Magnitude and Drivers of the Seasonal Cycle and Interannual Variability of pCO2 in the Washington Coast OOI Endurance Array

Abstract:

Fully resolving the variability of the partial pressure of CO₂ (pCO₂) in the ocean and the drivers of that variability is necessary for assessing current and future climate change and ocean acidification. One region of interest with relatively little documentation is the Washington coast off the northwestern United States. To study this, data was taken from the Ocean Observatories Initiative (OOI) Endurance Array, specifically the Washington Offshore Surface Mooring. The data show that the region is primarily a sink for carbon, with a net annual flux value of -1.9 ± 2.1 mol m^-2 yr^-1 in 2016 and -2.1 ± 1.9 mol m^-2 yr^-1 in 2017. The pCO₂ values for 2016 and 2017 range from 148 μatm to 436 μatm. Average pCO₂ tends to decrease in late spring and summer, while variability around that average increases in the documented upwelling season. During fall and winter, pCO₂ levels become less variable and rise to near equilibrium conditions with the atmosphere. A decomposition of pCO₂ into its thermal and non-thermal drivers was used to investigate seasonal forcing. The observed pCO₂ curve very closely follows that for non-thermal drivers, while thermal factors play a role in the winter, when they reduce the magnitude of pCO₂ and keep the area from becoming a source of carbon. This is in contrast with the OOI Pioneer Array in the coastal Atlantic Ocean, where pCO₂ generally follows thermal drivers, though thermal and non-thermal drivers tend to work against each other to diminish the seasonal cycle. Despite very different drivers, the Pioneer Offshore Surface Mooring has a similar annual flux value of -1.2 ± 2.2 m^-2 yr^-1 for 2016.