Influence of continental air mass transport on atmospheric CO2 in the western North Pacific

Since 1993, atmospheric carbon dioxide (CO2) has been continuously observed by the Japan Meteorological Agency at Minamitorishima station (24 degrees 18'N, 153 degrees 58'E), located about 2000 km off the Asian continent in the western North Pacific. The long-term record shows high-frequency measurements with interesting episodic events with extremely low CO2 mixing ratios 5-10 ppm below the background seasonal cycle. These extremely low CO2 (ELC) events occur several times each year, primarily in July, August, and September, although the number of events varies from year to year. The origins of air masses associated with the ELC events were defined by backward trajectory analyses as well as chemical characterizations using simultaneous observations of other trace gases (CO, CH4, and O-3). The results indicate that the air masses with extremely low CO2 were influenced by active biospheric uptake in summer over different continental sink regions in Siberia, northern Asia, and Southeast Asia due to rapid long-range transport driven by strong northerly or southerly winds. The spatial scale of the widespread low-CO2 distribution for the ELC events in 2001 was captured by a simulation experiment using a three-dimensional chemical transport model. It clearly revealed that the Intertropical Convergence Zone around 20 degrees N in the western North Pacific during summer blocked further southward intrusion of ELC events through the lower troposphere.