Investigating the Synoptic Component in Atmospheric CO2 Variability Using CO2 Mixing Ratio Observations and the Parameterized Chemistry Transport Model

A popular method for estimating the global distribution of terrestrial CO2 sources and sinks has been to invert atmospheric CO2 measurements through the use of atmospheric transport models. Past approaches often utilized monthly mean CO2 measurements in remote marine boundary layer environments. The global network of monitoring stations is growing, however, and continuous measurements are becoming more high frequency and continental. Inversions of continental data will help to improve the inversion results but taking advantage of the measurements requires a better understanding of the processes driving the high-frequency variability. This study uses CO2 mixing ratio observations and a forward model to investigate synoptic scale variability (order of days) of atmospheric CO2. There are two main goals: 1) evaluate model performance in capturing seasonal and synoptic scales of variability over the continent, and 2) determine the physical processes that occur on synoptic time scales during frontal passage events and how they contribute to patterns seen in continuous observations. To simulate the transport of CO2 in the atmosphere we use the Parameterized Chemistry Transport Model (PCTM) driven by surface CO2 fluxes and GEOS4 reanalysis. Terrestrial CO2 fluxes are provided by a new version of the Simple Biosphere Model (SiB3). The results are compared to a network of well-calibrated continuous CO2 mixing ratio measurements in North America. Our results show that PCTM does a reasonable job capturing seasonal and synoptic variability at the measurement sites. The seasonal cycle is well represented by the model, both in timing of spring drawdown and fall leaf senescence and seasonal structure. Synoptic variability is captured surprisingly well throughout the year at most of the stations. We find several cases where horizontal advection of remotely generated CO2 anomalies of biospheric and anthropogenic origin contribute significantly to the synoptic signal. We also find coherent events in the model and observations, where similar synoptic patterns are seen at adjacent sites in response to a common synoptic event. This adds an additional emphasis on the importance of horizontal advection.