Drivers and Variability of Intensified Subsurface Ocean Acidification Trends at Station ALOHA

Ocean carbon uptake, cycling and sequestration are variable on all time scales, and modulated by an interplay of complex physical and biogeochemical drivers, including anthropogenic CO₂increase and associated ocean acidification (OA). OA at Station ALOHA is intensified in the subsurface due to increases in both natural and anthropogenic carbon pools, and their interactions. Enhanced subsurface change is found for all OA indicator variables. This includes both the parameters who have previously been reported to be systematically impacted by nonlinear interactions between anthropogenic and natural carbon pools ([H⁺], pCO₂, Revelle Factor), but also those who do not show this generalized response in the ocean interior (pH, aragonite saturation state (Ω_Ar)). Different parameters have trend maxima in each of the three water masses in the upper 500 m, driven by different mechanisms. Enhanced acidification is noted in the North Pacific Tropical Water (NPTW) between 2015-2020. This steepening is due to the interplay of a circulation slowdown during a prolonged negative phase of the North Pacific Gyre Oscillation (NPGO) with other anomalous atmospheric forcing that altered source water chemistry, including large-scale freshening. Long-term sustained increased acidification is also associated with freshening and cooling in the Subsurface Salinity Minimum (SSM) over the whole time-series, with considerable oxygen loss and nutrient increases. In the North Pacific Intermediate Water (NPIW), a well-documented long-term circulation slowdown has led to enhanced CO₂ ingrowth from remineralization, buffered by increasing carbonate dissolution. Local changes seem to play a smaller role than circulation and source water changes. In two water masses, enhanced acidification is associated with cooling and freshening, providing new insights on how OA can accelerate beyond the well documented warming and souring of the ocean.