Empirical Algorithms to Predict pH and Aragonite Saturation State on SOCCOM Biogeochemical Argo Floats in the Pacific Sector of the Southern Ocean

The Southern Ocean plays a major role in the global uptake, transport, and storage of both heat and carbon, yet it remains one of the least-sampled regions of the ocean. The Southern Ocean Carbon and Climate Observations and Modeling (SOCCOM) project aims to fill the observational gaps by deploying over 200 autonomous profiling floats in the Southern Ocean over the next several years. Initial float deployments have greatly expanded our observational capability to include wintertime measurements as well as under-ice measurements, and many of these floats include novel biogeochemical sensors (pH, nitrate, oxygen). Here we present empirical algorithms that can be used to predict pH and Ω_Aragonite from other float-measured parameters (temperature, salinity, pressure, nitrate, oxygen). These algorithms were trained using bottle measurements from high-quality repeat hydrographic GO-SHIP cruises. We obtained R² values of 0.98 (pH) and 0.99 (Ω_Aragonite) and RMS errors of 0.007 (pH) and 0.052 (Ω_Aragonite) for data between 100–1500 m. These algorithms will allow us to both validate pH data from these sensors, as well as predict Ω_Aragonite and pH on floats that do not have pH sensors. Here we present estimated pH and Ω_Aragonite over 20 months of deployment for several SOCCOM floats in the Pacific Sector of the Southern Ocean. The results show seasonal ranges in surface pH and Ω_Aragonite of 0.05 and 0.1, respectively.