A broad swath across the North Pacific basin uptakes a disproportionately large amount of atmospheric CO2 every year, with the region of most intense uptake located in the North Pacific transition zone, from ∼30N to 40N-45N. Though a net carbon sink on a mean annual basis, the region varies seasonally between a strong sink in winter and a neutral to weak source in summer. Herein we use observational carbon data to investigate processes regulating air-sea CO2 flux in this region on seasonal and annual timescales by quantifying the impacts of temperature, biology, and physics on seawater pCO2. Temperature effects dominate the pCO2 signal seasonally, yet support only a portion of the annual CO2 uptake in the region, via their impact on the solubility of CO2 in seawater. Instead, processes removing carbon from surface waters dominantly support the region's uptake of CO2 on annual timescales: the vertical export of organic carbon to depth, and the geostrophic advection of dissolved inorganic carbon laterally out of the region. We find the location of this carbon sink region, traditionally attributed to a combination of biological and temperature effects, to instead be driven by the steady geostrophic divergence of DIC at these latitudes. Copyright 2012 by the American Geophysical Union.