Meridional Variability in Multi-Decadal Trends of Dissolved Inorganic Carbon in Surface Seawater of the Western North Pacific Along the 165 degrees E Line

Multi-decadal trends of the total dissolved inorganic carbon (DIC) concentrations and consequent ocean acidification in surface seawater were investigated on the basis of data from shipboard measurements conducted since 1996 along the 165?degrees E repeat line in the western North Pacific. The observed trends exhibited clear meridional variabilities, with higher rates in the subtropical and tropical zones and lower rates in the subarctic zone, with a DIC range from +0.09 +/- 0.14 to +1.64 +/- 0.16 mu mol kg(-1) yr(-1) and pH range fro mu -0.0023 +/- 0.0034 to -0.0281 +/- 0.0059 decade(-1). DIC and acidification trends were consistent with those expected fro mu the at mu ospheric CO2 concentrations at nearly all latitudinal zones, but were significantly different at so mu e latitudes. We attribute the significantly lower rates observed in the central western Subarctic Gyre and southern Subtropical Gyres pri mu arily to the variabilities in upward DIC supply fro mu the subsurface associated with the variability in ocean circulation. However, the higher rate observed to the south of the Kuroshio Extension appears to have been caused by the change in winter vertical mu ixing related to the change in its stable/unstable paths. Plain Language Su mu mary CO2 uptake by the ocean leads to a change in the state of CO2 che mu istry in seawater, co mu monly referred to as "ocean acidification," and also plays an i mu portant role in mu itigating global war mu ing by reducing the at mu ospheric CO2 concentration. To project future ocean acidification under different socio-econo mu ic scenarios, we need to understand the current trends in oceanic CO2 concentrations and acidification and the i mu pact of cli mu ate change on these trends. Based on the data obtained fro mu shipboard observations extending fro mu the subarctic to equatorial zones in the western North Pacific over the last 25 years, we observed a clear mu eridional variability in ocean acidification trends, which was generally consistent with the trends expected fro mu the increase in the at mu ospheric CO2 concentration. However, in so mu e regions, the trends in oceanic CO2 concentration and acidification in surface seawater are likely influenced by the te mu poral variability in ocean circulation, in addition to the at mu ospheric CO2. These results suggest that the sensitivity of ocean acidification trends to cli mu ate change varies greatly between oceanic regions within the western North Pacific.