Contrasting regional carbon cycle responses to seasonal climate anomalies across the east-west divide of temperate North America

Interannual variations in climate modulate the gross primary productivity (GPP) and net ecosystem exchange (NEE) of ecosystems across North America. However, on the scale of 100s–1000s km, interannual variations in these fluxes are poorly understood. Here, we examine the dominant modes of variability in GPP and NEE and their relationship to environmental drives using two state-of-the-science flux products: NEE constrained by surface-based and space-based atmospheric CO2 measurements over 2010–2015 and up-scaled GPP from FluxSat over 2001–2017. We find that, despite a more active carbon cycle in eastern North America, the arid western half of North America provides a larger contribution to IAV in GPP (104% of east) and NEE (127% of east) during the growing season (April-September). This occurs because flux anomalies in western North America are temporally coherent across the growing season leading to an amplification of GPP and NEE. Increased productivity and carbon sequestration in western North America is closely associated with cooler and wetter than average conditions. In contrast, IAV in eastern North America is dominated by seasonal compensation effects, wherein positive anomalies during April–June are compensated for by negative anomalies during July–September. These seasonal compensation effects are most closely linked to temperature, with a phase shift of the seasonal cycle to earlier in the year for warmer years. Terrestrial biosphere models in the MsTMIP ensemble partially capture these differences in IAV between eastern and western North America. However, the models underestimate the sensitivity of flux anomalies in western North America to variations in soil temperature and moisture by 25–100%. The results of this study suggest that ecosystems in western North America may be more sensitive to warming and increasing aridity than models predict, suggesting that reductions in growing season productivity and carbon sequestration under climate change may be larger than predicted by models.