Concentration variations of atmospheric CO₂ observed at Syowa Station, Antarctica from 1984 to 2000

Systematic and continuous measurements of the atmospheric CO₂ concentration have been carried out at Syowa Station, Antarctica since February 1984. The measurement system was renewed in 1995, but the continuity of the data from the two systems was confirmed by operating them simultaneously. The CO₂ data taken for 17 years from 1984 to 2000 showed clear evidence for a seasonal cycle, a secular trend and interannual variations. The seasonal cycle was variable from year to year, with especially larger amplitudes in 1992 and 1998 and a large phase delay in 1993. A rapid increase in the CO₂ concentration was observed in 1987, 1994 and 1998 in association with ENSO events. The average rate of the secular CO₂ increase for the last 17 years was calculated to be 1.49 ppmv yr^-1. Short-term CO₂ variations with amplitudes of around 1.0 ppmv were found in the austral summer season of several years after 1990, probably due to an intrusion of CO₂-depleted air mass into the Antarctic region.