Seasonal dissolved oxygen gas exchange in the California Current Upwelling System

Eastern boundary current systems are known for seasonal upwelling that brings water from below the mixed layer to the surface. The process deoxygenates the water column by bringing lower-dissolved oxygen water closer to the surface and oxygenates the water column by driving primary productivity due to the injection of nutrients from deep waters. Here, an observational dataset of 5 years of dissolved oxygen, temperature, and salinity measured to 500 m and from the coast to around 400 km offshore is used to study the seasonal evolution of air-sea exchange of oxygen in an upwelling region. The data were collected as part of the California Underwater Glider Network (CUGN) which monitors transect line 66.7 off of Monterey Bay, CA, transect line 80.0 off of Point Conception, CA, and transect line 90.0 off of Dana Point, CA year-round. A narrow band of coastal oxygen undersaturation drives dissolved oxygen uptake by the ocean in spring and summer, while in winter dissolved oxygen uptake predominates due to more widespread but less intense oxygen undersaturation. Oxygen outgassing occurs offshore of the nearshore uptake band in spring and summer, driven by warming and primary productivity, such that the cumulative net oxygen gas exchange integrated seaward from shore reaches zero once around one hundred kilometers from the coast. Ventilation of the upper ocean is observed as a result of shoaling isopycnals in the upwelling season. The balance between outgassing and uptake is such that the central and southern California Current System is a region of net outgassing of oxygen for a region from the coast to 400 km, 365 km, and 530 km for lines 66.7, 80.0, and 90.0 respectively. Competing factors causing uptake and outgassing and the seaward extent of the domain determine whether an upwelling zone is a net source or sink of oxygen gas.

Plain Language Summary

The oceanography of coastal California is such that a seasonal vertical and offshore movement of water in the nearshore 50 km reduces the dissolved oxygen concentration of the water. The dissolved oxygen changes in the ocean were observed with autonomous underwater vehicles that allowed scientists to observe seasonal changes. Dissolved oxygen gas was found to enter the ocean in the coastal region during times of upwelling while it was released from the ocean further offshore. Some dissolved oxygen that entered the ocean surface layer found its way to deeper waters. The amount of dissolved oxygen gas released from the surface ocean compensated for the amount of oxygen uptake that occurred. The region from the coast to around 400 km from shore released oxygen gas to the atmosphere overall.