Seasonal Variations in Atmospheric Carbon Dioxide from Observation-Based Flux Products

The ocean is the largest sink of anthropogenic CO2, but carbon exchange between the atmosphere and the ocean can be uncertain on seasonal and multi-year timescales due to sparse observations. Several different methods have been applied to interpolate surface ocean pCO2 observations to generate an ensemble of observation-based flux products. We used three observation-based products – Self-Organizing Map Feed-Forward Neural Network (SOM-FFN), JENA, and Copernicus Environment Monitoring Service (CMEMS) – as boundary conditions to chemical transport model runs to investigate how these different observation-based flux estimates drive variability in atmospheric CO2 mole fractions. We configured the transport model to tag CO2 exchanged from specific regions to identify which regions contribute most of the variability. We compared differences among the simulated atmospheric CO2 fields derived from the observation-based fluxes and compared the atmospheric total column (which can be measured by current satellite missions such as NASA’s OCO-2 and OCO-3) versus the surface level. We focused our analysis on seasonal and multi-decadal trends in atmospheric CO2.

We found that the seasonal cycle of surface CO2 generally ranged about 0.40 ppm from peak to trough (global average), with larger values in the mid-latitudes (30-60°N and 45-60°S). In contrast, the seasonal cycle amplitude in the column was about 0.14 ppm. The simulations generally agreed between the three observation-based flux products in terms of the seasonal amplitude. We also found a north-south gradient in the phasing of the seasonal cycle of tracers. High latitudes showed larger seasonal variation, but lower latitudes had peaks about 1-3 months after high latitudes. Our results reveal that the low-latitude tracers in both hemispheres were generally the dominant contributors to the seasonality of CO2. Our analysis provides a constraint on the flux differences that are detectable from direct atmospheric observations from atmospheric CO2 over the ocean.