The seasonal cycle is the dominant mode of variability in the air-sea CO2 flux in most regions of the global ocean, yet discrepancies between different seasonality estimates are rather large. As part of the Regional Carbon Cycle Assessment and Processes Phase 2 project (RECCAP2), we synthesize surface ocean pCO(2) and air-sea CO2 flux seasonality from models and observation-based estimates, focusing on both a present-day climatology and decadal changes between the 1980s and 2010s. Four main findings emerge: First, global ocean biogeochemistry models (GOBMs) and observation-based estimates (pCO(2) products) of surface pCO(2) seasonality disagree in amplitude and phase, primarily due to discrepancies in the seasonal variability in surface DIC. Second, the seasonal cycle in pCO(2) has increased in amplitude over the last three decades in both pCO(2) products and GOBMs. Third, decadal increases in pCO(2) seasonal cycle amplitudes in subtropical biomes for both pCO(2) products and GOBMs are driven by increasing DIC concentrations stemming from the uptake of anthropogenic CO2 (Cant). In subpolar and Southern Ocean biomes, however, the seasonality change for GOBMs is dominated by Cant invasion, whereas for pCO(2) products an indeterminate combination of Cant invasion and climate change modulates the changes. Fourth, biome-aggregated decadal changes in the amplitude of pCO(2) seasonal variability are largely detectable against both mapping uncertainty (reducible) and natural variability uncertainty (irreducible), but not at the gridpoint scale over much of the northern subpolar oceans and over the Southern Ocean, underscoring the importance of sustained high-quality seasonally resolved measurements over these regions. Plain Language Summary Changes in the seasonal cycle amplitude of surface ocean carbon dioxide partial pressure (pCO(2)) are described over the historical period spanning 1985-2018, using both observation-based and model-based estimates. We identify increasing pCO(2) seasonality over most regions due to increases in anthropogenic carbon in the surface ocean. Observation-based products indicate that there have been important high-latitude changes in pCO(2) seasonality due to perturbations to the climate system. We also find important discrepancies between observation-based and modeled pCO(2) seasonality over much of the globe that are likely associated with systematic biases in model representations of the surface dissolved inorganic carbon (DIC) seasonal cycle and the ratio of total alkalinity to DIC. Both reducible and irreducible forms of uncertainty associated with monitoring pCO(2) seasonality changes are quantified, highlighting the need for sustained, seasonally unbiased measurements over the high latitudes as part of an optimized marine carbon observing system.