Kazuhiro Nemoto

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.

Water masses labeled with global fallout 137Cs formed by subduction in the North Pacific

[1] We provide a first cross section of the 137 Cs concentration along 165°E longitude in the western North Pacific. The 137 Cs profile is characterized by several subsurface cores with high 137 Cs, including two 137 Cs concentration maxima at 20°N, one at 250 m (δ θ ≈ 25.5) and one at 400-500 m (σ θ ≈ 26.0) depths. The shallower maximum is in the density range of North Pacific Subtropical Mode Water (NPSTMW) and the deeper one is in the density range of Lighter Central Mode Water (LCMW). The main 137 Cs cores, therefore, were formed by movements of NPSTMW and LCMW in the interior ocean during the past four decades. The 137 Cs has been transported from subarctic region to subtropics and tropics as a result of subduction.

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.

Interannual variations of net community production and air-sea CO2 flux from winter to spring in the western subarctic North Pacific

The role of spring biological production for the air–sea CO2 flux was quantified in the Western Subarctic Gyre (48◦N, 165◦E), where the vertical profile of temperature revealed the existence of a temperature minimum (Tmin) layer in the North Pacific. The vertical profiles of temperature, salinity, dissolved oxygen, nutrients and dissolved inorganic carbon, DIC, in the upper water column were significantly variable year by year in spring, 1996–2000. Correspondingly, surface seawater at this site in spring was supersaturated with CO2 in 1997, 1999 and 2000, but was undersaturated in 1996 and 1998. The concentrations of DIC and nutrients in the winter mixed layer were estimated from those in the Tmin layer in spring with a correction for particle decomposition based on the apparent oxygen utilization. The net community production (NCP) and air–sea CO2 flux from winter to spring were calculated from the vertically integrated deficits of DIC and nutrients in the upper water column between the two seasons. The calculation of the carbon budget indicated large interannual variations of NCP (0–13 mmol m−2 d−1) and CO2 efflux (4–16 mmol m−2 d−1) for this period. The CO2 efflux was generally low in the year when NCP was high. The close coupling between biological production and CO2 efflux suggested the important role of the changes in the mixed-layer depth, as a key process controlling both processes, especially of the timing, so that a decrease in the mixed-layer depth could result in the activation of biological production. The early biological consumption of the surface DIC concentration could shorten the period for acting as a source for atmospheric CO2 and depress the CO2 efflux in the Western Subarctic Gyre from winter to spring in 1996 and 1998. On the contrary, in 1997, persistently deep vertical mixing until late spring could suppress the biological activity and give rise to long-lasting CO2 efflux.

85Kr measurement system for continuous monitoring at the Meteorological Research Institute, Japan

A 85Kr measurement system for continuous monitoring based principally on the BfS-IAR method (activity measurement of 85Kr by gas counting coupled with gas chromatographic separation, using pure CH4 as carrier and Counting gas) was implemented for the first time in Japan. In this paper, a detailed description of the system and procedures is given and the inter-comparison results of our system with the BfS-IAR system are presented. A consistent temporal concentration change with high accuracy and consistency of the respective data with the BfS-IAR data (maximum difference of 5%) were achieved with the Meteorological Research Institute (MRI) system, which shows that the system is valid and reliable for the purpose of background monitoring for 85Kr in air. Also, the 85Kr monitoring record at the MRI during 1995-2001 is described. The record distinctively shows the Northern Hemispheric background 85Kr concentrations at the mid-latitude and the elevated concentrations affected by the operation of a nuclear fuel reprocessing plant in Tokai-mura, Ibaraki. Japan.

Spatial and temporal variations of surface seawater fCO2 in the Kuroshio off Japan

To examine the annual growth rate of CO2 in the ocean, spatial and temporal variations of the fugacity of CO2 (fCO2) in surface seawater in the Kuroshio off Japan were surveyed, using the data obtained by the Japan Meteorological Agency in the period 1989–1995. Winter surface seawater fCO2 decreased distinctly in the south of the Kuroshio front, where sea surface temperature increased sharply. In other seasons, such a large spatial variability at the Kuroshio front was rarely found. The temporal variation in surface seawater fCO2 at the selected area could be expressed by a rather simple function consisting of annual and bi-annual harmonics and a linear trend. The seasonal variation has a sharp maximum peak in summer, but an indistinct minimum peak in the period from winter to spring. The annual growth rate was 1.5 μatm/yr, significant at the 95% level. The seasonal variation reproduced assuming a thermodynamic dependence of surface seawater fCO2 on water temperature and salinity explains only about ∼24% of the observed seasonal variation.