MRI measurements of partial pressure of CO₂ in surface waters of the Pacific during 1968–70: re-evaluation and comparison of data with those of the 1980s and 1990s

Abstract

During 1968 to 72, scientists of the Meteorological Research Institute/Japan Meteorological Agency measured CO₂ in the surface seawater and overlying air in the Pacific Ocean quasicontinuously to examine CO₂ exchange between the sea and the atmosphere. From the data remaining in our laboratory from that time, we re-evaluated the partial pressure of CO₂ in surface seawater (pCO^s₂) by taking into account pressure broadening effects due to the use of CO₂-in-N₂ standards, the use of chemical desiccant (Mg(ClO₄)₂), calibration using background air data, seawater temperature rise between the equilibrator and sea surface, the WMO CO₂ mole fraction scale, and differences in pressure during the oceanic CO₂ measurements from those of standards and background air in the nondispersive infra-red gas analyser cell. The overall uncertainty of pCO^s₂ measurements for the cruises from 1968 to 1970 was estimated to be less than 3.6 ¼atm, which allowed us to evaluate temporal variations in the carbonate system over a few decades, but uncertainty was higher (¾ 10 ¼atm) for the cruise from 1971 to 1972 because of serious malfunctions of the system. The re-evaluated pCO^s₂ data from 1968 to 1970 exhibit patterns similar to those observed at the same place and time of year (within 30 days) in 1982/83, 1988, 1995 and 1996, but compared with levels in the 1980s and 1990s the pCO^s₂ level was clearly lower in the wide area of the Pacific except south of the Subtropical Front (STF; 47°S) in the Australian sector. The observed pCO^s₂ increased by 34 ±5 ¼atm (n = 133) for the area 7°N to 35°N, 138°E to 147°E from February 1969 to February 1995, 29 ±5 ¼atm (n = 247) for the area 9°N to 35°N, 138°E to 165°E from February 1970 to February/March 1996, 26 ±7 ¼atm (n = 224) for the area 29°N to 51°N, 170°W from April 1970 to April 1988, and 41 ±9 ¼atm (n = 165) for the area 10°S to 45°S, 148°E to 166°E from January/February 1969 to January/February 1995. In the northern subtropics (7°N to 35°N, 138°E to 147°E), we estimated the long-term increase (35 ±6 ¼atm, n= 133) after removing seasonal variations that were obtained from the pCO^s₂-sea surface temperature (SST) relationship. Observed and seasonally adjusted increases were nearly equal to those of the partial pressure of CO₂ in the air (1.4 ¼atm yr^-1) over the same time intervals. South of the STF, pCO^s₂ increase as found in the subtropics was not detected, mostly due to the large variability of pCO^s₂ (250 to 380 ¼atm in 1968/69) on small spatial scales. The average pCO^s₂ south of the STF showed large variations on time scales of months and years that affect the estimation of the growth rate of atmospheric CO₂.