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

Abstract

This study seeks to identify drivers of differences in ocean acidification (OA) rates throughout the upper ocean at Station ALOHA in the North Pacific Subtropical Gyre (NPSG). OA is intensified in the subsurface due to increases in natural and anthropogenic carbon pools, and their interactions. Enhanced subsurface trends are found for all OA indicators. This includes parameters that have previously been identified to exhibit nonlinear interactions between anthropogenic and natural carbon ([H⁺], pCO₂, pH, Revelle Factor), as well as parameters that are not subject to this response (aragonite saturation state (Ω_Ar)). Different parameters have trend maxima in each of the water masses in the upper 500 m, driven by different mechanisms. Enhanced acidification is noted in the North Pacific Tropical Water (NPTW) from 2015 until 2020. This 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. 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 circulation slowdown has led to enhanced CO₂ ingrowth from remineralization, buffered by increasing carbonate dissolution. Local changes seem to play a smaller role. In the SSM and NPTW, 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.