Does GOSAT Capture the True Seasonal Cycle of Column-Averaged CO2?

Abstract:

Atmospheric carbon dioxide concentration varies seasonally as driven by biospheric productivity. The amplitude of the seasonal cycle strongly depends on location, especially latitude, because it is indicative of the local biospheric CO₂ fluxes but also a result of atmospheric transport. The Greenhouse Gases Observing Satellite (GOSAT) provides global data on column-averaged CO₂ concentrations (XCO₂) and now, after five years of operation, makes it possible to regionally estimate the amplitude of the XCO₂ seasonal cycles. However, satellite-retrieved XCO₂ can be subject to biases in the retrieval system, and these potential biases are best identified through validation against ground-based measurements and model data.

In this study, we quantitatively evaluate the XCO₂ seasonal cycle of GOSAT observations from the Atmospheric Carbon Observations from Space (ACOS) retrieval system, and compare the regional seasonal cycles to those directly measured by the Total Carbon Column Observing Network (TCCON) stations and those predicted by several carbon cycle models (e.g., MACC, CarbonTracker) that assimilate in situ measurements. We extend the analysis with a look to the latitudinal dependence of the seasonal cycle amplitude throughout the northern hemisphere. Although preliminary results show a generally good agreement between models and both satellite and surface observations, they also unveil some surprisingly large discrepancies between the biosphere models, and up to 10% differences in the seasonal cycle amplitudes seen by TCCON and GOSAT. This validation study lays ground work to the analysis of OCO-2 observations that also utilize the ACOS retrieval system, and are anticipated to be a major source of information on local XCO₂ seasonal cycles in the near future.