Extremes and Short‐Term Fluctuations in Coastal Ocean Acidification and Hypoxia

Abstract In Eastern boundary upwelling systems, such as the California Current System (CCS), seasonal upwelling brings low oxygen and low pH waters to the continental shelf, causing ocean acidification and hypoxia (OAH). The location, frequency, and intensity of OAH events is influenced by a combination of large‐scale climatic trends, seasonal changes, small‐scale circulation, and local human activities. Here, we use results from two 20‐year long submesoscale‐resolving simulations of the Northern and Southern U.S. West Coast (USWC) for the 1997–2017 period, to describe the characteristics and drivers of OAH events. These simulations reveal the emergence of hotspots in which seasonal declines in oxygen and pH are accompanied by localized short‐term extremes in OAH. While OAH hotspots show substantial seasonal variability, significant intra‐seasonal fluctuations occur, reflecting the interaction between low‐ and high‐frequency forcings that shape OAH events. The mechanisms behind the seasonal decreases in pH and oxygen vary along the USWC. While remineralization remains the dominant force causing these declines throughout the coast, physical transport partially offsets these effects in Southern and Central California, but contributes to seasonal oxygen loss and acidification on the Northern Coast. Critically, the seasonal decline is not sufficient to predict the occurrence and duration of OAH extremes. Locally enhanced biogeochemical rates, including shallow benthic remineralization and rapid wind‐driven transport, shape the spatial and temporal patterns of coastal OAH.