Shipboard Measurements of the Distribution of Methane Mixing Ratio and 13CH4 in the Western Pacific: The Impact of Tropical Meteorology

Trace gas transport in the troposphere from the Northern to Southern Hemisphere is profoundly influenced by the position and strength of two meteorological zones: the Inter-Tropical and South Pacific Convergence Zones (ITCZ and SPCZ). In the mid-Pacific these zones are well separated, vary with season and El Nino Southern Oscillation events, and are usually well defined. In the Western Pacific the situation is more complicated with the zones often joining in a wedge east of Indonesia or disappearing altogether. The situation is further complicated by complex tropical meteorology involving deep convection. Here we present the first results of a joint project aimed at examining the impact of the ITCZ and SPCZ on the transport of atmospheric trace gases from the Northern to the Southern Hemispheres in the tropical Western Pacific. The project (developed between the New Zealand National Institute of Water and Atmospheric Research - NIWA, and the Japanese National Institute of Environmental Studies - NIES) uses high precision measurements of methane mixing ratio and the stable methane carbon isotopic species 13CH4 as atmospheric tracers supplemented by measurements of related species such as CO, 13CO, and 14CO. The measurements are made on large clean air samples (up to 1~m3) collected aboard bulk carrier ships operated by the Japanese shipping company Toyofuji on voyages between New Zealand and Japan. These air samples provide excellent snapshots of trace gas mixing ratio and isotopic gradients between the hemispheres as well as in-depth information on changes through the convergence zones. We show the initial results from 6 voyages between May 2004 and January 2007 illustrating the clearly defined latitudinal gradients in mixing ratio and isotopic composition, and their seasonal variations. One particular voyage (December 2005) showcases the distinct "zones" in trace gas distribution as the ship moved from south to north through the SPCZ and ITCZ, and highlights the influence of Asian outflow on methane mixing ratio and isotopes.