast: Inferences from a high-resolution regional model

A 6 year, 2009-2014 simulation using a 2 km horizontal resolution ocean circulation model of the Northeast Pacific coast is analyzed with focus on seasonal and interannual variability in along-slope subsurface oceanic properties. Specifically, the fields are sampled on the isopycnal surface sigma = 26.5 kg m(-3) that is found between depths of 150 and 300 m below the ocean surface over the continental slope. The fields analyzed include the depth z(26.5), temperature T-26.5, along-slope current v(26.5), and the average potential vorticity PV between sigma = 26.5 and 26.25 kg m(-3). Each field is averaged in the cross-shore direction over the continental slope and presented as a function of the alongshore coordinate and time. The seasonal cycle in z(26.5) shows a coherent upwelling-downwelling pattern from Mexico to Canada propagating to the north with a speed of 0.5 m s(-1). The anomalously deep (220 m) z(26.5) displacement in spring-summer 2014 is forced by the southern boundary condition at 24 degrees N as a manifestation of an emerging strong El Nino. The seasonal cycle in T-26.5 is most pronounced between 36 degrees N and 53 degrees N indicating that subarctic waters are replaced by warmer Californian waters in summer with the speed close 0.15 m s(-1), which is consistent with earlier estimates of the undercurrent speed and also present v(26.5) analyses. The seasonal patterns and anomalies in z(26.5) and T-26.5 find confirmation in available long-term glider and shipborne observations. The PV seasonality over the slope is qualitatively different to the south and north of the southern edge of Heceta Bank (43.9 degrees N).