A wintertime uptake window for anthropogenic CO₂ in the North Pacific

An ocean model has been forced with NCEP reanalysis fluxes over 1948-2003 to evaluate the pathways and timescales associated with the uptake of anthropogenic CO₂ over the North Pacific. The model reveals that there are two principal regions of uptake, the first in the region bounded by 35-45°N and 140-180°E, and the second along a band between 10-20°N and between 120°W and 180°E. For both of these regions, the dominant timescale of variability in uptake is seasonal, with maximum uptake occurring during winter and uptake being close to zero or slightly negative during summer when integrated over the basin. A decadal trend toward increased uptake of anthropogenic CO₂ consists largely of modulations of the uptake maximum in winter. For detection of anthropogenic changes, this implies that in situ measurements will need to resolve the seasonal cycle in order to capture decadal trends in ΔpCO₂. As uptake of anthropogenic CO₂ occurs preferentially during winter, observationally based estimates which do not resolve the full seasonal cycle may result in underestimates of the rate of uptake of anthropogenic CO₂. There is also a sizable circulation-driven decadal trend in the seasonal cycle of sea surface ΔpCO₂ for the North Pacific, with maximum changes found near the boundary separating the subtropical and subpolar gyres in western and central regions of the basin. These changes are due to a trend in the large-scale circulation of the gyres, which itself is driven by a trend in the wind stress over the basin scale. This trend in the three-dimensional circulation is more important than the local trend in mixed layer depth (MLD) in contributing to the decadal trend in ΔpCO₂.