Estimating Air-Sea Carbon Flux Uncertainty over the Tropical Pacific: Importance of Winds and Wind Analysis Uncertainty

The tropical Pacific Ocean stands out as a significant natural source of carbon to the atmosphere – even rivaling U.S. emissions. Knowing how this source has changed over recent decades, and how it might change in coming decades is important to understanding and predicting net oceanic carbon uptake. Estimates of the effect that changes in the wind have had on regional air-sea carbon exchange depend strongly on the wind analysis product used. Fortunately, the array of approximately 70 moored-buoys spanning the tropical Pacific provides direct wind information critical to our ability to monitor the system for long-term trend. The analysis-product winds used most widely in previous calculations of basin-scale carbon flux are compared with mooring winds and found to exhibit significant differences in mean, variability, and trend. Earth system model calculations are in basic agreement with the mooring results and used to estimate effects of wind uncertainty on our knowledge of regional air-sea carbon exchange. Results show NCEP1 and NCEP2 winds contain biases large enough to obscure the interannual variability of CO₂ flux (RMSE ≈σ) and cause spurious 25-year (1992-2016) trend components in equatorial Pacific carbon flux of 0.038-0.039 and 0.016-0.021 PgC yr^-1 decade^-1. These spurious trends act to reduce by up to 50% the 25-year trend in equatorial Pacific carbon flux simulated by the Earth system model under increasing atmospheric CO₂ concentration. The Cross-Calibrated-Multi-Platform (CCMP) wind product tracks observed variability of equatorial Pacific wind better (interannual RMSE ≈ 0.4σ) than the NCEP reanalyses when site-sampled at mooring locations, yet still causes a spurious regional trend of 0.03 PgC yr^-1 decade^-1. The implication of our finding is that the wind-driven trend in air-sea carbon flux over the equatorial Pacific over the last 25 years is much smaller than previously thought.