Shu Saito

Decreasing pH trend estimated from 25-yr time series of carbonate parameters in the western North Pacific

We estimated long-term trends of ocean acidification in surface waters in latitudinal zones from 3°N to 33°N along the repeat hydrographic line at 137°E in the western North Pacific Ocean. Estimates were based on the observational records of oceanic CO2 partial pressure and related surface properties over the last two decades. The computed pH time series both for 25 yr in winter (late January.early February) and for 21 yr in summer (June.July) exhibited significant decreasing trends in the extensive subtropical to equatorial zones, with interannual variations that were larger in summer. The calculated rates of pH decrease ranged from 0.0015 to 0.0021 yr-1 (average, 0.0018 ± 0.0002 yr-1) in winter and from 0.0008 to 0.0019 yr-1 (average, 0.0013 ) 0.0005 yr-1) in summer. The thermodynamic effects of rising sea surface temperature (SST) accounted for up to 44% (average, 15%) of the trend of pH decrease in the subtropical region in winter, whereas a trend of decreasing SST slowed the pH decrease in the northern subtropical region (around 25°N) in summer. We used the results from recent trends to evaluate future possible thermodynamic changes in the upper ocean carbonate system.

Seasonal variation in total inorganic carbon and its controlling processes in surface waters of the western North Pacific subtropical gyre

Abstract Seasonal variation in total inorganic carbon (TCO 2 ) in surface waters of the western North Pacific (137°–152°E) subtropical gyre was analyzed on the basis of measurements of TCO 2 and partial pressure of CO 2 ( p CO 2 sw). The controlling processes including vertical mixing, horizontal advection, and net air–sea CO 2 transport, as well as biological activity, were quantified. The seasonal increase in normalized TCO 2 (NTCO 2 ) from autumn to winter, ranging from 19 to 37 μmol kg −1 in the northern part of the subtropical gyre between 24°N and 30°N, was predominantly accounted for by the upward supply of TCO 2 due to enhanced vertical mixing. The contribution of horizontal advection, estimated from monthly meridional NTCO 2 distributions and the monthly advection field of the Meteorological Research Institute (MRI)s 3D-ocean general circulation model, was insignificant. Analyses of the mixed-layer NTCO 2 budget revealed that biological activity was playing an important role in the decrease in surface NTCO 2 from winter to summer. Annual net community production reached 48±19 gC m −2 between 24°N and 30°N, and 19±16 gC m −2 between 15°N and 23°N.

Distributions and variations in the partial pressure of CO2 in surface waters (pCO2w) of the central and western equatorial Pacific during the 1997/1998 El Niño event

Measurements of partial pressure of CO2 in surface waters (pCO2w) and overlying air (pCO2a) were made in the central and western equatorial Pacific from October 1997 to February 1998 within the period of the 1997/1998 El Nino, which was reported to be the strongest El Nino event on record. The distribution of the pCO2w showed a pattern driven by the eastward movement of western Pacific warm pool and thermodynamic effects (temperature and salinity), which was different from those of the moderate 1986/1987 El Nino and non-El Nino periods. Due to the eastward movement of the warm pool with sea surface temperature (SST) higher than 28.5 °C and sea surface salinity (SSS) lower than 34.5, the pCO2w between 180° and 163°W (347–364 μatm) was almost equal to that of the air (351 μatm). Between 143°E and 180°, the pCO2w tended to increase toward the west (387 μatm at 0°, 144°E in December 1997) along with the SST and SSS. West of 143°E in January 1998, a steep change in pCO2w ranging from 320 to 365 μatm occurred while retaining high SST (>28.5 °C) and SSS (>34.5). This was caused by the advection of surface water from the southern low latitudes that had been affected by biological activity (New Guinea Coastal Current). From December 1997 to January/February 1998, the SSS was usually higher than 34.5 west of 180°, which was significantly high compared to the western equatorial Pacific warm pool. This was probably due to the decrease of the net fresh water input for the western equatorial Pacific and/or the northward migration of surface water from the Southern Hemisphere. The CO2 outflux from the central and western equatorial Pacific (5.5°S–5.5°N, 139.5°E–159.5°W) was estimated to be 0.027 Pg-C/year in December 1997 and 0.038 Pg-C/year in January/February 1998. This presents a significant decrease from the CO2 outflux during the non-El Nino periods (0.34 Pg-C/year in January/February 1989, 0.11 Pg-C/year in September/November 1990) and a slight one from the moderate 1986/1987 El Nino period (0.055 Pg-C/year in January/February 1987). Following the El Nino–Southern Oscillation phenomena, CO2 outflux varied largely in the central equatorial Pacific and little in the western equatorial Pacific.

Autumn CO2 chemistry in the Japan Sea and the impact of discharges from the Changjiang River

We made comprehensive surface water CO2 chemistry observations in the Japan Sea during each autumn from 2010 to 2014. The partial pressure of CO2 (pCO2) in surface water, 312–329 μatm, was 10–30 μatm lower in the Japan Sea than in the same latitude range of the western North Pacific adjacent to Japan. According to the sensitivity analysis of pCO2, the lower pCO2 in the Japan Sea was primarily attributable to a large seasonal decrease of pCO2 associated with strong cooling in autumn, particularly in the northern Japan Sea. In contrast, the lower pCO2 in relatively warm, fresh water in the southern Japan Sea was attributable to not only the thermodynamic effect of the temperature changes but also high total alkalinity. This alkalinity had its origin in Changjiang River and was transported by Changjiang diluted water (CDW) which seasonally runs into the Japan Sea from the East China Sea. The input of total alkalinity through CDW also elevated the saturation state of calcium carbonate minerals and mitigated the effects of anthropogenic ocean acidification, at least during autumn. These biogeochemical impacts of CDW in the Japan Sea last until November, although the inflow from the East China Sea to the Japan Sea almost ceases by the end of September. The long duration of the high saturation state of calcium carbonate benefits calcareous marine organisms. This article is protected by copyright. All rights reserved.