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

The tropical Pacific is a major natural source of CO2 to the atmosphere and contributor to global air-sea carbon flux variability. High time-resolution wind and CO(2 )measurements from equatorial Pacific moorings reveal the primary factor controlling mooring-observed flux variability to be near-surface wind variability, above CO2 variability, in this region over the last 10 years. 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 that NCEP1 and NCEP2 winds contain biases large enough to obscure the interannual variability of CO2 flux (RMSE approximate to sigma) and cause spurious 25-year (1992-2016) trend components in equatorial Pacific carbon flux of 0.038-0.039 and 0.016-0.021 Pg C yr(-1) per decade, respectively. 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 wind product tracks observed variability of equatorial Pacific wind better (interannual RMSE approximate to 0.4 sigma ) than the NCEP reanalyses when site sampled at mooring locations yet still causes a spurious regional trend (0.03 Pg C yr(-1) per decade) that masks 40% of the simulated 25-year trend in carbon flux. The mooring observations are fundamental to identifying the limitations of current wind products to characterizing long-term trends and understanding air-sea carbon exchange. Plain Language Summary 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 used. Direct wind observations from the array of approximately 70 moored buoys spanning the tropical Pacific are critical to our ability to monitor the system for long-term trend.