The changing patterns of ΔpCO₂ between ocean and atmosphere

The average difference between the partial pressure of CO 2 in ocean surface water and the overlying atmosphere (ΔpCO 2 = pCO 2,ocean ) ‐ pCO 2,atm ) has been changing ever since the beginning of the anthropogenic era due to the dumping of CO 2 as a waste gas into the atmosphere. However, the change in the difference has not been uniform over the surface of the oceans. This work assesses regional variations in the Earth scale patterns of sources (ΔpCO 2 >0) and sinks (ΔpCO 2 <0) during this anthropogenic transient. The regional correction, ΔpCO 2 * , is defined as the quantity that must be added to a region's preanthropogenic ΔpCO 2 to give its present value. Using a five‐box ocean model with special surface regions for the prominent equatorial Pacific source and north Atlantic sink, we show that a larger magnitude of ΔpCO 2 * is necessary for these two regions than for the average ocean. Analytical results with a multiple one‐and‐one‐half‐box model, in which horizontal water exchanges are neglected, indicate that the ratio Of ΔpCO 2 * values for any two regions should vary inversely with the ratio of their gas exchange coefficients and nearly directly with the ratio of their deep‐to‐surface water piston velocities. The ΔpCO 2 * values for regions of the five‐box model vary by as much as a factor of six; predictions of the multiple one‐and‐one‐half‐box model are within a factor of two. Discrepancies from these predictions are believed to be due to horizontal and intermediate‐depth water exchanges, which tend to equalize the ΔpCO 2 * values. Our best estimate is that regions with large vertical water exchanges (equatorial and high‐latitude zones) and/or low gas exchange coefficients (equatorial zones) have ΔpCO 2 * magnitudes at least twice as large as the magnitude of ΔpCO 2 * for the average global ocean.