The changing patterns of ΔpCO2 between ocean and atmosphere

Abstract

The average difference between the partial pressure of CO2 in ocean surface water and the overlying atmosphere (ΔpCO2 = pCO2,ocean) ‐ pCO2,atm) has been changing ever since the beginning of the anthropogenic era due to the dumping of CO2 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 (ΔpCO2>0) and sinks (ΔpCO2<0) during this anthropogenic transient. The regional correction, ΔpCO2*, is defined as the quantity that must be added to a region's preanthropogenic ΔpCO2 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 ΔpCO2* 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 ΔpCO2* 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 ΔpCO2* 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 ΔpCO2* 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 ΔpCO2* magnitudes at least twice as large as the magnitude of ΔpCO2* for the average global ocean.