Seasonal cycles of mixing ratio and 13C in atmospheric methane at Suva, Fiji

Abstract

A series of clean air samples has been collected at a coastal site near Suva, Fiji (18°08′S, 178°26′E) by researchers at the University of the South Pacific. These samples, covering the period 1994 to mid‐2002, have been analyzed for methane mixing ratio and δ13C and provide the first ever time series of these species reported for this part of the tropical South Pacific. The data show large variability when compared to similar time series of the same species measured farther south in the extratropical Pacific. In particular, summer variability at the Fiji site is high, especially through La Niña conditions. A modeling study was carried out using a modified version of the UK Meteorological Office's Unified Model (a general circulation model) and TM2 (a chemical transport model driven by stored meteorological fields). These showed that a large amount of the variability in the methane mixing ratio and its δ13C can be attributed to complex tropical meteorology in the region changing the rate of transport of methane from the Northern into the Southern Hemispheres. Enhanced interhemispheric transport occurred during the summer months, especially during La Niña conditions which lead to the suppression of expected minima in the methane mixing ratio caused by OH oxidation. Although enriched signals in δ13C were expected at the site caused by intrusions of methane emitted from tropical biomass burning in Indonesia, relatively few of these events could be identified in the time series.