Abstract
Oyashio Current-influenced northwestern North Pacific Ocean margin (the Oyashio Region, east of Japan) represents an important area for intermediate water mass formation, hence the transport of anthropogenic CO2 into the interior North Pacific. Currently, it is unclear how the air-sea CO2 fugacity gradient in the climatically sensitive Oyashio Region has been behaving in recent decades. Taking advantage of a community-based Surface Ocean CO2 Atlas (SOCAT, Version 5), we examined the sea surface fCO2 trends in 0.5° × 0.5° grids in the east Japanese margin (32.0°N–43.5°N, 140.0°E–147.0°E). In the Oyashio Region, seawater fCO2 increased significantly at a rate of 2.86 ± 0.92 µatm yr−1, greater than the atmospheric fCO2 increase rate (1.95 ± 0.03 µatm yr−1). Because of lower oceanic fCO2 values in this region compared with the atmosphere, the faster seawater fCO2 increase suggests that the Oyashio Region probably represents a decreasing CO2 sink in the past decades. Furthermore, the trend of the non-temperature controlled fCO2 change (3.22 ± 1.11 µatm yr−1) suggests that processes such as (1) enhanced respiration based on the increasing export of organic carbon from the neighboring Okhotsk Sea, and (2) reduced primary production in the Oyashio Region itself due to increased water column stratification, may have contributed to the faster surface fCO2 increase.
Published Version
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