Transport of carbon monoxide from the tropics to the extratropics

Abstract

Global observations of carbon monoxide (CO) from the Measurements of Pollution in the Troposphere (MOPITT) instrument on the NASA Terra satellite and three‐dimensional trajectories computed from analyzed winds are used independently to study the transport of air from the tropics to the extratropics. During southern hemisphere spring (September through November), biomass burning in the southern tropics produces large‐scale plumes of CO. These plumes can be easily distinguished from the clean air of the southern hemisphere extratropics. Both total column CO maps and latitude‐height cross‐sections of CO show a strong gradient of CO between 30 and 40°S. Climatological trajectory calculations show that air originating in the lower troposphere near the tropical biomass‐burning regions generally rises into the middle and upper troposphere, where it is entrained in the equatorward side of the subtropical jet. While the zonal dispersion of air parcels within the tropics and subtropics is relatively rapid, air disperses rather slowly across the jet. The MOPITT CO data thus confirm the results from the trajectory analysis that transport from the tropics to the extratropics is a comparatively slow process. This gives rise to the appearance of “transport barriers” in the subtropics.