Various observations show trends toward warmer and earlier springs in the Northern Hemisphere. We hypothesize that the positive trend in the winter Arctic Oscillation (AO) has led to higher winter temperatures, advanced spring, and increased seasonal amplitudes in atmospheric CO2. To test this hypothesis, we modeled leaf-out and terrestrial carbon fluxes using the Simple Biosphere model, Version 2 (SiB2) and the National Centers for Environmental Prediction (NCEP) reanalysis for 1958--2002. We found that our modeled leaf-out trends were consistent with observed phenology and that the winter AO trend can statistically explain 20--70% of the modeled leaf-out trends in the eastern United States and northern Europe. We also found that warmer winter temperatures associated with the positive trend in the winter AO increased winter respiration. At the same time, these warmer winter temperatures advanced the date of leaf-out, increasing the total spring uptake of atmospheric CO2 by plants. These seasonally asymmetric trends toward increased respiration in winter and increased photosynthesis in spring can help explain the trend toward increased seasonal amplitudes in observed atmospheric CO2 concentration.