We observe significant interannual variability in the strength of the seasonal cycle drawdown in northern midlatitudes from measurements of CO₂ made by the Total Carbon Column Observing Network (TCCON) and the Greenhouse Gases Observing Satellite (GOSAT). This variability correlates with surface temperature in the boreal regions. Using TCCON measurements, we find that the slope of the relationship between the X_CO2 seasonal cycle minima and boreal surface temperature is 1.2 ± 0.7 ppm K⁻¹. Assimilations from CarbonTracker 2011 and CO₂ simulations using the Simple Biosphere exchange Model (SiB) transported by GEOS-Chem underestimate this covariation. Both atmospheric transport and biospheric activity contribute to the observed covariation.