Abstract
Abstract. Satellite-based observations of atmospheric carbon dioxide (CO2) provide measurements in remote regions, such as the biologically sensitive but undersampled northern high latitudes, and are progressing toward true global data coverage. Recent improvements in satellite retrievals of total column-averaged dry air mole fractions of CO2 (XCO2) from the NASA Orbiting Carbon Observatory 2 (OCO-2) have allowed for unprecedented data coverage of northern high-latitude regions, while maintaining acceptable accuracy and consistency relative to ground-based observations, and finally providing sufficient data in spring and autumn for analysis of satellite-observed XCO2 seasonal cycles across a majority of terrestrial northern high-latitude regions. Here, we present an analysis of XCO2 seasonal cycles calculated from OCO-2 data for temperate, boreal, and tundra regions, subdivided into 5∘ latitude by 20∘ longitude zones. We quantify the seasonal cycle amplitudes (SCAs) and the annual half drawdown day (HDD). OCO-2 SCAs are in good agreement with ground-based observations at five high-latitude sites, and OCO-2 SCAs show very close agreement with SCAs calculated for model estimates of XCO2 from the Copernicus Atmosphere Monitoring Services (CAMS) global inversion-optimized greenhouse gas flux model v19r1 and the CarbonTracker2019 model (CT2019B). Model estimates of XCO2 from the GEOS-Chem CO2 simulation version 12.7.2 with underlying biospheric fluxes from CarbonTracker2019 (GC-CT2019) yield SCAs of larger magnitude and spread over a larger range than those from CAMS, CT2019B, or OCO-2; however, GC-CT2019 SCAs still exhibit a very similar spatial distribution across northern high-latitude regions to that from CAMS, CT2019B, and OCO-2. Zones in the Asian boreal forest were found to have exceptionally large SCA and early HDD, and both OCO-2 data and model estimates yield a distinct longitudinal gradient of increasing SCA from west to east across the Eurasian continent. In northern high-latitude regions, spanning latitudes from 47 to 72∘ N, longitudinal gradients in both SCA and HDD are at least as pronounced as latitudinal gradients, suggesting a role for global atmospheric transport patterns in defining spatial distributions of XCO2 seasonality across these regions. GEOS-Chem surface contact tracers show that the largest XCO2 SCAs occur in areas with the greatest contact with land surfaces, integrated over 15–30 d. The correlation of XCO2 SCA with these land surface contact tracers is stronger than the correlation of XCO2 SCA with the SCA of CO2 fluxes or the total annual CO2 flux within each 5∘ latitude by 20∘ longitude zone. This indicates that accumulation of terrestrial CO2 flux during atmospheric transport is a major driver of regional variations in XCO2 SCA.
Highlights
The changing climate influences carbon exchange in every ecosystem on the planet, and polar amplification is driving more rapid changes at higher latitudes (Smith et al, 2019; Park et al, 2018; Pithan and Mauritsen, 2014; Holland and Bitz, 2003; Manabe and Wetherald, 1975)
Agreement between model-derived and observed half drawdown day (HDD) is better for the single-point model estimates nearest the ground site versus noon ground-based (NNG) results in panel (d) of Fig. 2, and the scatter increases in the comparisons of HDD from spatially averaged model estimates versus spatially averaged Orbiting Carbon Observatory 2 (OCO-2) observations in panels (e) and (f) of Fig. 2
Our results show that the Asian boreal forest region is distinct from other northern high-latitude regions with larger seasonal cycle amplitude (SCA) and earlier half drawdown day (HDD), and gradients of increasing seasonal cycle amplitudes (SCAs) and earlier HDD span from west to east across the Eurasian continent
Summary
The changing climate influences carbon exchange in every ecosystem on the planet, and polar amplification is driving more rapid changes at higher latitudes (Smith et al, 2019; Park et al, 2018; Pithan and Mauritsen, 2014; Holland and Bitz, 2003; Manabe and Wetherald, 1975). Many studies have combined process-based and atmospheric transport modeling with in situ and airborne observations to infer long-term temporal trends and spatial distributions of seasonal CO2 exchange and concluded that boreal forest regions play an essential role in global carbon dynamics (Lin et al, 2020; Yin et al, 2018; Piao et al, 2017; Barlow et al, 2015; Bradshaw and Warkentin, 2015; Gauthier et al, 2015; Graven et al, 2013; Pan et al, 2011; Tans et al, 1990). Lin et al (2020) found that even though Siberia is a relatively small source region, fluxes from Siberia were the second most influential in determining SCA of in situ CO2 on a global scale, following those from Northern Hemisphere midlatitudes
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