Masao Ishii

Aircraft observation of carbon dioxide at 8–13 km altitude over the western Pacific from 1993 to 1999

Abstract The spatial and temporal variations of atmospheric CO2 at 8–13 km from April 1993 to April 1999 were observed by measuring CO2 mixing ratios in samples collected biweekly from a commercial airliner between Australia and Japan. The CO2 growth rate showed a considerable interannual variation, with a maximum of about 3 ppm yr−1 during late 1997. This variation is related to the El Niöo/Southern Oscillation (ENSO) events. A year-to-year change related to the ENSO events was also found in the latitudinal distribution pattern of the CO2 annual mean between 30°N and 30°S. The averaged CO2 seasonal cycle in the Northern Hemisphere gradually decayed toward the equator, and a relatively complicated variation with a double seasonal maximum appeared in the Southern Hemisphere. A significant yearly change of the seasonal cycle pattern was observed in the Southern Hemisphere. The impact of a tropical biomass-burning injection on the upper tropospheric CO2 was estimated on the basis of the CO data from the same airliner observation.

Close coupling between seasonal biological production and dynamics of dissolved inorganic carbon in the Indian Ocean sector and the western Pacific Ocean sector of the Antarctic Ocean

The distribution of total dissolved inorganic carbon (DIC) in surface sea water and the upper water column of the seasonal ice zone in the Antarctic Ocean between 30°E and 150°E was investigated in the austral summer 1992/1993. In February–March 1993, total DIC content of surface seawater in the seasonal ice zone showed large spatial variability, ranging from 2064 to 2166 μmol kg-1. Biological activity played an important role in the deficit of total DIC in Prydz Bay, in the marginal ice zones (MIZ) near Lutzow-Holm Bay and Casey Bay, and in the offshore regions near 63°S, 100°E, while the decrease in total DIC due to meltwater input from the receding sea ice was also significant in those MIZ and in the area off the West Ice Shelf. From the analyses of total DIC concentration in the coldest waters (t<−1.7°C) of the subsurface temperature minimum layer, we deduced a characteristic value of normalized total DIC concentration (2184.0±3.7 μmol kg-1 at S=34) for the winter mixed layer over the wide area we investigated. Seasonally integrated net community production (NCP) in summer and its ΔCT/ΔN/ΔP ratios were calculated on the basis of the difference in normalized total DIC and nutrients concentrations between the winter mixed layer and the Summer Surface Water. Large spatial variations in the NCP, ranging from 10 to 48 gC m-2, and different ΔCT/ΔN/ΔP consumption ratios, typically 58/9.2/1 and 84/9.0/1, suggest the high variability of organic matter production and of its impact on both the air-sea CO2 exchange and export of carbon from the photic layer to the deeper waters in the seasonal ice zone of the Antarctic Ocean.

The international at-sea intercomparison of fCO2 systems during the R/V Meteor Cruise 36/1 in the North Atlantic Ocean

The ‘International Intercomparison Exercise of fCO2 Systems’ was carried out in 1996 during the R/V Meteor Cruise 36/1 from Bermuda/UK to Gran Canaria/Spain. Nine groups from six countries (Australia, Denmark, France, Germany, Japan, USA) participated in this exercise, bringing together 15 participants with seven underway fugacity of carbon dioxide (fCO2) systems, one discrete fCO2 system, and two underway pH systems, as well as systems for discrete measurement of total alkalinity and total dissolved inorganic carbon. Here, we compare surface seawater fCO2 measured synchronously by all participating instruments. A common infrastructure (seawater and calibration gas supply), different quality checks (performance of calibration procedures for CO2, temperature measurements) and a common procedure for calculation of final fCO2 were provided to reduce the largest possible amount of controllable sources of error. The results show that under such conditions underway measurements of the fCO2 in surface seawater and overlying air can be made to a high degree of agreement (±1 μatm) with a variety of possible equilibrator and system designs. Also, discrete fCO2 measurements can be made in good agreement (±3 μatm) with underway fCO2 data sets. However, even well-designed systems, which are operated without any obvious sign of malfunction, can show significant differences of the order of 10 μatm. Based on our results, no “best choice” for the type of the equilibrator nor specifics on its dimensions and flow rates of seawater and air can be made in regard to the achievable accuracy of the fCO2 system. Measurements of equilibrator temperature do not seem to be made with the required accuracy resulting in significant errors in fCO2 results. Calculation of fCO2 from high-quality total dissolved inorganic carbon (CT) and total alkalinity (AT) measurements does not yield results comparable in accuracy and precision to fCO2 measurements.

Changes in longitudinal distribution of the partial pressure of CO2 (pCO2) in the central and western equatorial Pacific, west of 160°W

We describe spatial and temporal variations in the partial pressure of carbon dioxide (pCO2) in the central and western equatorial Pacific on the basis of measurements conducted for the periods between 1987 and 1994. Surface water pCO2 data indicate the significant differences in longitudinal distribution depending on the ocean conditions. We examine the relationship between the area showing higher surface pCO2 values and the El Nino/Southern Oscillation phenomenon by using the Southern Oscillation Index. Results indicate that the area showing higher surface pCO2 values correlates with the SOI, which suggests significant intra- and interannual fluctuations of CO2 outflux from the central and western equatorial Pacific.

Carbon monoxide in the upper troposphere over the western Pacific between 1993 and 1996

Air samples at 8.5–13 km were collected regularly using a commercial airliner between Australia and Japan, and they were measured for CO mixing ratios to obtain time series data from April 1993 to July 1996. When averaged over 12 latitudinal bands between 30°N and 30°S, two overall features emerge from these data. First, CO levels in the upper troposphere decreased in all latitudinal bands. Second, the seasonal cycle showed significant differences between the northern and southern hemispheres. In the southern hemisphere a strong maximum in the CO mixing ratio (up to around 90 ppb) was found in October-November. The most likely source for this enhanced CO is tropical biomass burning. Methane oxidation and transport of industrial CO from the northern hemisphere were estimated as relatively minor sources during the austral spring. Air mass trajectories indicate that an extremely high CO level of ∼130 ppb observed in November 1994 between 10° and 20°S was due to enhanced biomass burning in Southeast Asia and/or northern Australia. On the other hand, air mass trajectories for the 20°-30°S region indicate that CO-rich air from biomass burnings over southern Africa or South America was transported across the South Indian Ocean within ∼1 week by the strong westerly winds around the subtropical jet. Thus it is concluded that a rapid horizontal transport coupled with deep convection plays an important role in the appearance of the CO spring peak in the upper troposphere over the western South Pacific.

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.

Variations and trends of CO2 in the surface seawater in the Southern Ocean south of Australia between 1969 and 2002

Measurements of the partial pressure of CO2 in surface seawater (pCOsw 2 ) were made in the Southern Ocean south of Australia during four cruises in January to February 1969, December 1983 to January 1984, December 1994 to January 1995 and January 2002. The spatial distribution of pCOsw 2 for the four cruises showed the same pattern north of the Sub-Antarctic Front (SAF), while year-to-year changes were noted south of the SAF. We evaluated the long-term trend of the pCOsw 2 representative of the zone between oceanographic fronts by taking into account changes in the seasonal variation in pCOsw 2 and the long-term increase of the sea-surface temperature (SST) of the Southern Hemisphere. The observed growth rate of pCOsw 2 was 0.7 ± 0.1 μatm yr−1 at its minimum, which was observed at the SST of 15 ◦C north of the Subtropical Front (STF), 1.0 ± 0.5 μatm yr−1 in the Sub-Antarctic Zone (SAZ) between STF and SAF, 1.5 ± 0.4 μatm yr−1 in the Polar Frontal Zone (PFZ) between SAF and the Polar Front (PF) and 1.8 ± 0.2 μatm yr−1 in the Polar Zone (PZ) between PF and 62◦S, determined as the northern edge of the Seasonal Sea Ice Zone (SSIZ) on the basis of surface salinity and satellite images. These increases were caused by the uptake of anthropogenic CO2 as well as variations in the thermodynamic temperature effect, ocean transport and biological activity. In the SSIZ between 62 and 66.5◦S, we could not clearly evaluate the long-term trend of pCOsw 2 due to the remarkable CO2 drawdown due to biological activity in January 2002. The relatively low growth rates of pCOsw 2 close to the STF and in the SAZ are probably associated with the formation of Subtropical Mode Water and Sub-Antarctic Mode Water in their respective zones. Between the north of the STF and the PZ, the growth rate of total dissolved inorganic carbon was calculated to be about 0.5–0.8 μmol kg−1 yr−1 via the buffer factor.

Large injection of carbon monoxide into the upper troposphere due to intense biomass burning in 1997

AbstracL Air samples at 8-13 km were collected regularly using a commercial airliner to obtain long-term measurements of carbon monoxide (CO) mixing ratio in the upper troposphere over the western Pacific between Australia and Japan during April 1993 - December 1997. The measurements in 1997 clearly reveal an anomalous CO increase during September to November in the Southern Hemisphere, with a maximum of 320-380 ppb around 20°S in October. Tropical biomass burning, not urban/industrial emissions, was the main source for the enhanced CO in 1997. A similar southern-spring increase due to biomass burning was observed in previous years. The peaks showed a large interannual variation associated with the El Nino/Southern Oscillation (ENSO) events. The largest CO spring peak appeared during the strong El Nino event in 1997, while the weak La Nina year of 1996 was marked by a largely suppressed CO spring peak. The outgoing longwave radiation (OLR) anomaly is largest during the El Nino events indicating that the events cause a longer drought in the tropics and significantly influence the enlargement of biomass burning in tropical Southeast Asia. Thus the most likely cause for the ENSO-cycle CO variability is a year-to-year change of biomass-burning emissions mainly from Southeast Asia. The appearance of the CO spring peak in the southern subtropics is discussed on the basis of the possible long-range transport of biomass-burning CO from Southeast Asia to the upper troposphere over the western South Pacific.

Basin-scale pCO2 distribution using satellite sea surface temperature, Chl a, and climatological salinity in the North Pacific in spring and summer

[1] An empirical method is presented for the estimation of basin-scale distribution of partial pressure of carbon dioxide (pCO 2 ) in the North Pacific using satellite-derived sea surface temperature (SST), chlorophyll-a concentrations (chl a), and climatological sea surface salinity (SSS). In this approach, multiple regression equations were developed to compute mixed layer dissolved inorganic carbon (DIC) based on SST, SSS and Chl a, whereas mixed layer total alkalinity (TA) was linearly regressed with SSS. The DIC-SST relation exhibited three different slopes at SST 27.5°C. Therefore data have been grouped with reference to SST. Regression equations were developed for two seasons (spring and summer). The regression errors for DIC and TA were 10.5 and 5 μmol kg -1 , respectively. The pCO 2 was computed from the estimated DIC and TA using dissociation constants given by Mehrbach et al. (1973), refit by Dickson and Millero (1987). The derived pCO 2 agreed with the shipboard pCO 2 observations within an error of 17-23 μatm. The sensitivity test on the regression equations for DIC estimation indicated that SSS is the most influencing parameter, followed by SST and Chl a. Using the monthly average SST and Chl a fields derived from the Advanced Very High Resolution Radiometer (AVHRR) and SeaWiFS (Sea-viewing Wide Field of view Sensor), respectively, and climatological SSS, monthly basin-scale pCO 2 fields were computed. The statistical model derived pCO 2 results are in agreement with underway pCO 2 in the North Pacific. This study strongly suggests that satellite-based techniques are promising tools for estimation of pCO 2 fields on a basin scale but the associated error bars are larger than required to study anthropogenic carbon uptake by the oceans. Incorporation of more in situ shipboard data may help in refining the estimating equations and reducing the errors further.