Cross-shore Advection by Subthermocline Eddies

Eastern boundary current systems have a poleward undercurrent along the coastal boundary in addition to a wind-driven equatorward current. The subsurface poleward current in the California Current System is the California Undercurrent (CUC). Through over 10 years of repeat glider observations, we find evidence of exchange of subsurface poleward CUC water with the interior ocean through subthermocline eddies, predominantly anticyclonic eddies that transport water with CUC signature off the coast towards the subtropical gyre. The glider data were collected as part of the California Underwater Glider Network (CUGN) with three glider lines along traditional California Cooperative Oceanic Fisheries Investigations (CalCOFI) hydrography lines, Line 66.7, Line 80.0, and Line 90.0. Along with temperature and salinity from 2006-present, dissolved oxygen concentration is available from an oxygen optode sensor from 2017-present. We find that the eddies transport water with a high salinity anomaly, high temperature anomaly, and low dissolved oxygen anomaly. Specifically, PDFs of height between a layer defined as σ_θ 25.7-26.8 and that layer’s average salinity demonstrate that the variables are dependent. The same is true for the layer’s height and dissolved oxygen concentration. On Line 67, from Hovmoller diagrams of objectively-mapped salinity anomalies, we see evidence of regular propagation cross-shore, with a subthermocline eddy present on the line almost every year. We estimate the cross-shore speed of the eddies to be around 0.03 m/s. There appears to be variation along the coast in the prevalence of subthermocline eddies, with less defined cross-shore propagation along Line 80 and Line 90. However, we show that in regions where they are prevalent, they are important to the cross-shore exchange of tracers. Finally, we use an empirical relationship to estimate pH, calcite saturation, and aragonite saturation. Anticyclonic subthermocline eddies transport water with low pH anomalies cross-shore and raise the depth of the aragonite saturation state threshold (𝝮_arag=1) while lowering the depth of the calcite saturation threshold (𝝮_calc=1). We suggest subthermocline eddies are an important component of the volume transport and the balance of tracers in the coastal ocean.