No abstract

Half‐century‐long observations at the 137°E repeat hydrographic section across the western North Pacific have been analyzed to demonstrate remotely forced decadal physical and biogeochemical variability of Subtropical Mode Water (STMW) over the last 40 years. During unstable periods of the Kuroshio Extension (KE) that lagged the warm phase of the Pacific Decadal Oscillation by 3–4 years, high regional eddy activity reduced the formation rate and salinity of STMW in its main formation region south of the KE. At the 137°E section south of Japan, decreasing southwestward advection of oxygen‐rich STMW from the formation region resulted in decreases of its cross‐sectional area, dissolved oxygen, pH, and aragonite saturation state and increases of nutrients and dissolved inorganic carbon, among which changes of the carbon system parameters accelerated their long‐term trends. Such changes reversed and acidification slowed down during stable‐KE periods, especially in the current period since 2010 exhibiting a hiatus of acidification.

Improved spatial and temporal representation of total alkalinity (TA) is expected to be an important component in monitoring changes in the oceanic carbon cycle and acidification over the coming decades. For this reason, previous authors have sought to develop and apply empirical methods to characterize TA in the surface ocean. However, there are regions such as the North Pacific that have proven difficult to successfully represent through empirical relationships based on temperature and salinity with linear regression. Here we propose a new empirical approach for reconstructing TA for the Pacific basin using sea surface salinity and sea surface dynamic height (SSDH). We propose five zones of the Pacific basin where the empirical relationships are applied separately. The root-mean-square error of the fittings of these equations to the measured TA is 7.8 lmol kg 21 . The SSDH-based empirical equation helps especially to represent the TA in the North Pacific subtropical-subarctic frontal zone where salinity-normalized TA as well as other oceanographic variables exhibits a large meridional gradient and sizeable formation of Central Mode Water and Subtropical Mode Water occurs.

Because annual anthropogenic CO2 emissions have grown rapidly over the past decades, there is a concern that anthropogenic CO2 invasion into the ocean may also have caused the rate of ocean acidification to increase. Here, we report the decadal and longer‐term variability in the rates of change of inorganic carbon variables since the early 1980s in surface seawaters of various oceanic regions along the 137°E repeat line in the western North Pacific. In the subtropical frontal zone, we found that the mean rate of acidification tracked the acceleration of the atmospheric CO2 increase; during 2008–2017, the rate of acidification was 30% faster than during 1983–2017. In the Kuroshio Recirculation and tropical zone, acidification trends were clear, but the trends were modulated by decadal variability associated with temporal variability in regional ocean circulation.

Identifying ocean acidification and its controlling mechanisms is an important priority within the broader question of understanding how sustained anthropogenic CO2 emissions are harming the health of the ocean. Through extensive analysis of observational data products for ocean inorganic carbon, here we quantify the rate at which acidification is proceeding in the western tropical Pacific Warm Pool, revealing −0.0013 ± 0.0001 year−1 for pH and −0.0083 ± 0.0007 year−1 for the saturation index of aragonite for the years 1985–2016. However, the mean rate of total dissolved inorganic carbon increase (+0.81 ± 0.06 μmol · kg−1 · year−1) sustaining acidification was ~20% slower than what would be expected if it were simply controlled by the rate of atmospheric CO2 increase and transmitted through local air‐sea CO2 equilibration. Joint Lagrangian and Eulerian model diagnostics indicate that the acidification of the Warm Pool occurs primarily through the anthropogenic CO2 that invades the ocean in the extra‐tropics is transported to the tropics through the thermocline shallow overturning circulation and then re‐emerges into surface waters within the tropics through the Equatorial Undercurrent from below. An interior residence time of several years to decades, acting in conjunction with the accelerating CO2 growth in the atmosphere, can be expected to contribute to modulating the rate of Warm Pool acidification.