Auxiliary material for Paper 2007GB003167 Changes in the North Atlantic Oscillation influence CO2 uptake in the North Atlantic over the past 2 decades Helmuth Thomas Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada A. E. Friederike Prowe Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada Marine Biogeochemistry Research Division, Leibniz-Institut für Meereswissenschaften, IFM-GEOMAR, Kiel, Germany Ivan D. Lima and Scott C. Doney Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA Rik Wanninkhof Ocean Chemistry Division, Atlantic Oceanographic and Meteorological Laboratory, NOAA, Miami, Florida, USA Richard J. Greatbatch Department of Oceanography, Dalhousie University, Halifax, Nova Scotia, Canada Theoretical Oceanography Department, Leibniz-Institut für Meereswissenschaften, IFM-GEOMAR, Kiel, Germany Ute Schuster School of Environmental Sciences, University of East Anglia, Norwich, UK Antoine Corbiere Laboratoire d'Oceanographie et du Climat: Experimentation et Approches Numeriques, IPSL, Universite Pierre et Marie Curie, Paris, France Thomas, H., A. E. Friederike Prowe, I. D. Lima, S. C. Doney, R. Wanninkhof, R. J. Greatbatch, U. Schuster, and A. Corbiere (2008), Changes in the North Atlantic Oscillation influence CO2 uptake in the North Atlantic over the past 2 decades, Global Biogeochem. Cycles, 22, GB4027, doi:10.1029/2007GB003167. Introduction In the following section we report results of the pCO2-decomposition analysis. We have decomposed the evolution of the pCO2 into contributions of salinity, temperature, DICnorm and AT,norm to the model pCO2 trends according to equation (1): We show the results of the decomposition analysis in Figure S1 for the 1979-2004 simulation, representing multidecadal trends. Altered evaporation/precipitation and ocean circulation patterns lead in both the model and observations to more saline surface water conditions in the subtropics and western subpolar gyre and fresher conditions along the North Atlantic Current (NAC), eastern subpolar gyre and polar seas [Curry et al., 2003; Curry and Mauritzen, 2005]. The addition (removal) of freshwaters correspondingly dilutes (enhances) surface water DIC and AT. Because changes in DIC and AT have opposite effects on pCO2, the direct net effect of freshwater variations is small to moderate; regions of freshening exhibit a small drop in pCO2 and the reverse in saltier region (Figure S1a). Increasing temperatures [Fung et al., 2005; IPCC, 2001] raise the surface ocean pCO2 almost throughout the entire basin, particularly in the mid-latitude and high-latitude regions. It has to be noted that despite the general trend of global warming, on shorter timescales the evolution of temperature might not necessarily show uniform characteristics, and shows regionally varying trends at multiannual scales, which primarily depend on the NAO phase. Our simulations reproduce the reported spatio-temporal pattern of temperature evolution (Figure S2) [see, e.g., Greatbatch, 2000]. The evolution of DICnorm (Figures S1c and S1d) has been discussed in detail in the main document. AT,norm also increases in the eastern subpolar gyre and decreases in the western subpolar gyre; the resulting pCO2 trends partially counteract the DICnorm signal. The similarity of combined DICnorm and AT,norm regression analysis confirms the dominant role of DICnorm in controlling the pCO2 evolution (Figures S1e and S1f). In support of our line of argument, pursued in the main document, we furthermore show the regressions for surface temperature (Figures S2a-S2c) and surface salinity (Figures S2d-S2f) for three periods. Mean distributions of salinity, DICnorm, AT,norm and deltapCO2 are shown in Figure S3. The mean annual anomalies of DICnorm concentrations and temperature are shown in Figures S4 and S5, respectively. Figures S6-S8 of provide details in order to underpin our analysis of the main drivers of the surface ocean CO2 variability as discussed in the main document in section 3.5. 1. 2007gb003167-fs01.eps Trend regression analysis of the decomposed pCO2 changes for anthropogenic conditions for the 1979-2004 period according to equation (1). Regression is shown for the pCO2 (a) and for the changes in the pCO2 driven by temperature (pCO2, Temp.) (b), by salinity (pCO2, Sal.) (c), by DICnorm. (pCO2, DICnorm.) (d), by ATnorm. pCO2, Alk.,norm.) (e) and simultaneously by DICnorm. and ATnorm. (pCO2, DICnorm.&Alk.,norm.). The black diamonds indicate the locations at 56.1ºN/18.5ºW and 45.8ºN/43.6ºW (see Figures 8 and 9 in the main document). 2. 2007gb003167-fs02.eps Evolution of surface temperature and salinity in the North Atlantic Ocean. Trend regressions of temperature are given for the 1979-2004 (a), 1991-1996 (b) and 1997-2004 (c) periods. Please note the change of scale in Figures S2a and S2d. Trend regressions of salinity are given for the 1979-2004 (d), 1991-1996 (e) and 1997-2004 (f) periods. The black diamonds indicate the locations at 56.1ºN/18.5ºW and 45.8ºN/43.6ºW (see Figures 8 and 9 in the main document). 3. 2007gb003167-fs03.eps Mean distributions of selected parameters. The distribution of salinity (a), DICnorm (b), AT,norm (c), and deltapCO2 (d) are given for 1979. 4. 2007gb003167-fs04.eps Annual averages of anomalies of DICnorm for the years 1980-2004. 5. 2007gb003167-fs05.eps Annual averages of anomalies of sea surface temperature (SST) for the years 1980-2004. 6. 2007gb003167-fs06.eps We compare our simulations to the observations reported by Lüger et al. [2006]. (a) The average decrease of the deltapCO2 is 0.12 ppm a-1, the decrease of the annual minimum deltapCO2 is 1.9 ppm a-1. (b) The mean temperature rises by 0.08 ºC a-1, while the annual minimum temperature decreases by 0.1 ºC a-1. 7. 2007gb003167-fs07.eps Simulated mechanistic details for an eastern subpolar gyre location at 56.1ºN/18.5ºW for the 1995-2004 period. We compare our simulations to the observations reported by Schuster and Watson [2007]. (a) The average increase of the deltapCO2 is 0.6 ppm a-1, the increase of the annual minimum deltapCO2 is 1.2 ppm a-1. (b) The mean temperature rises by 0.02 ºC a-1, the annual minimum temperature rises by 0.04 ºC a-1, while the annual maximum temperature decreases by 0.07 ºC a-1. 8. 2007gb003167-fs08.eps Comparsion of model results for a western Atlantic Ocean location at 39.4ºN/61.3ºW for the 1995-2004 period, located at the intergyre boundary. Validation of model results for a northern subpolar gyre location 55.0ºN/43.1ºW for the 1995-2004 period, located at the intergyre boundary. We compare our simulations to the observations reported by Corbière et al. [2007]. (a) The average increase of the deltapCO2 is 1.1 ppm a-1, the mean temperature rises by 0.09 ºC a-1 (b). (c) The evolution of salinity (see main document for details.) References: Corbiere, A., N. Metzl, G. Reverdin, C. Brunet, and T. Takahashi (2007), Interannual and decadal variability of the oceanic carbon sink in the North Atlantic subpolar gyre, Tellus, 59(2), 168-179, doi: 10.1111/j.1600-0889.2006.00232.x. Curry, R., and C. Mauritzen (2005), Dilution of the northern North Atlantic Ocean in recent decades, Science, 308, 1772-1774. Curry, R., B. Dickson and I. Yashayaev (2003), A change in the freshwater balance of the Atlantic Ocean over the past four decades, Nature, 426, 826-829. Fung, I., S. C. Doney, K. Lindsay, and J. John (2005), Evolution of carbon sinks in a changing climate, Proc. Nat. Acad. Sci. (USA), 102, 11,201-11,206, doi:10.1073/pnas.0504949102. Greatbatch, R. J. (2000), The North Atlantic Oscillation, Stoch. Environ. Res. Risk Assessment, 14, 213-242. IPCC (2001), The scientific basis, in Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, edited by C. A. Johnson, Cambridge Univ. Press, New York. Luger, H., R. Wanninkhof, D. W. R. Wallace and A. Kortzinger (2006), CO2 fluxes in the subtropical and subarctic North Atlantic based on measurements from a volunteer observing ship, J. Geophys. Res., 111, C06024, doi:10.1029/2005JC003101. Schuster, U. and A. J. Watson (2007), A variable and decreasing sink for atmospheric CO2 in the North Atlantic, J. Geophys. Res., 112, C11006, doi:10.1029/2006JC003941. Takahashi, T. et al (2008), Climatological mean and decadal change in surface ocean pCO2, and net sea-air CO2 flux over the global oceans, Deep Sea Res. II, accepted.