Spatiotemporal heterogeneity in the increase of ocean acidity extremes in the Northeast Pacific

Abstract. The acidification of the ocean (OA) increases the frequency and intensity of ocean acidity extreme events (OAXs), but this increase is not occurring homogeneously in time and space. Here we use daily output from a hindcast simulation with a high-resolution regional ocean model coupled to a biogeochemical-ecosystem model (ROMS-BEC) to investigate this heterogeneity in the progression of OAX in the upper 250 m of the Northeast Pacific from 1984 to 2019. We focus on the temporal and spatial changes in OAX using a relative threshold approach and using a fixed baseline reflecting the initial conditions. Concretely, conditions are considered extreme when the local hydrogen ion concentration ([H⁺]) exceeds the 99^th percentile of the [H⁺] distribution of the baseline simulation where atmospheric CO₂ was held at its 1979 level. Within the 36 years of our hindcast simulation, the increase in atmospheric CO₂ causes a strong increase in OAX throughout the upper 250 m, but most accentuated near the surface. On average across the entire Northeast Pacific, for every additional 10 μatm of CO₂ in the atmosphere, OAXs occupy an additional 6.3 % of the upper 250 m depth, last 7.6 days longer, and are 0.18 nmol L⁻¹ (~ −0.006 pH units) more intense. This causes the OAXs to occupy at the end of the simulation a more than 10-times larger volume. The more than 11-fold increase in length, and the strong increase in the number of extreme days per year causes 88 % of the surface area in 2019 to experience near permanent extreme conditions. Finally, the model simulates a more than 6-fold intensification of the OAXs, causing also the intensity of the events with return periods of 10 years or more to increase by more than 80 %. Superimposed on these overall trends are very substantial spatial and temporal differences in these changes. The fraction of the volume identified as extreme across the top 250 m increases in the Central Northeast Pacific up to 160-times, while the deeper layers of the nearshore regions experience "only" a 4-fold increase. Throughout the upper 50 m of the Northeast Pacific, OAXs increase relatively linearly with time, but sudden rapid increases in yearly extreme days and OAX duration are simulated to occur in the thermocline of the Central Northeast Pacific. These differences largely emerge from the large spatial differences in the magnitude and nature of variability in [H⁺], with the transition between the rather variable thermocline waters of the Offshore Northeast Pacific and the very stable waters of the Central Northeast Pacific causing a very sharp transition in the occurrence of OAX. This transition is caused by the limited offshore reach of offshore propagating eddies that are the dominant driver of OAX in the Northeast Pacific. As the OAXs become more extreme, more of them also become undersaturated with respect to aragonite (Ω_A < 1), i.e., become corrosive. In the final year of our hindcast, we find that below 100 m OAXs are characterized by corrosive conditions across a wide stretch of the region offshore of the U.S. and Canadian Coasts. The spatially and temporal heterogeneous increases in OAX, including the abrupt appearance of extremes, likely have negative effects on the ability of marine organisms to adapt to the progression of OA and its associated extremes.