GLOBAL BIOGEOCHEMICAL CYCLES, VOL. 3, NO. 2, PAGES 179-189, JUNE 1989 THE CHANGING PATI'E•S OF ApCO2 BETWEEN OCEAN AND ATMOSPHERE Tyler Volk Earth Systems Group,Department of Applied Science, New York University, New York Robert Bacastow Geological Research Division, Scripps Institution of Oceanography, La Jolla, California Abstract. The average difference between the partial pressure of CO2 in ocean surface waterand the overlying atmosphere (ApCO2 = pCO2,ocean- pCO2, atm) has been changing ever sincethe beginning of the anthropogenic era due to the dumpingof CO2 as a wastegasinto the atmosphere. However, the changein the difference has not been uniform over the surface of the oceans. This work assesses regional variations in the Earth scale patterns of sources (ApCO2>0) and sinks (ApCO2<0) during this anthropogenic transient. The regional correction, ApCO2*, is defined asthe quantity that must be added to a region's preanthropogenic ApCO2 to giveits present value. Usinga five-boxocean modelwith special surface regions for theprominent equatorial Pacific source andnorthAtlanticsink, we show that a largermagnitude of ApCO2* is necessary for these two regions thanfor the average ocean. Analytical results with a multiple one-and- one-half-box model,in whichhorizontal water exchanges areneglected, indicate thatthe ratioof ApCO2* values for any two regionsshouldvary inverselywith the ratio of theirgasexchange coefficients andnearly directly with the ratio of their deep-to-surface waterpiston velocities. The ApCO2*values for regions of thefive-box model varyby asmuchasa factor of six;predictions of themultiple one- and-one-half-box model are within a factor of two. Discrepancies from thesepredictions are believedto be dueto horizontal andintermediate-depth waterexchanges, Copyright 1989 by the American Geophysical Union. Paper number 89GB01528 0886-6236/89/89GB-01528510.00 which tend to equalize the ApCO2* values. Our best estimate is thatregions with large vertical water exchanges (equatorial and high-latitude zones) and/or low gas exchange coefficients(equatorial zones) have ApCO2* magnitudes at least twice as large as the magnitude of ApCO2* for the average global ocean. INTRODUCTION One of the major discoveries duringthe International Geophysical Year 30 years ago was that the oceanand atmosphere display large scale patterns of disequilibrium in CO2 [Keeling, 1968]. Since then many more measurements have addeddetail to the picture of where the oceans outgas CO2 to the atmosphere (source regions, where ApCO2 > 0; ApCO2 = pCO2,ocean- pCO2,atm) and whereingassing occurs (sink regions, whereApCO2< 0). Maps of globalpatterns have beenconstructed [Keeling, 1968; Takahashi et al., 1983; Broecker et al., 1986], and seasonality andother details of the Earth scale source/sink system have been elucidated [Weiss et al., 1982; Takahashi et ai., 1983; Chen, 1984; Smethie et al., 1985; Takahashi et al., 1985; Andri6 et al., 1986; Brewer, 1986; Takahashiet al., 1986; Peng et al., 1987; Takahashiet al., 1988;Volk andLiu, 1988]. The carbon chemistry of the global ocean surface is in a transient due to anthropogenic CO2 entering from the atmosphere. Because the patterns of the source and sink regions give clues to the operation of thecarbon cycle, it is important to inquire into how an½ why the patternsof ApCO2 have changedsince the preanthropogenic oceanic state, and how they will presumably continue to change. Even thoughboth large-scalesource and large-scale sink regions are obsen•ed, the globaloceans must be a net sink for atmospheric CO2 because of the knownsources