Inter-hemispheric Exchange in the Troposphere by an Atmospheric Transport Model Based on Observed Winds

Abstract

A global atmospheric transport model was developed using a simple semi-Lagrangian method and the inter-hemispheric exchange was investigated. The time evolution of the mixing ratio of an artificial tracer at grid points with resolution of 2.5°horizontal and 14 vertical pressure levels is obtained using three-dimensional wind fields analyzed at the European Centre for Medium Range Weather Forecasts. A steep horizontal gradient zone was observed in the equatorial region of the lower troposphere and indicates a boundary between the hemispheres. The latitude of this boundary changed with longitude and the seasons ; it was located north of the equator, on the average. The boundary coincided with the Inter Tropical Convergence Zone in the Pacific throughout the year and shifted to 30°N in Asia in the northern summer and to 30°S in South America in the northern winter. Monthly flux of mass across this zone was estimated. The maximum mass exchange from south to north occurred in December and the minimum in May ; the maximum from north to south was in May and the minimum in October. The interhemispheric exchange time was about one year. The results of this experiment suggest that sources and sinks of atmospheric constituents along the Indo-China peninsula may affect air quality in the Southern Hemisphere during the northern summer while such sources and sinks in the Amazon Basin may affect air quality in the Northern Hemisphere during the northern winter.