Abstract
Ocean acidification is punctuated by episodic extremes of low pH and saturation state with regard to aragonite (ΩA). Here, we use a hindcast simulation from 1984 to 2019 with a high-resolution regional ocean model (ROMS-BEC) to identify and track ocean acidification extremes (OAX) in the Northeast Pacific and the California Current System (CCS). In the first step, we identify all grid cells whose pH and ΩA are simultaneously below their first percentile over the analysis period (1984-2019). In the second step, we aggregate all neighboring cells with extreme conditions into three-dimensional time evolving events, permitting us to track them in a Lagrangian manner over their lifetime. We detect more than twenty-two thousand events that occur at least once in the upper 100 m during their lifetime, with broad distributions in terms of size, duration, volume and intensity, and with 26% of them harboring corrosive conditions (ΩA < 1). By clustering the OAXs, we find three types of extremes in the CCS. Near the coast, intense, shallow, and short-lasting OAXs dominate, caused by strong upwelling. A second type consists of large and long-lasting OAX events that are associated with westward propagating cyclonic eddies. They account for only 3% of all extremes, but are the most severe events. The third type are small extremes at depth arising from pycnocline heave. OAX potentially have deleterious effects on marine life. Marine calcifiers, such as pteropods, might be especially impacted by the long-lasting events with corrosive conditions.
Plain Language Summary
The emission of carbon dioxide by human activities causes ocean acidification, i.e., the decrease of the pH and saturation level of seawater with respect to the carbonate mineral aragonite. Episodic events of unusually low pH and aragonite saturation level punctuate these long-term declines, potentially intensifying stress on marine plankton. Particularly prone to extremes is the California Current System off the U.S. West coast due to its naturally low pH-aragonite waters and its strong variability. We use a high-resolution numerical model to identify and characterize extreme events associated with ocean acidification in this region, and understand their drivers. We find extremes to have a broad range of volumes, durations and strengths, with a quarter of them carrying corrosive conditions for shelled organisms, i.e., aragonite saturation levels below 1. The largest and longest-lived events are associated with cyclonic eddies (whirls of approximately 50 to 100 km in diameter) that trap upwelled low pH-aragonite waters near the coast. Although representing only 3% of the events, they cause most of the total excess of acidity induced by all identified extremes. The vertical extent and duration of extremes with corrosive mean conditions are expected to impact calcifying organisms, such as pteropods.
Desmet F., Gruber N., Köhn E. E., Münnich, M. & Vogt M., 2022. Tracking the space-time evolution of ocean acidification extremes in the California Current System and Northeast Pacific. Journal of Geophysical Research: Oceans 127: e2021JC018159. doi: 10.1029/2021JC018159. Article.
