Temporal Variations of Inorganic Carbon Pools and Air-Sea CO2 Fluxes in the North Pacific Subtropical Gyre Evaluated from 30 Years of Sustained Field Observations

Among the primary objectives of the Hawaii Ocean Time-series (HOT) ocean monitoring program is to measure the time-varying concentrations of carbon dioxide in the upper water column and estimate the annual air-to-sea gas flux. Here, we report on the results of thirty years of direct measurements of dissolved inorganic carbon (DIC) system parameters at the deep ocean Station ALOHA (22°45’N, 148°W) in the subtropical North Pacific. We present updated (through 2018) assessments of the rate of accumulation of upper ocean DIC and the associated rise in pCO₂ and decline in pH (i.e., ocean acidification). We also examine higher-frequency variability in CO₂ system constituents that complicates the assessment of long-term trends within shorter time-series datasets. Our analysis reveals strong seasonality and a significant role for interannual variability consistent with an 11-year cycle, suggesting that solar forcing may play a role in oceanic physical and biogeochemical dynamics at this location. Despite the seasonal and interannual contributions, the overwhelming signal is one of progressive DIC accumulation in response to an increasing atmospheric CO₂ burden and the maintenance of an associated air-to-sea net CO₂ flux.