Drivers of Surface Ocean Acidity Extremes in an Earth System Model

Abstract

AbstractOceanic uptake of anthropogenic carbon causes acidification, a process that describes the increase in hydrogen ion concentrations ([H+]) and decrease in calcium carbonate mineral saturation states (Ω). Of particular concern are ocean acidity extreme (OAX) events, which pose a significant threat to many calcifying marine organisms. However, the mechanisms driving such extreme events are not well understood. Here, we use high‐frequency output from a fully coupled Earth system model of all processes that influence the surface ocean temperature and carbon budgets and ultimately [H+] and Ω anomalies to quantify the driving mechanisms of the onset and decline of high [H+] and low Ω extreme events. We show that enhanced temperature plays a crucial role in driving [H+] extremes, with increased net ocean heat uptake being the dominant driver of the event onset in the subtropics. In the mid‐to‐high latitudes, decreased downward vertical diffusion and mixing of warm surface waters during summer, and increased vertical mixing with warm and carbon‐rich subsurface waters during winter are the main drivers of high [H+] extreme event onset. In the tropics, increases in vertical advection of carbon‐rich subsurface waters are the primary driver of the onset of high [H+] extremes. In contrast, low Ω extremes are driven in most regions by increases in surface carbon concentration due to increased vertical mixing with carbon‐rich subsurface waters. Our study highlights the complex interplay between heat and carbon anomalies driving OAX events and provides a first foundation for more accurate prediction of their future evolution.