Temporal variations of atmospheric carbon dioxide in the southernmost part of Japan

We present analysis of the temporal variation of atmospheric CO₂ in the subtropical region of East Asia, obtained aboard a ferry between Ishigaki Island and Hateruma Island, Japan for the period June 1993-April 2005. The annual mean CO₂ concentration increases from 360.1 ppmv in 1994 to 378.4 ppmv in 2004, showing an average growth rate of 1.8 ppmv yr^-1. The growth rate shows interannual variations with high values during ENSO events. The average seasonal CO₂ cycle reaches the maximum in early April and the minimum in mid-September, with a peak-to-peak amplitude of 8.5 ppmv. Numerical simulations using a three-dimensional atmospheric transport model show interannual variations of the CO₂ growth rate similar to the observation, but the amplitude of the seasonal cycle is larger, with maximum concentration appearing earlier than the observation by 1 month. Low CO₂ values observed during the spring of 1998 are likely associated with the 1997/1998 ENSO event. A backward trajectory analysis suggests that they were due to changes in atmospheric transport whereby maritime air masses from the Pacific Ocean dominated over polluted air masses from the Asian Continent. Extreme values (either high or low) of CO₂ are also occasionally observed. A comparison of backward trajectories of air parcels with CO₂ concentration fields calculated using the atmospheric transport model shows that these unusual CO₂ concentrations result from the transport of air affected not only by anthropogenic CO₂ emissions but also by terrestrial biospheric activities mainly in China.