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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°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 μmol kg−1 yr−1 and pH range from −0.0023 ± 0.0034 to −0.0281 ± 0.0059 decade−1. DIC and acidification trends were consistent with those expected from the atmospheric CO2 concentrations at nearly all latitudinal zones, but were significantly different at some latitudes. We attribute the significantly lower rates observed in the central western Subarctic Gyre and southern Subtropical Gyres primarily to the variabilities in upward DIC supply from 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 mixing related to the change in its stable/unstable paths. Plain Language Summary: CO2 uptake by the ocean leads to a change in the state of CO2 chemistry in seawater, commonly referred to as "ocean acidification," and also plays an important role in mitigating global warming by reducing the atmospheric CO2 concentration. To project future ocean acidification under different socio‐economic scenarios, we need to understand the current trends in oceanic CO2 concentrations and acidification and the impact of climate change on these trends. Based on the data obtained from shipboard observations extending from the subarctic to equatorial zones in the western North Pacific over the last 25 years, we observed a clear meridional variability in ocean acidification trends, which was generally consistent with the trends expected from the increase in the atmospheric CO2 concentration. However, in some regions, the trends in oceanic CO2 concentration and acidification in surface seawater are likely influenced by the temporal variability in ocean circulation, in addition to the atmospheric CO2. These results suggest that the sensitivity of ocean acidification trends to climate change varies greatly between oceanic regions within the western North Pacific. Key Points: Meridional variability was found in the trends of dissolved inorganic carbon in the surface layer along the 165°E repeat lineExtremely slow rates of increase observed in the subarctic and tropics are attributed to ocean circulation variabilityA fast increase in the south of the Kuroshio Extension is likely associated with the variability in its path [ABSTRACT FROM AUTHOR]

To quantitatively assess the inorganic carbon cycle in the eastern Indian sector of the Southern Ocean (80–150°E, south of 60°S), we measured ocean surface temperature, salinity, total alkalinity (TA), the partial pressure of carbon dioxide (pCO2), and concentrations of chlorophyll‐a (chl a), dissolved inorganic carbon (DIC), and nutrients during the KY18 survey (December 2018–January 2019). The sea–air CO2 flux in this region was −8.3 ± 12.7 mmol m−2 day−1 (−92.1 to +10.6 mmol m−2 day−1). The ocean was therefore a weak CO2 sink. Based on the DIC and TA in the temperature minimum layer, we estimated the change of pCO2 from winter to summer (δpCO2) due to changes in water temperature, salinity, and biological activity (photosynthesis). The spatial distribution of pCO2 in the western part (80–110°E) of the study area was mainly driven by biological activity, which decreased pCO2 from December to early January, and in the eastern part (110–150°E) by temperature, which increased pCO2 from January to February. We also examined the changes in the CO2 concentrations (xCO2) over time by comparing data from 1996 with our data (2018–2019). The oceanic and atmospheric xCO2 increased by 23 and 45 ppm in 23 years, respectively. These changes of ocean xCO2 were mainly driven by an increase in CO2 uptake from the atmosphere as a result of the rise in atmospheric xCO2 and increase in biological activity associated with the change in the water‐mass distribution. Plain Language Summary: Determining the drivers of seasonal changes in ocean CO2 is important for estimating the global carbon cycle. Seawater samples were collected in the eastern Indian sector of the Southern Ocean to investigate the carbonate chemistry of the ocean surface water. During winter and summer, the partial pressure of carbon dioxide (pCO2) at the ocean surface decreased and increased due to photosynthesis and temperature changes, respectively. Surface water pCO2 was lower than the pCO2 of the atmosphere in summer: the result was CO2 uptake by the ocean. A comparison of data from 1996 with our data (2018–2019) indicated that the CO2 concentration (xCO2) of the seawater had increased by 23 ppm. This increase was due primarily to an increase in uptake of CO2 from the atmosphere as a result of the rise of atmospheric xCO2. Key Points: The eastern Indian sector of the Southern Ocean absorbs atmospheric CO2 weakly in summerBiological activity and increases in water temperature change the pCO2 from winter to summerChanges in atmospheric pCO2 and water masses explained the interannual variations of dissolved CO2 in the ocean [ABSTRACT FROM AUTHOR]

We evaluated the contribution of net biological effects (photosynthesis + respiration + decomposition) to the carbonate parameters and air–water CO2 fluxes in Tokyo Bay, Ise Bay, and Osaka Bay in Japan. The carbonate parameters (fugacity of CO2, total alkalinity, and dissolved inorganic carbon) were measured mainly by cargo ships traveling between Japan and other countries. We used the measurement data from three inner bays and surrounding outer bays in Japan along with reference data from previous studies for complementary analysis. We found that (a) the inner and outer bays in this study were strong annual atmospheric CO2 sinks, (b) the annual biological effect on the air–water CO2 fluxes was about 6%–27% of the measured CO2 fluxes, and it affected the seasonal variation of the CO2 flux, and (c) the biological effect was largest in Tokyo Bay, and almost the same in Ise and Osaka Bays. The intensity of the biological effect corresponded mainly with nutrient concentrations, which seemed to be controlled by the wastewater treatment in urbanized areas around the bays. Our results suggest that labile carbon/nutrient ratio of wastewater should be a major consideration for evaluating the biological effect on the carbon cycle in urbanized bays, which will continue to expand globally. Plain Language Summary: We analyzed the biological effect (photosynthesis, respiration, and decomposition) on air–water CO2 exchange in Tokyo Bay, Ise Bay, and Osaka Bay in Japan using data from cargo‐ship measurements and previously published reports. We concluded that (a) bay water strongly absorbs atmospheric CO2, (b) biological effects accounted for 6%–27% of the evaluated CO2 absorption and had significant effects on its seasonal variation, and (c) the biological effects seemed to be mediated mainly by the low degradable carbon/nutrient ratio in wastewater. This study should improve our understanding of the carbon flow in urbanized coastal areas, which are expanding globally. Key Points: Urbanized bays in Japan are reportedly annual atmospheric CO2 sinks but comprehensive measurements are fewWe quantified biological effects on carbonate parameters and CO2 flux using an empirical relationship with riverine and oceanic endmembersThe biological effect was regulated mainly by nutrient concentrations related to wastewater treatment [ABSTRACT FROM AUTHOR]