Characteristics of Interannual Variability in Space-based XCO₂ Global Observations

Abstract. Atmospheric carbon dioxide (CO₂) accounts for the largest radiative forcing among anthropogenic greenhouse gases. There is, therefore, a pressing need to understand the rate at which CO₂ accumulates in the atmosphere, including the interannual variations (IAV) in this rate. IAV in the CO₂ growth rate is a small signal relative to the long-term trend and the mean annual cycle of atmospheric CO₂, and IAV is tied to climatic variations that may provide insights into long-term carbon-climate feedbacks. Observations from the Orbiting Carbon Observatory-2 (OCO-2) mission offer a new opportunity to refine our understanding of atmospheric CO₂ IAV since the satellite can measure over remote terrestrial regions and the open ocean where traditional in situ CO₂ monitoring is difficult. In this study, we analyze the IAV of column-averaged dry air CO₂ mole fraction (XCO₂) from OCO-2 between September 2014 to June 2021. The amplitude of IAV variations is up to 1.2 ppm over the continents and around 0.4 ppm over the open ocean. Across all latitudes, the OCO-2 detected XCO₂ IAV shows a clear relationship with ENSO-driven variations that originate in the tropics and are transported poleward. The XCO₂ IAV timeseries shows similar zonal patterns compared to ground-based in situ observations and with column observations from the Total Carbon Column Observing Network (TCCON). At lower degrees of aggregation (i.e., 5°x5° grid cells), there are larger inconsistencies with TCCON suggesting that one or both of the observing systems are affected by bias or systematic retrieval issues that are of a similar magnitude to the IAV signal. Our results suggest that OCO-2 IAV provides meaningful information about climate-driven variations in carbon fluxes and provides new opportunities to monitor climate-driven variations in CO₂ over open ocean and remote regions.